If it is a small area of light it is probably a plane or helicopter.

When a plane is high in the atmosphere we hear no sound but it appears bright as the Sun is shining on it.

Weather balloons are usually high in the atmosphere and we see them as the Sun is reflected off their shiny surface.

When a meteorite of sufficient size hits the upper atmosphere, it creates a steak across the sky that is bright enough to be seen in the daytime. This is called a ‘fireball’. A fireball often leaves a trail of cloud or smoke.

Sunrise is defined as the time when the Sun is just about to rise for a person at sea level whose horizon is at sea level (i.e. a boat at sea). This means that the top limb of the Sun is level with the horizon. Similarly, Sunset is when the Sun has just disappeared from view.

It is interesting to note that the calculated Sunrise and Sunset times may differ from the actual times by up to 3 or 4 minutes due to variations in the Earth’s atmosphere.

Moonrise is defined as the time when the Moon is just about to rise for a person at sea level whose horizon is at sea level (i.e. a boat at sea). This means that the top limb of the Moon is level with the horizon. Similarly,

Moonset is when the Moon has just disappeared from view.

It is interesting to note that the calculated Moonrise and Moonset times may differ from the actual times by up to 3 or 4 minutes due to variations in the Earth’s atmosphere.

Planets are objects that orbit a star but are too small the have nuclear fusion and therefore do not emit light or radiation of their own. We only see planets because the starlight (or in our solar system – Sunlight) is reflected from them. (Jupiter, our solar system’s largest planet, would just be a star if it was about 80 times more massive).

Moons are objects that orbit planets. Again they are too small the have nuclear fusion and therefore do not emit light or radiation of their own.

Moons are obviously smaller than the planet they orbit. However, there are several moons, including our Moon, which are larger than our solar system’s smallest planet, Pluto.

• Heat generated during the formation of our planet by the infall of material. (Energy of motion, or Kinetic Energy being converted to Heat).
• The ongoing decay of radioactive elements in the core. (Nuclear Fission).
• The fact that rock is a very poor conductor of heat means that it has been able to escape from the core only slowly. (The Surface Area to Volume Ratio means that as the diameter of a body like a planet doubles, the surface area increases as the square of the diameter and the volume goes up as the cube. This means that small worlds can cool much quicker than large ones.)

For this reason the Moon has cooled off inside while the Earth is still hot. The fact that the Earth has a hot mobile mantle means that as the heat escapes to the surface it does work. This work takes the form of convection currents in the Earth¡¦s mantle. These convection currents move very slowly at speeds of about 5 cm per year. It is these slow moving convection currents that tear and buckle our planet¡¦s crust moving the landmasses and their tectonic plates, opening and closing oceans, forming hills and valleys. Sea floor spreading from mid oceanic ridges and subduction at continental margins also creates volcanic activity and major mountain chains such as the Rocky Mountains.

Another important factor is Gravity. The Moon has only 1/6th of the Earth¡¦s gravity and this means that it has not been able to retain an atmosphere or any water. On Earth the presence of an atmosphere and water produces erosion and transportation of material to areas of deposition such as the oceans and lakes. In this way the surface of our planet is slowly changing all the time and tectonic activity is constantly pushing up new regions on which erosion can act. As a result the surface features on our planet have a relatively short life expectancy. On the Moon there is no tectonic or volcanic activity to drive change from the inside and no atmosphere or water to cause erosion. For this reason the surface of the Moon has not suffered change since volcanic activity died out, perhaps 3,600,000,000 years ago and impacting comets and asteroids have left impact craters which remain to the present day.

