Sky Map
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INTRODUCTION
The Earth is only a very small part of the entire Universe, and we have now come to appreciate that what goes on up in the sky is not only very interesting, but that it can also have an effect on life on our planet.
Some people have likened the Earth to a ship travelling through the immensity of space. A ship that, fortunately for us, carries with it our life support system, and all the necessities for our existence. We are fortunate that our spaceship has an enormous window that allows us to see out into the space around us, and to study the various objects with which we share the universe.
Western Australians are particularly fortunate to be living in a part of the Earth that has wide open spaces with only a small population. This means that there are many locations which have low light pollution, allowing views of the night sky that are limited only by the sensitivity of the human eye, and are unaffected by large city lighting. Moderate to low cloud cover over much of the state also means that celestial wonders can be seen on a large number of nights.
In today’s modern world, many city dwellers have never experienced the full wonder of a truly dark sky. In a dark country place, when there is no moon, thousands of individual stars can be seen in their undiminished glory and the glow of millions more can be seen as a white glow (the Milky Way – our galaxy) arcing across the sky. In the Southern Hemisphere we are particularly favoured as we see more stars than those in the Northern Hemisphere (about 3000 compared to about 2000 in the north), and the centre and brightest parts of our galaxy cross overhead in the winter months.
Although the night sky can be appreciated as a work of art, there is so much more to be discovered through continued observation, and by learning the name and locations of a few stars and constellations. Much of this can be done without any equipment, using only the naked eye, and later a set of binoculars. This can be supplemented by a visit to some of the places around the state that study various aspects of the space environment. The following pages are devoted to explaining various features of this environment and how you can explore and learn more about this realm outside the Earth, and in some cases, how the Earth is affected by what goes on in space.
A SKY MAP
Finding our way around the Earth can be done in a number of ways. For local travel, we know a few local landmarks and we use roads to find our way to the supermarket, our place of work or other venue to which we wish to travel. Outside our local area we use maps. These maps usually indicate our travel by interconnected roads. However, if we travel across the sea or in the air, or even in the outback, we eventually need to make reference to a coordinate system. The usual coordinate system on the Earth uses latitude (north-south direction) and longitude (east-west direction) to specify our position on the surface of the Earth, which is of course a sphere.
When we look up at the sky we can imagine that we are at centre of a large sphere upon which the stars and other celestial objects are located. Of course, we now know that each of these objects are at different distances from us, but that is not immediately apparent to us. This large sphere is called the Celestial Sphere, and like the Earth’s spherical surface, we can assign two coordinates to locate any point on this sphere. These coordinates are called declination (equivalent to celestial latitude), and right ascension (celestial longitude). Astronomers use these two coordinates to locate all objects in the sky.
The Celestial Sphere appears to rotate above our heads (whereas it is actually the Earth that is rotating beneath), and our view of the stars changes from hour to hour. Although this means that we get to see different stars at different times of year, it does make it more difficult to find things in the sky from our vantage point stuck somewhere in Western Australia. A fixed observer prefers to use what we call a horizon coordinate system. In this system, the position of a sky object is given by an elevation above the horizon (in degrees from 0 to 90), and an azimuth (or bearing) from north (in degrees from 0 to 360).
One of the simplest ways to convert from celestial coordinates to horizon coordinates is to use a planisphere. These are sold at the Perth Observatory and the Horizon Planetarium. A rotatable disk is set for the month and time of night, and the appearance of the sky with the position of the brightest stars is shown inside a window.
When you become familiar with the names of the brightest stars and some of the constellations, you can also navigate your way around the sky without any help whatsoever. A good sky map can help you move from those stars you do know to those you don’t.
Also, just like GPS, telescopes can be purchased that will point to whatever coordinates or stars you specify. These are called GOTO telescopes.
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TIME and DISTANCE
In our daily life we use zonal time to determine when we should go to school or work or eat and play. This is a time such that the Sun passes overhead at around noon time.
This time is no good to astronomers. Imagine trying to report an astronomical event (such as a star blowing up), using zonal time. An astronomer in Australia might say that the event occurred at 9pm whereas one in New Zealand would say it happened at 1am. To avoid this problem, astronomers use Universal Time (UT), which is the zonal time at 0 degrees longitude. This used to be called Greenwich Mean Time (GMT). Universal Time is specified using a 24 hour clock and is written as 14:30 UT.
For some purposes UT is not suitable, because UT is based on solar time or the apparent movement of the Sun. Stars, on the other hand complete a full circuit of the sky about 4 minutes quicker than does the Sun. A star day (usually referred to as a sidereal day) is 23 hours and 56 minutes (using solar time). To find the exact right ascension that will be overhead at a particular place, astronomers use sidereal time.
The Universe is a very large place, and our normal units of distance measurement are quite inadequate to cope with space distances. In fact, the unit of distance is dependent on whether we are talking about near space or deep space.
On Earth, we use metres and kilometres to specify distance. In the solar system, we use a unit called the Astronomical Unit (AU), which is equal to the average distance between the Earth and the Sun. This is about 150 million kilometres.
For the stars, which are so much further away than the planets, we commonly use a unit called the Light Year (ly), which is the distance light travels, in a year. Since light travels at about 300,000 km per second, and knowing that there are about 32 million seconds in a year, we find that 1 ly = 9.5 trillion kilometres.
Astronomers also use another unit called a parsec where 1 pc = 3.26 ly.
THE ASTRONOMICAL BRIGHTNESS SCALE
The most noticeable difference between stars when you look at them is their brightness. Astronomers use what they call a magnitude scale to specify the brightness of a star. This scale originated in Greece over 2000 years ago.
Greek astronomers assigned a magnitude of zero to the brightest stars and a magnitude of six to the faintest stars that could be seen. This is the reverse of what we might expect and leads to bright objects (such as the Moon) having negative magnitudes.
When instruments became available, astronomers also found that there was considerable difference between the brightest stars, and this led to the brightest star in the sky (Sirius) coming out with a magnitude of -1.6.
