## Finding the Moon in the sky

It's very satisfying to be able to work out where you have to look to see the Moon on a particular day. But, to a casual observer, it can easily seem as if there isn't much rhyme or reason to when and where the Moon appears in the sky. Partly this is because, if the sky is overcast for several days in a row, we may miss seeing it altogether, and won't be able to see where each phase appears in the sky in relation to the others. In fact, the Moon moves across the sky like the Sun, rising in the...

## The ecliptic

There are two fundamental planes of reference in the sky the ecliptic and the celestial equator. Both are consequences of the Earth's various motions. The relationship between them is the key to understanding the apparent motion of the Sun, Moon and planets. Let's begin with the ecliptic. This is the plane of the Earth's orbit about the Sun or, to put it another way, the plane in which the Earth orbits the Sun. It is called the ecliptic because it is the plane that the Moon must occupy if it is...

## Sidereal and synodic months

The Moon takes 27.32 days to make a complete orbit around the Earth. This period is known as the sidereal month. 'Sidereal' means 'relative to the Figure 9.13 Synodic and sidereal months. The length of a lunation is greater than the time it takes the Moon to complete an orbit of the Earth because the Moon must complete more than one orbit to bring it back in line with the Sun, i.e. the time taken from one new Moon (at A) to the next (at C) is a few days more than the time taken to complete an...

## Explaining a solar eclipse

Although an eclipse of the Sun can occur only at new Moon, solar eclipses do not occur every time the Earth and Moon are in syzygy because, as we have seen, the Moon's orbital plane is inclined at an angle of 5 to the plane of the ecliptic. The two points at which its orbital plane crosses the ecliptic are known as nodes. The Sun can be eclipsed by the Moon only if the new Moon occurs when the Moon itself is at, or quite close to, one or other of these nodes. A further complication is that the...

## Anomalous rainbows

Although the scientific theory of the rainbow is more or less complete, it has to be applied cautiously to natural rainbows. The idealised assumptions on which the theory is based seldom, if ever, exactly match the conditions in which rainbows are formed in nature. Hence, very occasionally, you may see, or hear about, a rainbow with features that appear to defy explanation. Such bows are known as anomalous rainbows. Broadly speaking, anomalous rainbows fall into three broad categories. Most...

## Watching a solar eclipse

From an astronomical point of view, an eclipse is the result of the relative positions and relative sizes of the Earth, Moon and Sun. But, for most of us, the attraction is not the astronomical event, the syzygy, it's the spectacle in the sky. In an eclipse, the Moon's shadow overtakes the Earth and races across the Earth's surface. Since the Earth spins in the same direction as that in which the Moon orbits it, the ground speed of the shadow is less than it would be if the Earth were...

## Solar eclipses

Much is made of the coincidence between the apparent size of the Sun and Moon. On closer examination this turns out not to be quite true. The Earth's orbit around the Sun is almost circular, which means that the Sun's apparent diameter of half a degree remains more-or-less the same throughout the year. On the other hand, the Moon's orbit around the Earth is markedly elliptical, and so its apparent diameter changes significantly compared with that of the Sun. In fact, the difference in their...

## Where to look for an inferior planet

There are three factors that determine where in the sky we should look to see an inferior planet. The first is that an inferior planet orbits much closer to the Sun than the Earth does. Hence it is always seen near to the Sun, in either the evening or the morning sky, and never in the middle of the night, except at very high latitudes. The second factor that affects the visibility of an inferior planet is the inclination of the ecliptic to the horizon. Since all planets orbit the Sun more or...

## Notes for ice halo observers

Although, in temperate climates, halos occur in clouds high above your head, in very cold climates they sometimes form at ground level. Often these ground-level halos are not noticed because they are not bright, since the depth of the cloud of ice crystals in which they are formed may not be very great. If you are somewhere where ice crystals can occur near to the ground, it's worth looking at the ground, as well as in the sky, for halos. 1 Make an annotated sketch showing the Sun, the halo (or...

## Halo

Far and away the most common type of halo is one that encircles the Sun or the Moon. If you are prepared to look at the sky every time conditions are right, you should see it several dozen times a year. When fully formed it appears as a diffuse ring of light, about 1.5 wide, with an angular radius of approximately 22 , around either the Sun or the Moon. Sometimes the inner edge of the halo seen around the Sun is reddish. You can quickly make a rough and ready determination of its radius by...