There is no one agreed reason why planets have rings, but there are several possibilities. It is probable that both possibilities can produce a ring system:

• An asteroid or comet passed so close to the planet that the tidal forces were able to tear the comet or asteroid to pieces. These pieces spread out round the planet to form the rings. We saw an example of the way a comet can be torn apart by a planet when Comet Shoemaker Levy 9 broke into about 22 pieces during a very close approach to Jupiter in 1993. The comet fragments later crashed into Jupiter in July 1994 after completing one more orbit of the planet. On this occasion no rings were formed because the fragments of Comet Shoemaker Levy 9 approached Jupiter at the wrong angle.
• One of the planet’s moons may have come to close as a result of orbital decay and broken up to form a ring system. For all planets there is a distance that can be calculated from gravitational theory for which any body of a reasonable size approaching closer than this distance will be torn apart by tidal forces. This distance is known as “The Roche Limit”.
• Because material from a ring system is slowly falling into the upper atmosphere of the planet all the time, the rings are slowly being destroyed. It used to be thought that the rings of Saturn were formed at the same time as the planet but this now seems unlikely because the rings could not survive this long. Probably the very thin rings of Jupiter, Uranus and Neptune are examples or ring systems that are much older than Saturn's. Jupiter's rings are composed of rocky, dusty material, which suggests that they probably originated from an asteroid instead of a comet.

All stars, including the Sun, are born in great clouds of dusty Hydrogen and Helium that lie between the stars of the Milky Way. As young stars they pass through an early stage when they are somewhat unstable before settling down to an “adult phase” during which they change very slowly. This is known as the “Main Sequence Stage” of a star’s life. In this adult phase they increase their output of energy very slowly. This is the longest stage of a star’s life and varies in length depending on the star’s mass. The larger the star, the shorter will be its adult life. During adulthood a star is producing energy from fusing four atoms of Hydrogen into one atom of Helium. This is the same process as the Hydrogen Bomb and releases a huge amount of energy for a given amount of Hydrogen fuel. This method of producing energy is called “Nuclear Fusion”. Unfortunately it has not been possible to use nuclear fusion to solve our energy crisis on Earth because of the very high temperatures and pressures needed to make it work.

Gradually, as the star uses up its supply of Hydrogen fuel, it slowly increases its output of energy. For a star like the Sun its fuel supply should last about another five to six billion years before it enters the old stage and becomes a red giant. Life on Earth, however, will not be able to survive this long as the slow rise in temperature will make the Earth uninhabitable in about one to two billion years.

Everyone is familiar with seeing the tides rise and fall twice a day. The Moon’s gravity acting on our oceans causes this. What you probably won’t have noticed is that the Earth’s surface also heaves up and down just like the sea. These movements are called “Earth Tides” and represent about 30cm. of up and down motion twice a day.

These tidal movements need energy and most of this energy comes from slowing down the Earth’s daily rotation by a very small amount. Geologists have found evidence that a very long time ago, early in the Earth’s history, our planet seems to have taken about 16 hours to rotate on its axis instead of the 24 hours it takes today. The reason why we only see one side of Moon lies in the fact that the Earth is much heavier than the moon and used to raise “Moon Tides” on our satellite, which were much bigger than those the Moon raises on Earth. The energy for these came from the Moon’s rotation. Because the Moon is only 0.0123 (1.23%) as heavy as the Earth it’s rotation period has been slowed down much more quickly than ours. For this reason the Moon’s rotation period has slowed to the point where it is exactly the same as its orbital period round the Earth. This is a stable state for the Moon and will not change any more. The Earth will, on the other hand, continue it’s very gradual spin down until eventually our day will also be 27.322 of our present days, just like the Moon.

Tidal locking of rotation is found quite frequently when a smaller body orbits a larger one. Where two bodies are of equal size their rotation rates will be slowed identically and they will eventually orbit each other with the same sides always facing each other.

Because it is the air that makes the sky look blue, in space where there is no air, it looks black. Unless you happen to be close to the Moon or a planet there is nothing in space for the Sun to shine on and this is why space looks dark. You are just looking into empty space between the stars.

1. Gravity always acts from the centre of an object equally in all directions. This means that as material falls in to build the planet it is pulled in from all directions equally. In this way the planet grows into a spherical shape.
2. There is a second factor involved as well. Although rock seems very hard and inflexible even the hardest rock bends if put under great pressure, particularly if heated. When a planet forms the material that arrives first soon gets covered by later infalling material. As the planet grows the deeply buried material is subjected to heat and great pressure. This results in the bending and, at times, flowing of this material under the pressure induced by gravity pulling on the overlying material. (That rock can flow will be obvious when we look at the lava from most volcanoes). Because gravity always pulls evenly in all directions from the centre of a body, the force of gravity is strong enough to pull any body into a spherical shape if it is larger than about 150km in diameter.