A difference of 5 magnitudes is equivalent to a brightness ratio of 100. The magnitude scale is thus a logarithmic scale which makes it possible to specify a very wide range of brightness (from the Sun at magnitude -27 to the faintest object seen by the Hubble telescope of about magnitude 30) using only a small range of numbers instead of talking in billions and trillions. This goes well with our eye, which responds in a logarithmic way to light intensity, as does our ear to sound intensity (which is measured in decibels, another logarithmic scale). A logarithmic scale is one that has equal intervals for equal ratios. Thus a change from 10 to 100 and a change from 100 to 1000 would both be the same on such a scale.
The scale opposite shows the magnitude scale with the brightness of various celestial objects indicated.
In a remote country region, the human eye can see stars down to magnitude 6 (although some humans, with very acute eyesight claim to be able to see magnitude 7 objects in a dark sky location). However, in the glare of city lights, you might be lucky to see down to magnitude 3. This severely limits the number of stars you can see, as the number of stars increases by a factor of ~10 for each magnitude increase.
VISION - OUR PERCEPTION OF THE UNIVERSE
Our understanding of history is tainted by the amount and quality of knowledge we have of any particular era. The further back we go in time, the smaller the number of hard facts we have, and the greater the likelihood that our preconceived ideas will have in shaping our understanding of the past. The same is true in most sciences and our knowledge of the space environment is no exception. When we view the sky with our own eyes, they are the filter through which we view the universe and which limit our perception.
Although the eye can see over a wide range of brightness, it cannot do so instantly. We have all had the experience of being ‘blinded’ by a flash of intense light. When we go out to look at the night sky it takes a while before our eyes become ‘dark adapted’ and we are able to perceive the faintest of objects. Some of this adaption is due to a change in the size of our pupils. In bright sunlight, the pupil constricts to about 1 mm diameter. In dark conditions it dilates to almost 8 mm and this variation increases the sensitivity of the eye by 64 times. Other changes in the retina account for the rest of the adaption. This all takes times. The adaptation curve opposite shows the first adaptation (to an intermediate level of sensitivity) occurs in about 10 minutes, but that the second and deeper stage of adaptation requires around half an hour. To avoid the loss of dark adaptation, astronomers use soft red light to read sky charts.
There are two sorts of visual sensors in the retina. These are called cones and rods. The cones are the sensors that are used in bright light, whereas the rods come into play in low light. Only the cones are colour sensitive, which is why the stars tend to show little colour to our eye, whereas a camera will show major colour differences between stars.
Averted vision is a trick that can be used to see really faint objects. Don’t look straight at the object you wish to see. Instead, look directly off to the side, and the object you wish to see will appear brighter. The reason is that there are few rods in your central vision (this region is mainly populated by cones).
Another limit to our vision is termed resolution. This is the smallest angular distance over which we see two objects as being separate. Resolution is mainly determined by the diameter of our pupil (or telescope aperture). Humans can see down to a limit of about one minute of arc (1/60th of a degree). Larger telescopes, with a larger aperture (the size of the objective lens or mirror) can do much better, although the atmosphere places a lower limit of around one second of arc (1/3600th of a degree). Telescopes in space can do much better, and the Hubble Telescope, with a 2.4m main mirror, has a resolution of around 0.05 seconds of arc.
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CONSTELLATIONS
The night is full of stars. Stars are distinguishable from one another by eye mainly in their brightness. However, stars are not distributed uniformly across the sky, but occur in groupings that form recognisable features and it this apparent grouping that makes it possible for us to navigate our way through the various patterns from one grouping to another.
The ancient Greeks divided the sky into various groupings that they called constellations. Sometimes the constellation looked like its name, but often it did not. Modern day astronomers used this concept and divided the sky into 88 separate constellations. These cover the entire sky.
The most famous constellation in the southern sky is the Southern Cross. This is always above the horizon for observers in southern Australia. Most other recognisable constellations rise and set at different times of year. The constellation of Scorpius looks like its name and is the most prominent constellation of the winter months. On the other hand Orion (the hunter), doesn’t really look like its name, but is the most prominent constellation of the winter months.
Non-astronomers often view the night sky and can see recognisable patterns in the groupings of stars. These often acquire popular names. They cannot be termed constellations, as there are only 88 precisely specified such groupings. Instead these popular groupings are termed ‘asterisms’.
One of the most famous asterisms is called the ‘saucepan’ and consists of the central stars in the constellation of Orion. Another is an asterism that many Aboriginal groups recognise as the ‘emu’. This is unique in that it does not consist of a grouping of individual stars, but rather it is formed from the light and dark patches (dust clouds) of the Milky Way. Feel free to invent your own asterisms to help you find your way around the night sky.
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STARS
The night sky is full of stars. The brightest of these are given Arabic names (in contrast to constellations which have Greek names). Some examples are Sirius, Canopus, Rigel, Deneb, and Betelgeuse. Stars which do not have Arabic names (and even ones that do) are given names that consist of a Greek letter followed by the constellation in which they reside. The brightest star in a constellation is given the letter alpha (the first letter of the Greek alphabet), and beta is given to the second brightest and so on. For instance, the brightest star in the constellation Scorpius is called both Antares (its Arabic name) and alpha-Scorpii. The nearest star to the Earth is called alpha-Centauri. This is the brightest star in the constellation Centaurus.
Stars vary in their apparent brightness. However, because they are at different distances from us, their apparent brightness is a combination of their actual luminosity and their distance. The two pointer stars (to the Southern Cross) are named alpha and beta Centauri. To our eyes, alpha is somewhat brighter than beta. However, alpha lies 4 light years away, whereas beta is around 300 light years distant. Beta Centauri is thus intrinsically much brighter than alpha. Intrinsic brightness depends upon a star’s size and its temperature. The temperature of a star determines its colour. Astronomers classify a star according to its colour in a sequence labelled OBAFG KM – a classification that can be remembered with the mnemonic “O, Be A Fine Girl, Kiss Me”. O-type stars are blue and M-type stars are red. The Sun is right in the middle of this sequence as a yellow G-type star.