## Looking at the Moon without a telescope

The thing that makes the Moon stand out from all the other things you can see in the night sky is that it is the only celestial body whose surface is visible to the naked eye. Not only that but, if you have good eyesight, under favourable conditions its great lava plains, the so-called 'maria' (lunar 'seas'), can be seen more distinctly with the naked eye than the features of any of the planets seen through a good telescope. These 'maria' give the Moon its spotty appearance. Nevertheless,...

## The apparent motion of the

Curiously, the Sun, the epitome of light, is the key to the night sky. This is because its position along the ecliptic determines when the planets can be seen, what stars are visible during any given night, the phase of the Moon, and where that phase will occur on the ecliptic. This means that, if you know where the Sun is on the ecliptic, you can work out, without Sun's position on the ecliptic at sunrise - Sun's path across the sky during the day Figure 8.3 The daily path of the Sun across...

## Inferior mirages

Inferior mirages are extremely common. Their most striking feature is a resemblance to a shimmering patch of water in the middle distance, but, unlike real water, these disappear when approached. They can be seen by anyone walking or driving along a straight road on a hot day. They are particularly obvious as you approach the top of a gentle incline ahead of which stretches a straight, horizontal section of road. Your eye is then placed just above the layer of hot air next to the surface of the...

## Eclipses of the Moon

A lunar eclipse occurs when the Moon passes through the Earth's shadow. This can only happen at full Moon when the Earth lies between the Moon and the Sun. However, the Earth's shadow is so much wider than the Moon's shadow that anyone who is on the night side of the Earth during the eclipse will be able to see at least some phase of the event. This means that many more people have seen a lunar eclipse than a solar one. Despite this, lunar eclipses are not quite as frequent as solar ones...

## The Moons phases

At any given moment, half the Moon's surface is illuminated by the Sun. However, the side of the Moon that always faces the Earth, the 'near side', becomes visible to us on Earth in stages from one day to the next because of the Moon's orbital motion about the Earth. These stages are, of course, the phases of the Moon. These phases are direct proof that the Moon is spherical. No other shape could account for the same sequence, from crescent to full Moon and back again, that we observe in...

I recommend to those who are new to these games the entertainment of watching the gyrations and transformations of their own shadows while walking at night along a lamplit road. As you pass close to the lamp your shadow will appear short and squat by your side, and slowly turn in the direction of your walk while growing longer and narrower, till the bright lamp of the next lampost will replace it by the shadow that is now behind you. E.H. Gombrich Shadows The Depiction of Cast Shadows in...

## Nakedeye astronomy

To our ancestors, the sky was both a calendar and a clock. For better or worse, improvements in the calendar and the invention of clocks have rendered even the most rudimentary knowledge of the stars, or the movements of the Sun and the Moon, more or less superfluous. These days, there is no practical incentive for anyone, other than an astronomer, to take an active interest in what is going on in the night sky. Consequently, most of us are not in the habit of looking at it closely as a matter...

## Clouds at sunset

Clouds are not themselves the cause of sunset colours. They can, however, make all the difference to a sunset by reflecting the rosy hues of the Sun just after it has set. Arguably, the most eye-catching sunsets are those in which the sky is covered with ribbons of altocumulus cloud. Sunlight reflected by these clouds not only creates a glorious spectacle in the sky, it also bathes your surroundings in a diffuse amber glow. Without a layer of clouds to scatter the Sun's reddened light, this...

## Glossary

Ablation The erosion of a surface, say through melting, vaporising or weathering. absorption When an atom absorbs an electromagnetic wave, the energy of the atom changes and the wave ceases to exist. Depending on the nature of the atoms of which an object is composed, the energy of the absorbed wave may increase its temperature, bring about a chemical reaction or be emitted as electromagnetic energy of a different wavelength. Visible wavelengths that are not absorbed are responsible for the...

## Constellations

Constellations are areas of the sky within which the brighter stars have been grouped together to form distinctive patterns to make it easier for us to find our way around the sky. In the western tradition, this practice began with the Babylonians. Modern star maps divide the celestial sphere into 88 areas of differing shapes and sizes, within each of which is to be found a number of bright stars that have been linked together to make a pattern that can, with some effort of imagination on your...

## Looking for rainbows

Even if you are prepared to scan the sky every time it rains, you will be lucky to see more than a handful of really memorable rainbows in the course of a year. In fact, unless you live somewhere where short-lived rain showers are common, like Hawaii, you probably won't see many rainbows at all. Looking for rainbows demands patience, and a willingness to take advantage of every opportunity when conditions look promising. Rainbows are rare because they form only if several conditions are...