The fact that small asteroids and moons are often not round is because they are not large enough to have sufficient gravity to pull themselves into a round shape. Small asteroids are often chips that have been knocked off larger ones in collisions, so they started life with funny shapes.

Most stars are spherical in shape like our Sun. The trouble is that stars are so far away that no matter how big a telescope you use you can never magnify a star enough to be able to see it as more than a point of light. This is not strictly true as with modern equipment some of the close stars that are large have been resolved).

When you look at a bright star with just your eyes you are dependent on the quality of the lenses in your eyes to give you a sharp image of the point of light that is the star. Looking at the point images of bright stars is the hardest eye test you can have and you can pick up the smallest imperfections in your eyes if you know what to look for.

Very few people have optically perfect eyes. Even those lucky people who don¡¦t have to wear spectacles usually have small eye imperfections that don¡¦t matter under normal conditions. If you have ¡§perfect vision¡¨ you will see a star as a minute point of light without any spikes. Most people have a small amount of what the optician calls ¡§ASTIGMATISM¡¨. This means that instead of focusing the light of a bright star into a sharp point the lenses of the eye focus it into a small blob with one or more spikes. This is how most people with so-called ¡§normal vision¡¨ see a bright star and why stars are usually portrayed as having spikes.

Have you ever noticed that the smoke from the wick of a candle that has just been blown out looks blue in sunlight? You may also notice that thin smoke from a fire also looks blue. If the smoke gets very thick you will see that it looks brown or even black. The smoke does not change colour, as it gets denser. In fact the smoke particles are microscopic fragments of partially burnt fuel, usually rich in Carbon, and range in colour from brown to black.

The tiny smoke particles are so small that they are about the same size as the waves of blue light. This means that the blue light waves bounce off them in all directions. This is called scattering. The longer light waves of green, yellow and red light can get round the smoke particles and carry on in a straight line.

The Earth’s atmosphere contains a lot of small dust and smoke like particles. These scatter the blue component of the white light coming from the Sun. (Remember that white light is a mixture of all the colours of the rainbow). This is why the Sun’s blue light is scattered by the particles in the air and you can see it as a blue sky.

At Sunrise and Sunset the light from the Sun has to pass through a lot of dense air before it gets to your eyes. This means that almost all the blue light has been scattered away leaving only the longer yellow and red light waves. This is why you often get lovely golden red colours lighting up the clouds and sky at the beginning and end of the day.

As the Moon gets closer to the horizon, the observer has landmarks to relate the size too, whereas when it is high in the sky, there is nothing to relate it too, so it looks smaller.

The Moon¡¦s size does actually vary through the month, but this is not the reason for the illusion. The Moon orbits the Earth in an ellipse not a circle, which means that the distance to the Moon varies by about 10% from about 360,000 km, at its closest, to about 400,000 km, at its furthest. This distance variation equates to a size variation of from about 30„S (minutes of arc) to 33„S.

Since the Apollo Space Programme successfully travelled to the Moon and back in the late 1960’s and early 1970s, there have been a number of people who have asked whether these events had actually taken place. Some have suggested that it was all a propaganda hoax and that it was all filmed on location in a place called: “Area: 51”. This ‘hoax’ theory keeps coming up.
The TV programme called, Conspiracy Theory: Did We Land on The Moon?, hosted by X-Files actor, Mitch Pileggi and was aired by the FOX network. The programme itself must have been a hoax! As one commentator put it: “ Fox should stick to making cartoons, I’m a big fan of the Simpsons!” It was supposed to have been a factual documentary, but it was a distortion in that only one side, one point of view was presented. Even a biased reporter could have verified the facts as a number of commentators have.

Looking at some of the points raised in the programme.