O-type stars have a surface temperature of around 30,000 Kelvin (which is similar to the familiar Celsius scale except that 0 C = 273 K), and M-type stars have a surface temperature of 2000 – 3000 K. Stars also vary in size, and large O-type stars are called Blue Giants whereas large M-type stars are called Red Giants. The constellation of Orion has examples of both these types. Rigel is a blue giant star, whereas Betelgeuse is a red giant. Small stars are called dwarfs. The larger the stars’ mass, the shorter its lifetime. Stars like the Sun are believed to live for about ten billion years, whereas a blue giant may only live for ten million years. Stars sometime come in pairs, or even triplets. Double stars are called binaries. Stars also are not always constant in brightness, and these are called variable stars.
Looking at stars which are at different distances is a form of time travel. For if a star lies at a distance of 1000 light years, the light from that star has taken one thousand years to reach us. So we are looking at the star as it was 1000 years ago, not as it is now.
GALAXIES
Stars are not randomly distributed throughout the universe but rather clump together in groups called galaxies. Galaxies come in different forms. There are elliptical galaxies, spiral galaxies and irregular galaxies. The solar system, in which we live, is in a spiral galaxy, although living inside, we only see a diffuse band of light spanning the sky. We call this the Milky Way. This is the integrated light from the billions of individual stars that are too faint to be seen separately.
It is estimated that there are in excess of one hundred billion stars in our galaxies, and similar numbers in other galaxies. Astronomers also estimate that there are over one hundred billion galaxies in the observable universe.
All the stars we see in the night sky are contained within our own galaxy, and most are relatively close to us, with respect to the size of the Milky Way, which is over one hundred thousand light years in diameter. The Sun lies in the Orion spur which is a branch of the Sagittarius arm, and is about 50,000 light years from the centre of the galaxy. Most of the Milky Way is concentrated in the form of a disc or pancake, but there is spherical halo that surrounds it (and most other galaxies) in which a lower density of old stars and dense clusters of stars live.
We can only see three other galaxies with our naked eyes. The Andromeda Galaxy, also known as M31 (after a designation by the astronomer Charles Messier), lies near the square of Pegasus, and is visible to unaided vision as a faint diffuse point of light. In the southern hemisphere we can also see two other galaxies. These are the large and the small Magellanic Clouds (LMC and SMC). Both of these are irregular galaxies, and look like small diffuse patches of light not far from the south celestial pole. They are often referred to as satellite galaxies of the Milky Way.
To see other galaxies we must use a telescope, and when we do we see some of the most spectacular images that the night sky has to offer. Galaxies not only consist of stars, but many also contain large dust clouds that cut through star fields. Stars also appear in clusters, some in very dense clusters and some in open clusters.
Astronomers now believe that most galaxies contain a massive black hole at their centre. Such objects are so massive that light cannot escape from them. Although they cannot be seen, the effects they have on their environment can. Black holes are now thought, in some as yet unknown way, to be essential to the existence of a galaxy.
THE UNIVERSE ZOO
As we explore more and more of the deep space environment we find it has many strange and sometimes not understood objects. These are not only stars, but clouds of gas and clouds of dust and jets of material that moves extremely rapidly. Some stars feed on others, drawing their matter from them. Some people have likened the diversity of objects in the universe to a zoo. There is even a website where you can help to identify and name new objects in images taken by large ground and space telescopes
Some stars tend to live in clusters. Two types of clusters are common; open clusters and globular clusters. Open clusters tend to exist in the galactic disc. The best naked eye open cluster is the Pleiades, also known as the Seven Sisters. Several other examples of open clusters may be seen with a pair of binoculars around the constellations of Scorpius and Sagittarius.
Globular or dense clusters tend to live in the galactic halo and thus away from the plane of the Milky Way. The southern hemisphere has two good examples of globular clusters. The first, which can be seen with the unaided eye, is called 47 Tucanae. This is relatively close to the Magellanic Clouds. The second, called Omega Centauri, is best seen with binoculars and lies in the constellation of Centaurus, to the north-west of the Southern Cross.
The best example of a small dust cloud is called the Coal Sack and this nestles close to the Southern Cross in its south-west corner. Larger dust clouds can be seen obscuring parts of the Milky Way, particularly as we look toward the centre of the galaxy in the direction of the constellation Sagittarius.
With binoculars we can see faint patches of light that are called nebulae. These are sometimes galaxies and sometimes the remnants of exploded stars. The brightest ones were given numbers by the astronomer Charles Messier, originally as objects not to be mistaken by hunters after comets. There are about one hundred of these objects, numbered M1 through M100 etc. Sky maps are available (particularly via computer) which show the position of these objects together with a description of their nature.
All of the objects we have listed so far are in the region of the stars. Closer to home is the space environment we call the solar system. This is dominated by the Sun, our daytime star. Around this star orbits the planets, asteroids and comets, and a multitude of dust particles. All these can be seen at some time by the naked eye.
There has been an argument recently over what constitutes a planet. The International Astronomical Union made a decision on this a few years ago, but it is far from complete and far from satisfactory. However, in general planets in our solar system are categorised into three groups: (1) Terrestrial or rocky planets like the Earth, Mercury, Venus and Mars, (2) Gas giants, which include Jupiter, Saturn, Uranus and Neptune, and (3) Dwarf planets, which include Pluto, Ceres and many others that have recently been discovered beyond Pluto. Note that all these are planets. A dwarf human is still a human, despite its shortened stature.
Five of these planets are visible to the naked eye at some time during a year (six if we include the Earth, which we see every day, although not in its entirety). These are Mercury, Venus, Mars, Jupiter and Saturn. A few people claim to see Uranus, whose magnitude hovers around the limit of visibility.
Venus is the brightest of all the planets and reaches magnitude -4. It is bright because it is shrouded completely in a layer of clouds that reflect sunlight well. Jupiter is the second brightest planet followed by Mars. Mars is bright enough to activate the cones in our retina, and shows an orange-red colour. Jupiter is the largest planet in our solar system, and Saturn has an awe-inspiring band of rings circling it, although these require binoculars to see, and a moderate size telescope (e.g. 150mm aperture) to see properly.
The solar system is also populated by smaller bodies such as comets, asteroids and meteoroids. These are discussed in later pages.
The Sun is the king of the solar system. Everything else is bound into the solar system by the gravitational attraction of the Sun. This is because the Sun is so much more massive than any other body in the solar system, even the giant planet Jupiter. The Sun is one million times the volume of the Earth and one thousand times the volume of Jupiter.
The Sun is a giant ball of gas held together by its own gravitational force. The surface is a scorching 6000 degrees Celsius, whereas the centre is believed to be around 15 million degrees Celsius. This is sufficient for nuclear fusion to occur. It is this energy that makes its way to the surface of the Sun and which then radiates out into space to make life possible on the Earth.
The Sun is divided into three interior zones: (1) the core where nuclear reactions occur, (2) the radiative zone, through which the core energy passes very slowly, and (3) the convection zone, which boils and which carries heat very quickly to the surface. The surface of the Sun is termed the photosphere, and is not really a (solid) surface at all. It is simply the lowest layer of the Sun that we can see directly. The inner atmosphere of the Sun is called the chromosphere (or colour sphere), and it is this layer that radiates a deep red colour that can be seen in the transitional stages of a solar eclipse. Astronomers also use a special H-alpha (the H is for hydrogen) to monitor the chromosphere for flare and other interesting activity. The outer atmosphere is called the corona.
The Sun and the Moon are the only two bodies in the sky which do not appear to the eye as points, and on which detail can be seen. Because the Sun is so bright (magnitude -27) it is dangerous to look at it directly, and the safest way to observe the Sun is to project its image onto a white background. This can be done using a small telescope or an old pair of binoculars (only one side of the binoculars are used. A shading screen must be placed at the objective of the optics (see diagram below).
The most interesting features on the Sun’s surface are sunspots. These grow and decay and show the Sun’s slow rotation. Try and trace these on successive days.
For those who can’t observe themselves, there are many web sites that show solar images.
We are all familiar with weather on the Earth’s surface: clouds, wind, rain, heat, cold and more extreme weather such as hail, tornados and cyclones. All of this derives from the heat energy radiated by the Sun and modified by the Earth’s lower atmosphere. If there was no Sun, the surface of the Earth would be at a temperature of around -270 C and there would be no weather – no wind, no clouds and no rain.
The Sun also produces a different kind of weather in space. This space weather comes not from the Sun’s heat, but from X-rays, coronal mass ejections (CMEs) and high energy solar particle events (SPEs).
The Sun’s outer atmosphere, the corona is at a temperature of 2 millionK and this continually boils off a stream of plasma called the solar wind. This wind travels out into the solar system. When it passes the Earth it changes the shape of the Earth’s magnetic field. However, every so often a much denser cloud of coronal material (a CME) is blasted out from the Sun. These clouds are millions of kilometres in size and if they hit the Earth they can inject some of their matter into the Earth’s atmosphere and cause a geomagnetic storm. A large geomagnetic storm can damage power lines, disrupt communications and cause low altitude satellites to lose height and decay more rapidly. But the most interesting effect of this type of space weather is to produce bright aurora. People who live in the southern part of New Zealand regularly see aurora, but because even the south coast of Western Australia is further away from the South Pole, it only experiences aurora from the largest geomagnetic storms.
If you live on the south coast of WA and are interested in looking out for aurora, an alert service is offered at
Other types of space weather are caused by X-ray emission from solar flares. This can cause shortwave fadeouts and disrupt high frequency communications.
High energy SPEs are the most damaging form of space weather. The radiation from these can damage satellite electronics and make astronauts sick. Space weather alerts are available from the Australian Space Weather Services, a branch of the Australian Bureau of Meteorology
The moon is the second body in the sky (after the Sun) on which detail can be seen with the naked eye. The most discernable features are dark areas, which are called ‘maria’ or ‘mare’ (pronounced MAR’-AY) in the singular. This is a Latin word which means sea, because our ancestors thought they were similar to the Earth’s oceans. Unfortunately, the moon is extremely dry, and the colour difference is due to the different types of rock that exist in these regions.
Those of us who were alive in 1969 during the first moon landing by NASA may remember that the Apollo 11 astronauts landed their lunar module ‘Eagle’ in the sea of Tranquillity. Learning to recognise the names of the major ‘seas’ on the moon, is a good way to explore our nearest space neighbour.
One of the interesting facets of observing the Moon is the differences that are visible as the Moon goes through its phases in a nearly 30-day cycle. Different craters and mountains are visible at different phases. In general, these features are most visible when they are near the lunar terminator – the dividing line between day and night, or light and dark on the lunar surface. This is because the low elevation of the Sun near the daylight side of the terminator produces long shadows and high contrast.
To view craters you really need a pair of binoculars to supplement your vision, and even with low magnification you will find more craters than you care to count. You may like to memorise ten of the largest craters, such as Copernicus, Kepler, Tycho and Plato. Observe on several nights to see the differences that lunar phase makes, and come back and make follow-up observations a month later.
Although craters are much less visible at full moon, bright rays extending out from some craters are easily seen. The best example is the rays from Tycho, some of which extend across nearly the entire visible disc. Copernicus has rays of smaller extent. These rays are due to bright material which has been thrown out of the crater at the time of formation (impact).
Although they are actually vastly different in actual size, the Sun and Moon actually appear to be almost identical in diameter. This is because they lie at different distances from the Earth. In fact the ratio of their distance is closely the inverse of the ratio of their diameters. This coincidence allows the moon to pass in front of the Sun (from our point of view) and produce what we know as a total solar eclipse.
Although one or two total solar eclipses generally occur somewhere on the Earth each year, the probability of viewing such an eclipse from any given point on the Earth is quite low. On average you will have to wait about 176 years between total solar eclipses at any one place. Total solar eclipses only last a few minutes (generally between 2 and 7 mins) even in the most favourable situations.