No blast crater from the Lunar Module landing. The gravitational force on the Moon is 1/6 that of the Earth, it wasn’t necessary to use the full 10,000 pound thrust available to land. They only needed to use 3,000 pounds of thrust.
The engine bell at the base of the engine is about 5 feet across, thus the blast pressure on the Moon’s surface equated to about 1 pound per square inch. On Earth a similar rocket with a 60,000 pounds pressure blast produced a mark on the dessert floor that was barely recognisable!

Pictures of space suited crewmembers inside a building. These pictures were taken by NASA during crew training and were never purported by NASA to be anything else. It was the programme that made those claims!

No rocket plume in the video of the ascent stage lift off. Of course there wasn’t! The fuels used were a combination that ignites when making contact with each other. They don’t need an ignitor and produce only a very faint visible exhaust plume. Only if you could look UP the bell would you see a bright blue light! Why didn’t the producers of the programme get this one right? Maybe they should have looked it up!

Flags waving in the breeze. So would you if you were being twisted vigorously by the astronaut trying to get the flagpole into the ground! They look as if they are waving because of the metal rod that runs along the top of the flag, which holds it out as if being blown in a breeze.

Poor quality video. On the early missions, NASA did not realise the importance of video footage and the gear then tended to be bulky and used a lot of power. Weight and power were at a premium. Thus poor footage on Apollo 11.

Absolutely perfect photographs. This one is easy to explain. The astronauts were the top operators of this gear in the world at the time, having practised thousands of times. Also, NASA did not release all the photographs taken; only the best photographs were made public.

No stars in the photographs. Most photographers know the answer to this one. It is very hard to take an image of a very bright surface AND of faint objects at the same time, without super-imposing. In the case of the photographs shown, the starlight is simply too faint to be recorded on the film.

Radiation is too high; the astronauts would have been cooked. This problem had been worked out well before hand. The Van Allen belts are two doughnut-shaped belts in the Earth’s magnetosphere between about 1,000km and 20,000km above the Earth’s surface, where many energetic charged particles from the solar wind are trapped in the Earth’s magnetic field. The Apollo missions flew through them at very high speeds, around 40,000 km/hr. Their total exposure was about 2rems, the equivalent of 100 X-rays or 40% of the maximum permissible dosage; fatal doses are at about 300rems. There simply wasn’t enough time to get a lethal dose and the metal hull of the spaceship blocked most of the radiation.

Crosshairs on the photographs. A number of claims were made on the programme that NASA had ‘touched up’ photographs and manipulated them in some way. Again, if the producers of the show had talked to any knowledgeable photographer they could have learnt that when a photograph is over-exposed, the white parts bleed into the film around them, making all those surrounding images white also. If you looked at the images shown on the TV programme, the missing black cross hairs are surrounded by glaring white light from The Moon’s surface. The white light has bled the cross hairs out! As one commentator put it: “It’s a matter of contrast: the crosshair becomes invisible because the white part overwhelms the film.”

Rocks brought back from the Moon. Apollo astronauts brought back 380 kg of Moon rocks to Earth. They are unique and differ from earth rocks in many respects. Lunar samples have almost no water in their crystalline structures; Earth rocks have a lot of water there. Common substances like clay minerals on Earth are totally absent from the Moon rocks. There were particles of fresh glass found in Moon rocks that were produced by explosive volcanic activity and meteorite impacts over 3 billion years ago. Such rocks on Earth are rapidly broken down by water in only a few million years. These Apollo rocks are peppered with tiny craters from meteoroid impacts. These microscopic specks would burn up in the Earths atmosphere but being no air on the Moon they do impact and cause these craters as they fly through space at speeds often exceeding 50,000km/hr. To make a rock like that, you would need a bigger atomic nuclear accelerator than any existing on Earth today!

One scientist reports that he has a 3m high pile of research papers on Moon rocks in his office. Not a single paper challenges their origin!

With this insurmountable amount of evidence there seems little doubt that the Apollo missions visited the Moon and that astronauts set foot on the surface.

Gravity is a very difficult subject to explain and scientists are still trying to work out exactly what it is and how it operates.