Much more common than a total solar eclipse is a partial solar eclipse. This is where the Moon appears to only occult (or hide) a fraction of the solar disc. There are also lunar eclipses where the Earth hides some or all of the Sun’s light. Again these can be total or partial. Lunar eclipses last much longer than solar eclipses because the Earth is much larger than the Sun as seen from the Moon.
Precautions must be taken when observing solar eclipses. Never look directly at the Sun. The safest way is to use projection (see solar page). Alternatively, you can use a shade 14 welding filter (do not use anything lower), preferably mounted in a mask held to the face.
During the short time of totality it is quite safe to look directly at the moon and view the spectacularly beautiful corona that surrounds the eclipsed sun. This is a sight you will never forget.
Although five planets can be seen with the unaided eye, they can be seen only as points of light. So how do we know which points of light in the sky are stars and which are planets? The old adage that stars twinkle while planets do not is not always reliable. Planets can twinkle when near the horizon, and stars high in the sky may not twinkle on clear, calm nights.
Two of the planets, Venus and Jupiter are often brighter than any of the stars. This is a big help in identification. Mars also has a reddish tinge. Mercury never strays far from the Sun and can only be seen just after sunset or just before sunrise. Saturn is normally about a magnitude zero object, and can easily be confused for a star.
Probably the best way to identify planets is by continued observation of the night sky. By this you will learn where the brighter stars are in the sky (with respect to each other), and will easily be able to identify when a foreigner is present.
The name planet is Greek for wanderer, for the fact that they move among the stars. So this is another way to identify planets. See if they are in the same place from night to night and week to week. This is not easy for Saturn, the furthest planet which only moves very slowly through the constellations.
Binoculars are another good aid to help identify planets. All of the planets will show some form of disc (or at least appear larger than a star), albeit small. Venus will show phases like the moon. Jupiter is accompanied by up to four nearby starlike points, which move around Jupiter (even from hour to hour). These are the four largest moons of Jupiter, often called the Galilean moons after Galileo, who was the first to observe them. The reddish colour of Mars is more apparent through binoculars. Saturn appears to have ears through binoculars – a larger telescope is required to show that these are really rings around the planet.
Superb close-up images of all these planets can now be seen at various internet sites due to both amazing advances in image processing of ground based telescopes, the Hubble space telescope, and of course the various spacecraft that have visited these bodies.
Planets are dynamic bodies, and changes can be seen from night to night even in a small telescope. Venus will change phase, Mars and Jupiter show rotation, and storms may be seen on a seasonal basis on the latter two planets. The aspect of Saturn’s rings changes from year to year, and sometimes they appear to vanish, when they are aligned edge-on to the Earth.
The Earth has only one natural satellite, the Moon. Mercury and Venus have no moons. Mars has two very small moons (Phobos and Deimos) that are irregular in shape, and are now thought to be captured asteroids. There is a very interesting story about these moons, for in the novel Gulliver’s Travels, by Jonathon Swift, not only the existence of two moons, but also their orbits were predicted well before they were discovered by Asaph Hall in 1877. If you want to read more about this strange story go to Mars Moons.
Jupiter has four large satellites (Io, Europa, Ganymede and Callisto) that were discovered by Galileo in 1609 when he first turned a telescope toward the planet. These are the only moons, other than the Earth’s Moon, that can be seen in binoculars. Many more moons have been discovered. For many years the number stood at twelve, but now it is around 60. Many of these are very small and are probably captured asteroids. The inner of the four Galilean moons, Io, is the most volcanically active body in the solar system.
Saturn has a very large moon (Titan), which is larger than Mercury and which has an atmosphere of its own. Titan has over 18 smaller siblings.
Uranus has five major satellites (Ariel, Umbriel, Titania, Oberon and Miranda), and Neptune has one (Triton). Even Pluto has several moons, one moon (Charon), almost as large as itself.
In some ways the planetary moons of the four gas giant planets are more interesting than the planets themselves. Because they have solid surfaces, unlike their parents, they have greater potential for exploration by space probes, for the possible existence of life, and for potential human colonisation in the distant future.
Since the discovery of the first planet around another star in 1995, the number of exoplanets (planets outside our own solar system) discovered has risen dramatically. In 2014 (when this was written) there were almost 1000 known exoplanets, with many more awaiting confirmation.
Exoplanets may be discovered by two main techniques. If the planet passes in front of its parent star, as seen from Earth, the occluded light will cause a drop in the brightness of the star. A second technique uses the fact that when a planet orbits a star, the star also shows a small orbital motion. In fact it is correct to say that the star and planet orbit around their common centre of mass. Because the star is much more massive than the planet, the star moves only slightly. This wobble can be detected by a highly sensitive instrument called a spectroscope which shows a Doppler shift in the spectral line wavelengths emitted by the star.
Most of the exoplanets so far discovered have been larger than the Earth, and closer to their host star than the Earth is to the Sun. There is a good reason for this. It is much easier to detect large planets close to their stars than it is to detect smaller planets that are large distances from their host.
Many stellar systems have been detected with multiple planets, and astronomers have been very surprised at the composition of these systems. All sorts of descriptions and theories have been invented to account for these discoveries. In fact, imagination has outstripped fact in many instances, as guesses have abounded to feed our desire to find answers and similar systems to our own.
One feature of a stellar system is the ‘goldilocks zone’. This is a ring around a star where there should be a possibility of finding liquid water on a planet that lies within this zone (that is, where the planetary surface temperature should lie between 0 and 100 Celsius). A dwarf star will have a small goldilocks zone that is close to the star, whereas a giant star will have a large zone which lies at a large distance from the star.
The first asteroid was discovered in 1801 by the Celestial Police, a group of European astronomers, who set out to find the planet that Bode’s Law had predicted should lie between the orbits of Mars and Jupiter. Imagine their surprise when they found not one, but several such objects, and continued to find more year by year.
They were called asteroids (star-like) because they were points of light in telescopes, showing no disc like a planet. However, they are not at all like stars, and only appear as points of light because they are small. It would be much better to call them planetoids (which some astronomers have done). Unfortunately, the popular name has stuck.
There are now over 100,000 asteroids known, most of them very small. Spacecraft flybys have shown that many are irregular in shape, and all are heavily cratered. Only the largest of the asteroids have a circular shape, and these now fall into the dwarf planet category. Recent research has given rise to the idea that many asteroids may not be solid bodies, but rather a rubble pile of many rocks loosely held together.
Most asteroids travel around the Sun, between the orbits of Mars and Jupiter, but many are known to exist in orbits closer to the Sun, and some further away. Some asteroid’s orbits cross the orbit of the Earth and have been called Near Earth Asteroids (NEAs) and even Potentially Hazardous Asteroids (PHAs) if they appear to have the potential to collide with Earth in the future.
Most asteroids have a rocky composition, although differences have led to an asterological (like geological for the Earth) classification scheme.
Vesta is the only asteroid that gets bright enough (magnitude 5) to be seen with the naked eye (although you need to know precisely where to look). This is not because Vesta is the largest asteroid, but because it has a very bright surface, much brighter than its cousins.
Comets are mystery objects that sometimes make their way into the inner solar system and can be seen as a glowing ball with a long tail. They have been referred to as ‘broom stars’, and through the centuries have inspired awe and often fear.
The US astronomer Fred Whipple described a comet as a ‘dirty snowball’. This is the nucleus of the comet, a mixture of dust and ices. As a comet moves along its orbit closer to the Sun, the surface layer of ices melt, releasing dust particles and molecules of gas which first cluster around the nucleus forming the coma. This gas and dust is acted upon by the solar wind and by the pressure of light from the Sun and sends then streaming out from the comet forming a tail which is many millions of kilometres long.
A comet will often have two tails, a dust tail and a gas or plasma tail. The dust tail always points away from the Sun – it is subject to the pressure of sunlight, which always travels in a straight line away from the Sun. The plasma tail lies close to the dust tail, but it is curved, following the path of the solar wind, which streams away from the Sun in a spiral, like water flung out from a rotating garden sprinkler.
Comets are thought to live in a region called the Oort cloud which lies at the very limits of the solar system, about 50,000 Astronomical Units away from the Sun. About one billions comets are thought to exist there. We cannot see them, but every so often, gravitational forces of passing stars push one into the inner solar system where we can see it. Many comets are only seen once, and return to the Oort cloud. However, some are captured by the gravity of Jupiter or another planet and are perturbed into a short period orbit where they return to the inner solar system every few years.
Comet Halley is the most famous periodic comet, with a time between reappearances of 76 years, the last in 1986.
If you lie on your back and look up into the night sky for an hour, you will undoubtedly see a meteor, a fast streak of light across the sky, sometimes called a ‘shooting star’. However, meteors have nothing to do with stars. They are caused by very small particles that burn up as they enter the Earth’s upper atmosphere at speeds between 11 and 72 kilometres per second. The average meteor is caused by a meteoroid (the name for the body in space) the size of a grain of sand, and these may have a brightness magnitude of one or two.
It is now believed that most of the meteors you observe visually are due to grains of dust that have been cast off by comets as they travel around their orbits.
In the evening sky, you may see about one to three sporadic meteors, and these can appear anywhere in the sky. If you watch in the morning hours (say around 3 or 4 am), you will see more meteors, maybe from 4 to 6. This is because the Earth is running away from most cometary particle orbits in the evening and running into them in the morning.
During certain nights of the year you will observe many more meteors than on an average night. These are called meteor showers, and the meteors all appear to come from (or radiate from) one small area of the sky. This location is called the radiant of the shower. Meteor showers can usually be traced back to individual comets, and are due to the passage of the Earth through part of the orbit that the comet has previously travelled along. Showers are named after the constellation in which the radiant is located (e.g. the Taurids radiate from the constellation of Taurus the bull).
Most meteors are no brighter than the stars, but occasionally you will see a stunningly bright meteor that flashes across the sky and may even light up the entire night sky. These types of meteors are called fireballs. (The current astronomical definition of a fireball is that it has a magnitude brighter than -4. This is Venus at its brightest.)
Most fireballs are not due to cometary material, but rather are due to pieces of asteroids that have been smashed off during collisions. Fireballs very rarely appear as showers. If you ever see a fireball that leaves you wanting to tell someone about it, go to the web site
Most meteors start to burn up at altitudes of 100 to 130 km. Meteoroids with higher speeds tend to start at higher latitudes. The meteoroid is normally totally consumed (ablated away) at a height of 70 to 100 km. The greater the initial mass of the meteoroid, the further into the atmosphere it penetrates.
Fireballs are due to meteoroids of larger mass, and these will penetrate down to 50 or even 20 kilometres altitude. A fireball is thus not only brighter, but can be seen for a longer time than the average meteor. As the mass of the large meteoroid is consumed in the ablation process it finally starts to lose velocity and decelerate. This deceleration can be so great that the forces on the meteoroid cause it to implode. This implosion is seen by the observer as a terminal explosion. Several pieces may be seen to fly out of the explosion.
A meteoroid of initial mass of one kilogram will invariably be all consumed by atmospheric ablation. However, an initial mass of 100 kg will normally partially survive the re-entry process and drop a meteorite onto the surface of the earth. The mass of such a meteorite will be only about one kilogram (i.e. 1% of its parent), and its very high space velocity will have been reduced to a terminal velocity of only 100 metres per second.
A meteorite is what is left of a meteoroid (the body travelling through space before it hits the Earth) if it survives the ablation process, which is seen from Earth as a fireball.
Meteorites are nearly all the remnants of asteroids. They come in three main types:
Most meteorites are denser than most Earth rocks and these and other properties allow a rock to be identified as a meteorite, even when it has not been seen to fall from a fireball. Such meteorites are termed finds. However, a meteorite that can be associated with a definite fireball is termed a fall. Falls tends to have larger numbers of stones, whereas finds (which may have laid weathering in the ground for quite some time) are richer in irons. This is because irons are easier to identify as meteorites. They tend to be different from iron- rich Earth rocks, because they always have a significant amount of nickel associated with the iron.