In simple terms gravity is a force (push or pull), which acts between all objects. Small objects like houses or humans produce so little gravity that it cannot be measured. Objects like asteroids which are made of rock and iron and are perhaps from a few to several tens of kilometres in diameter might have a gravitational pull of only a gram or two on a human being.

The Earth has a lot more matter in it than an asteroid so the pull of gravity is much stronger here. The Moon is 0.0123 of the Earth’s mass and has 0.17 of the gravity produced by our planet. If you could stand on the surface of the Sun, which is 332,946 times more massive than the Earth, you would weigh 27.9 times your normal weight.

The amount of gravity which is produced at the surface of a body depends on:

• Its Mass: The amount of matter the body contains.
• The Density: The amount of matter in a given volume. E.g.: grams per cubic centimetre. The Earth has a density of 5.5 grams per cubic centimetre.
• Weight: The force with which gravity attracts a body. Because the Moon has a much lower mass than the Earth you would be attracted to it with a force of one sixth that on Earth. BUT your mass would be the same as on Earth.

Saturn weighs 95.1592 times the Earth, but its density is very low, 0.76 grams per cubic centimetre. This is because it is a gas planet rather than a rock planet. As a result the surface gravity on this planet is only a little more than on Earth. On Saturn you would only weigh 1.15 times more than your usual weight.

Gravity is an amazing force because when we look at clusters of galaxies we find that gravity can act across millions of light years holding the galaxies together for billions of years.

• Until the late 1700’s astronomers had no idea about how far the Sun, the Planets or Stars were from the Earth. Because the planets appear to move against the background of stars, it was known that they were closer than the stars but no one knew how far away they really were.
• One of the main reasons why Captain Cook came to New Zealand was to try to find out how far away the Sun is from Earth. It had just become possible to make very accurate clocks and astronomers had worked out that the planet Venus was going to pass in front of the Sun. This is called a “Transit of Venus”. Astronomers knew that if they could accurately time the transit of Venus from a number of widely spaced locations on Earth, then it would be possible to work out the distance between the Earth and the Sun.
• Having found the distance to the Sun (149,600,000km) it then became possible to calculate the distance to each planet using Newton’s Laws of Gravity. We call the average distance between the Earth and the Sun the “Astronomical Unit”.
• To find the distances to the stars astronomers had to be able to measure VERY SMALL ANGLES. Telescopes were not accurate enough or big enough to do this unti11838 when the German astronomer F.W. Bessel managed to measure the distance to some of the nearest stars.
• Once astronomers had been able to work out how far the Sun was away from the Earth, it became possible to use the value of an astronomical unit to work out the distances to the stars.
• Bessel would measure the direction to a star very accurately against the background stars near by in the sky. Six months later when the Earth had moved half way around the Sun, he repeated the measurements and looked to see if the star appeared to have changed its position against background stars. The more the star had appeared to move, the closer the star was to us on earth.
• Using the value of the astronomical unit, Bessel was able to work out how far away some of the closer stars were using trigonometry. He found that they were much, much further away than any object in our Solar System.
• Light travels at 300,000 km. per second. To travel from the Sun to Earth, light takes 8.3 minutes. The closest star system to our Sun is one of the pointers to the Southern Cross, called Alpha Centauri. Light from this star system takes 4.3 years to reach Earth! This corresponded to a minute shift in position on the sky (parallax angle) of just 0.76 seconds of arc! Bessel was able to find the distances to stars out to 9 light years. Beyond this the parallax angles were too small to measure with his equipment.
• What is a light year? A light year is the distance light, travelling at 300,000km per second, covers in one year! (To work it out multiply 300,000 by 60. This will tell you how far light will travel in a minute. Now multiply that answer by another 60. This will tell you how far it travels in an hour. Now multiply that product by 24. That tells you how far it travels in a day. Now multiply that product by 365.25 and that will tell you how far it travels in a year! That distance is a light year in km.! (9,467,280,000,000km).
• Whereas Bessel with his equipment could only measure distances out to 9 light years, modern astronomers have extended this method to about 10,000 light years, using a special satellite, called Hipparchos.
• Within this distance there are examples of almost every type of star that exists. By knowing how far these stars are away from us, it has been possible to calculate how much light they are giving off! The Hubble Space Telescope can now identify stars in Galaxies that are about 100,000,000 (one hundred million) light years away. It is now possible to work out their distances quite accurately because we know how bright each type of star is. (If we know how bright a star is actually, and we can measure how much fainter it is, we can work out how far it is away).
• The most distant objects in the universe, which we can currently observe with the most powerful telescopes, are thought to be 12,000,000,000(12 billion) light years away!
• It is interesting to realise that Captain Cook and the astronomers on board were some of the people who made it possible for us to determine astronomical distances.

If you were on the Moon looking back at the Earth you would find that the Earth also goes through a cycle of phases in 29.531 Earth days as the Moon orbits our planet. The main difference is that the phases are absolutely opposite as seen from each world. When we are having a new moon on Earth an observer on the Moon would be seeing a “Full Earth”.

When you go outside on a night with a full Moon it is quite bright enough to see clearly. You can even read a book if the print is clear. The Earth would look about four times larger from the Moon than the Moon does from Earth. This means that the full Earth has about sixteen times more surface area than the Full moon. Another factor is that the Earth has a dense atmosphere with a lot of bright, white clouds floating in it. This means that the Earth is able to reflect much more Sunlight back to the Moon than the Moon can reflect back to Earth. A full Earth is therefore far brighter than a full Moon.

When we see a crescent Moon with the rest of the Moon faintly visible like a ghost, what we are really seeing is full Earth light illuminating the night side of our Moon. When this occurs in the evening sky it is often referred to as “The Old Moon in the New Moon’s Arms”. You will notice that if you watch the crescent Moon growing from night to night, the Earth light gets fainter each evening because, as the Moon advances towards full, the Earth phases will be progressing in the opposite direction towards a crescent Earth. The Sun light part of the Moon is also getting brighter and larger, making the faint Earth light area smaller and harder to see. If you watch a crescent Moon in the morning sky you will see exactly the same things happening only in the reverse order.

The three outer planets, Uranus, Neptune and Pluto are too distant from the Sun and the Earth to be visible without a telescope. This is why they were not known in ancient times.

The first new planet to be discovered in recent times was Uranus. This was discovered by William Herschel using a 16cm diameter reflecting telescope of his own manufacture in the back garden of his home in Bath, England. Herschel had been making a careful and systematic survey of the sky and was rewarded by the discovery of a new planet on the 13th March 1781. At first it was thought that the new object was a comet, but further study of its orbit showed it to be a planet and it was eventually decided to name it Uranus.

By 1790 astronomers had found that Uranus was not following the orbital path that it should be. After 1830 there was increasing speculation that this was because the gravity of an unknown planet was causing the difference between the observed and computed position of Uranus. By 1846 John Adams in England and Urbain Leverrier in France had calculated where the new planet should be in the sky. Unfortunately astronomers in neither country seemed willing to carry out the search and it was left to Johann Galle at the Berlin Observatory to locate the new planet on 23rd September 1846 using Leverrier’s calculations. In this way Neptune, as the new planet was named, was discovered jointly by two mathematicians, and an astronomer using a telescope. Subsequently it was realised that Galileo had observed Neptune and failed to recognise it as a planet on 28th December 1612 and 28th January 1613.

As time passed astronomers began to think that they were starting to observe tiny differences between the observed and calculated positions of Uranus and Neptune. Using the methods of Adams and Leverrier two well-known American astronomers, William Pickering and Percival Lowell came up with approximately the same position in the sky for the new planet. In 1929 a special survey was started at the Lowell Observatory using a special wide field photographic telescope. Clyde Tombaugh discovered the new planet on 18th February 1930 by comparing photographic plates taken of the same area of sky on different nights. The new planet showed up because of its movement against the background stars.