Contrary to belief, when a meteorite has just fallen it is not particularly hot. The short time of re-entry only heats up a thin surface layer which quickly ablates away. Stones however, tend to show an outer black fusion crust, while irons often show strange thumbprint depressions over the surface called regmaglypts.
Although most meteorites come from asteroids, two classes of meteorites have been identified which do not. One comes from the moon (lunar meteorites), and shows similarities with lunar rocks brought back by the Apollo astronauts, and the other is from Mars (Martian meteorites) and show oxygen isotope ratios identical to that in the Martian atmosphere. In both cases, these meteorites must have been blasted off their respective bodies during the impact of an asteroid.
Meteorites are normally named after the place near which they fell (e.g. the Mundrabilla meteorite which fell on the Nullarbor).
A one tonne meteoroid will probably drop a meteorite about 100 kg in mass, and even this will only hit the Earth a low speed (about 100 km/s). The largest known meteorite is the Hoba meteorite in Namibia with a mass of 60 tonnes. It only made a small pit in the ground where it fell.
As the initial mass of a meteoroid increases, the Earth’s atmosphere becomes less able to decelerate the body and consume its kinetic energy. Somewhere between a mass of 10 to 1000 tonnes (depending on the density and angle of incidence), a meteoroid will strike the Earth’s surface with little less than its initial space velocity. Any speed over3 km/s is termed a hypervelocity. At these speeds the kinetic energy carried by a meteoroid is greater than the energy of an equivalent mass of high explosive.
When an object hits the Earth at hypervelocity (and for large bodies this energy may be more than a nuclear bomb), this energy is all released. An impact crater is excavated in the Earth’s surface. Material is thrown out of the crater. Some of the soil may be vapourised, melted and/or fused, and most of the meteorite will be vapourised (and thus will never be found).
Australia has over 25 impact structures. Not all of these are recognisable as craters because some have been extremely weathered or have been buried by later geological activity. The best impact crater in Australia lies in the north of Western Australia in the Kimberleys. This is called Wolfe Creek Crater. It is a little under a kilometre in diameter, and is thought to have been produced by a hypervelocity impact about 60 million years ago.
As a rule of thumb, the diameter of a crater is around 10 to 20 times the diameter of the asteroid that produced it.
Often when observing the sky for the first hour or two after sunset (or the first hour or two before sunrise) you will see a star moving slowly across the sky. This is not a star but an artificial satellite launched into orbit by a rocket. Or more frequently, it may be a piece of space debris – a satellite that has come to the end of its life, a rocket body that launched a satellite into orbit and has remained in its own orbit, or a fragment of either created by an explosion or a collision between two satellites.
Most of the satellites that you observe are in low Earth orbit (LEO), at altitudes of between 200 and 2000 km. The lowest of these move very rapidly and orbit the earth in under 90 minutes. They will move across the sky fairly rapidly and will only be visible for a few minutes. Satellites that are in higher orbit may take 105 minutes to orbit the Earth, and will move more slowly. Some may be visible for up to 15 minutes.
Most visible satellites have diameters between 1 and 10 metres. A few, such as the international space station are larger and consequently brighter. A satellite is only visible because it reflects sunlight. This is why LEO satellites can only be seen just after sunset and just before sunrise. The observer must be in darkness but the satellite must still be in sunlight. An hour or two after sunset the satellites will also be in darkness. You will not see a LEO satellite at midnight.
Satellites that are in geosynchronous orbits at 36,000 km altitude can be seen all through the night, although they are so faint that you will need a telescope to see them.
These satellites tend to vary from magnitude 10 to magnitude 16.
Sometimes a satellite will be in just the right position and just the right orientation that the Sun reflects off their solar panels like a mirror, and a bright flash of light will be seen on the ground. In such cases the satellite can briefly become as bright as the moon.
To identify a satellite you have just seen, or to plan a time when you should go outside to see a certain satellite, visit the web site Heavens Above.
As there is pollution on Earth so there is pollution in space. The problem of orbital space debris grows greater each year. There are currently around 20,000 objects in orbit with a size greater than 10 centimetres. Of these only about 2000 or 10% are active useful satellites. The rest are hazards.
Orbital space objects travel at hypervelocity (in low Earth orbit typically around 7 km/s) and in consequence carry an enormous kinetic energy by virtue of their motion. A piece of space debris only 1 cm in size can render a satellite useless or kill an astronaut if it hits. To avoid collisions with high value active satellites, their ground controller will sometimes move them out of the way of a potential collision. But this costs fuel, and decreases the useful life of a satellite.
Space debris consists of old satellites, rockets bodies and fragments created when satellites accidently collide, are deliberately blown up, or even explode of their own accord (e.g. due to volatile fuel which has been left in the tanks of old rockets).
There have been many suggested ways to remove space debris from orbit, but they are all very costly and often involve political problems as well. One technique is to use a ground-based laser to lower the orbit of debris so that it burns up in the atmosphere. There is an Australian company working on this idea. They are based in Canberra but have a field test station in Western Australia. But firing laser beams into space upsets some people, and what if you accidentally cripple or deorbit an active satellite?
Although space is pretty close to a vacuum there are enough air molecules in low Earth orbit (up to 1000 km altitude) to cause enough drag through air resistance to an orbital satellite, that its orbital altitude will be slowly lowered.
The average time that it takes for a satellite to decay and then burn up in the atmosphere is:
A satellite reentry will appear like one or more bright points moving across the sky. It can be distinguished from a meteor or fireball by the time it is visible. If it is visible for less than 10 seconds it is probably a meteor or fireball. If it visible for more than 10 seconds it is most likely the reentry of a piece of space debris. A reentry will typically appear as several lights moving together (although a fireball may also).
Like a fireball, a large piece of space debris may survive the reentry process and make it to the ground. Approximately one to two pieces of space debris larger than 10cm are estimated to reenter the Earth’s atmosphere every day. Of these, about 20% (1 in 5) are estimated to survive reentry and make it to ground somewhere on the Earth. Of course, because the Earth is 70% ocean, most of these will impact over water. Even so, there should be one land impact every 15 days or so. Keep your eyes open. One person has been reported struck by a piece of reentry debris (insulation from a US military satellite).
We have tried to show throughout this booklet that your own eyes are all you need to discover a lot about our space environment.
When optical aid is needed, it is much more economical to buy a pair of binoculars rather than a telescope. The best size for astronomical use is 10x50. The first number refers to the magnification (10) and the second number is the size of the objective lenses (50 mm). The larger the objective aperture, the more light is collected and the fainter the objects that can be seen. Aperture also governs the ultimate resolution. The best magnification for binoculars or a telescope is the aperture in millimetres divided by 5. Thus 10 is the optimum magnification for an objective of 50 mm. The highest magnification that should ever be used is equal to the aperture in millimetres. However, at this magnification, the image will be much fainter and the field of view will be quite narrow. And what is known as the exit pupil of the instrument will be very small so that only a portion of the eye’s retina will be illuminated. Besides all this, a magnification of 10 is the largest than can be used when holding the binoculars with the hands. Even at 10, to achieve the best results, it is advisable to mount the binoculars on a tripod or other solid stand. Only then can the finest details be seen. Any magnification over 10 absolutely requires a solid stationary mount.
Do not purchase a telescope until you have spent some time at an observatory looking through various types of telescope and have received advice from a knowledgeable person. Good telescopes are expensive and a purchase decision should not be made lightly. Avoid cheap telescopes, particularly ones that advertise high magnification. A 300x50 telescope is absolutely useless for astronomical observation. Even bright stars will appear faint, you will never see more than one star at a time, and you will appear to be looking down a very small barrel. Purchasing such a telescope as a gift for a young person is the best way to turn them off astronomy!
Telescopes come in two major varieties: (1) refractors, which have an objective lens at the far end and an eyepiece lens next to the eye, and (2) reflectors, which employ an objective mirror and an eyepiece lens. There are many different varieties of reflecting telescopes all with their good and bad points. You may not need to purchase a telescope to discover the space environment, but if you do, try for some time (in the field, not a shop) before you buy, and take advice from several different people, books and astronomy magazines. Learn the night sky beforehand so you know what sort of objects are best suited for a telescope and which are not.
These are the ten brightest stars that can be seen from Australia.
DISTANCE is the distance of the star from the Earth in light years, where one light year is about 9.5 trillion kilometres.
BRIGHTNESS is the magnitude brightness of the star.
The star TYPE is from the OBAFGKM astronomical classification, where O type stars are the hottest and M type stars are the coolest. "var" indicates that the star is a variable magnitude star.
Of the 88 constellations only 19 contain a bright star and those 19 are the ones listed above.
There are many sites in Western Australia that have a connection to the space environment. The list below is a sample only.
Formal Astronomy Education
Observatories and Telescopes
Space Weather Monitoring
Space Object Tracking
Impact Craters
Space Communication
NASA Tracking Station Sites (Historical)
There are many ways to discover the space environment. Here are some suggestions.
THE SOLAR SYSTEM
THE SUN
SPACE WEATHER
THE MOON
ECLIPSES
PLANETS
Name Distance Diameter Density) Year Day
(AU) (km) (gm/cc) (Earth days) (Earth days)
Mercury 0.39 4880 5.4 88 59
Venus 0.72 12100 5.2 225 243R
Earth 1.0 12800 5.5 365 1
Mars 1.5 6400 3.9 687 1
Jupiter 5.2 143000 1.3 4331 0.4
Saturn 9.6 120000 0.7 10727 0.4
Uranus 19.2 50000 1.3 30718 0.7R
Neptune 30.1 49500 1.6 60216 0.7
PLANETARY MOONS
EXOPLANETS
ASTEROIDS
COMETS
METEORS
Shower Date Local Time Duration Velocity
Name (Transit) (days) (km/sec)
Lyrids 21 Apr 0410 2 48
Eta Aquarids 4 May 0735 10 64
Delta Aquarids 29 Jul 0210 20 41
Orionids 21 Oct 0420 5 66
Leonids 16 Nov 0026 4 72
Phoenicids 5 Dec 2000 1 13
Geminids 13 Dec 0205 6 35
FIREBALLS
METEORITES
(1) stones (2) stony-irons and (3) irons. Each of these three types is subdivided into further classes. For instance, many stones are found to have chondrules (see opposite page) in them. These are called chondrites, whilst stones without chondrules are called achrondrites.
IMPACT CRATERS
SATELLITES
SPACE DEBRIS
REENTRIES
Height(km) Decay
200 1 day
300 1 month
400 1 year
500 10 years
700 100 years
TELESCOPES
THE TEN BRIGHTEST STARS
NAME CONSTELLATION DISTANCE BRIGHTNESS TYPE
Sirius Canis Major 8.6 -1.5 A
Canopus Carina 1200 -0.7 F
Alpha Centauri Centaurus 4.3 -0.3 K/G
Arcturus Bootes 36 0.0 K
Vega Lyra 26 0.03 A
Capella Auriga 42 0.08 G
Rigel Orion 900 0.10 B
Procyon Canis Minor 11.4 0.4 F
Achernar Eridanus 85 0.5 B
Betelgeuse Orion 310 0.5 var M(giant)
SOME IMPORTANT CONSTELLATIONS
NAME TRANSLATION BRIGHTEST STAR
Aquila Eagle Altair
Auriga Charioteer Capella
Bootes Herdsman Arcturus
Canis Major Great Dog Sirius
Canis Minor Little Dog Procyon
Carina Keel Canopus
Centaurus Centaur Alpha Centauri
Crux Australis Southern Cross Acrux
Cygnus Swan Deneb
Eridanus River Achernar
Gemini Twins Pollux
Leo Lion Regulus
Lyra Lyre Vega
Orion Hunter Rigel
Piscis Australis Southern Fish Antares
Taurus Bull Aldebaran
Virgo Virgin Spica
WESTERN AUSTRALIAN SPACE PLACES
General Space Knowledge
Undergraduate and postgraduate courses in astronomy & radio astronomy
Undergraduate and postgraduate courses in astronomy & radio astronomy
SPACE ACTIVITIES
Australian Space Academy