As it turned out Pluto is a very small world, only 2,245 kilometres in diameter; actually this is smaller than our Moon, which is 3,476 kilometres in diameter. At an average distance from the Sun of 5,963,000,000 kilometres (40 times the Earth - Sun distance) Pluto is extremely cold and composed mostly of ice. Pluto is such a small world that astronomers started wondering if it was really the missing planet. Until recent space craft visits to Uranus and Neptune it was thought that these planets, as well as Pluto, might be wandering slightly from their calculated positions, there by suggesting the existence of a 10th planet. The results obtained from these space craft have, however, enabled very accurate masses to be found for Uranus and Neptune and when these new values are used in the calculations the need for a 10th planet to explain their motions, and those of Pluto vanish.

It can now be shown that the calculations of Pickering and Lowell were in error and did not assist in Pluto’s discovery. The discovery of the 9th planet was lucky chance combined with dedicated effort on the part of Clyde Tombaugh.

A few hundred years ago, when Europeans started to sail South to explore the world and look for new lands, they thought of themselves as starting from the top of the world. All their maps showed their homelands as on top, which was natural from their point of view. As they sailed South they thought of themselves as sailing round the Earth towards the bottom of the planet. In this way the Southern Hemisphere became thought of as down. There is no absolute reason for doing this; it’s just a convention that has become part of our culture and way of thinking.

If you think about it, you are weightless in space so that up and down have no meaning once you leave the Earth, or any other world for that matter. As there is no up or down in space the idea of a planet like the Earth having a top or bottom really has no meaning either. When you travel round the Earth you will find that you always seem to on top of the world where ever you happen to be.

Many stars do vary in brightness, but none, that are visible to the naked eye, vary rapidly enough to give this effect. If you were able to observe the stars from space you would find that they shine with a steady, unblinking light. Next time you go outside on a clear night have a careful look at the stars. You will find that straight over-head the stars are twinkling much less, if at all, compared to similar stars close to the horizon. This is because when you look close to the horizon you have to look through a lot more air than when you look straight up.

Astronomers use the expression “Air Mass” for the amount of atmosphere that we look through. When we look straight up (zenith), we are looking through the minimum of atmosphere or one air mass. As we look lower down towards the horizon we are looking though many air masses.

You will also notice that on windy nights the stars are likely to be twinkling much more than on a still, calm evening.

So why do stars twinkle?

On a hot summer day, if you look down a long, straight stretch of tar sealed road you will see that everything is shimmering and blurry. This is because the air above the hot road surface is being heated to a considerably higher temperature than the surrounding air. A similar effect is also often seen above a fire when there is no smoke. The hot air above the fire is less dense than the surrounding air and is very turbulent. Air bends light and hot air bends light less than cold air. Because the air above the road or fire is very turbulent the hot and cold air bends the light from distant objects in rapidly changing ways, as they mix, making distant objects appear blurred and simmering.

When we look up at the stars we see them through air that is moving and usually turbulent. In summer, warm air from close to the ground is often mixing with cold air from higher up, while in winter the reverse may be the case. High winds and hilly or mountainous country all add to the amount of turbulence in the wind flow. These fast moving cells of warm and cool air act like rapidly changing lenses or crinkly glass. This means that starlight, as it passes through the atmosphere, gets bent and distorted by the Earth’s air. It has two effects on the star image. Firstly these atmospheric irregularities cause fluctuations in brightness of the star known as scintillation and secondly the light path is irregularly distorted making the star image fuzzy and extended. The scintillation can be seen with the naked eye but the image distortion, known as astronomical seeing, is usually only evident in a telescope.

On a clear night when there is a lot of wind look at bright stars near the horizon and you may even find, that as well as twinkling they are flashing different colours. This is because the seeing conditions are so bad that as well as acting as lenses, the cells of hot and cold air are also acting as prisms and splitting the star light into its constituent colours.

Planets do not twinkle much, because they are extended objects; the twinkling of each part of the planet’s disc averages out to give a steady image. However, planets near the horizon change colours as explained above.

If you look at the location of big, modern observatories have been built over the last 70 years you will find that almost all have been sited on the tops of tall mountains in areas of low rainfall. In this way the telescopes have clear skies and are above most of the atmospheric turbulence. The Hubble Space Telescope is in orbit above the atmosphere and therefore enjoys perfect observing conditions and can function 24 hours a day.