## Apparent changes in brightness of superior planets

The brightness of a superior planet varies partly because it goes through a partial cycle of phases, and partly because its distance from the Earth changes enormously throughout its orbit. At conjunction, the planet is full, but because it is then at its maximum distance from the Earth, its apparent brightness is least. At quadrature, the planet is gibbous but since it is now closer to the Earth, it is brighter than at conjunction. At opposition, the planet is once again full, and since it is...

## Circumzenithal and circumhorizontal arcs

The type of crystal responsible for parhelia can also form circumzenithal and circumhorizontal arcs. Circumzenithal arcs are much more frequent than circumhorizontal ones, though neither of them is as frequent as parhelia. Figure 7.10 Circumzenithal arc. A circumzenithal arc is often mistaken for a rainbow, but, as you can see in this photograph, it differs from a bow in three important ways. In the first place a circum-zenithal arc is on the same side of the sky as the Sun. Secondly it curves...

## The Purkinje effect

A curious effect can be noticed at twilight, or whenever the intensity of illumination is greatly reduced. The effect is named after the man who first brought it to public attention. His name was Evangelista Purkinje, and he was the Professor of Physiology at the University of Prague until his death. He discovered many interesting eye-related optical effects, one of which has come to be known as the Purkinje effect. He noticed that, when the level of illumination is reduced, red objects appear...

## Circular rainbows

If we were birds, and spent our days on the wing, then every rainbow we saw might be circular. As it is, earthbound, the ground gets in the way, preventing the lower portion forming, and we never see more than the upper half. But mimic the birds, and raise yourself high above the ground, either in a Figure 5.7 Circular rainbow. You can see a circular bow if you are able to look down on a shower of drops and are close to it. The gap at the bottom of the bow is due to the object on which the...

## Meteors

The Earth in its orbit about the Sun does not move through a pristine void it battles its way though large numbers of tiny asteroid fragments and vast amounts of dust, much of which has been shed by active comets. Collectively these particles are known as meteoroids. Most meteoroids resemble granules of freeze-dried coffee in both consistency and density, and range in size from a microscopic speck through to a pea. There is no defined upper limit to the size of a meteoroid, though meteoroids...

## Superior mirages

When you see an inferior mirage of an object, at least part of the object is directly visible to you. This, of course, is only possible if it lies somewhere Figure 3.5 Superior mirage. If there is a temperature inversion a few metres or more above the ground an inverted image of a distant object may be seen. This is due to rays that are refracted towards the ground as they pass through the temperature inversion. Under such circumstances some rays reach the observer from the sky and in reverse...

## Lunar puzzles

The fact that we observe the Moon from the surface of another world that is itself in motion makes the Moon behave in ways that can seem puzzling. Here are two such puzzles. The first of these is the way in which the Moon appears to rotate as it moves across the sky. An observer at mid latitudes sees the full Moon rise in the east with its north-south axis inclined to the horizon. As it moves across the sky, the Moon appears to rotate in a clockwise direction so that it crosses the southern sky...

## The purple light

On some evenings - sometimes for several successive evenings - you may just be able to notice a faint lilac tinge in the twilight glow above the western horizon. This is the purple light. It usually makes its appearance just above the twilight arch some 15 or 20 minutes after sunset, and spreads throughout the twilight glow over the next quarter of an hour, reaching its maximum brightness towards the end of this period - i.e. about half an hour after sunset. It then fades away, and the last...

## Comets

There is a long association between comets and disastrous events on Earth. Over the ages, they have been variously blamed for floods, droughts and pestilence. As far as the ancients were able to tell, comets appear unpredictably, seemingly from nowhere, and after a few weeks vanish, never to be seen again. They were seen as a challenge to the harmony and predictability that normally reigns in the skies, and were excluded from the heavens on the authority of Aristotle. He argued that the fact...

## Atmospheric refraction

When looking at something, we not unnaturally assume that what we see is exactly as we see it and precisely where we see it. In almost all situations in which we find ourselves this assumption is perfectly justified because we are close enough to the things that we see for their light to reach us by travelling in a straight line to our eyes through a homogeneous atmosphere. Over larger distances, however, the atmosphere is not homogeneous because its density is not uniform. As you might expect,...

## Moonrise and moonset

A lunation lasts, on average, 29.5 days and so the Moon appears to move across the celestial sphere at an average rate of approximately 13 per day. In other words, on each successive day, the Moon will rise with those stars that were 13 to the east of it at the previous rising. At the same time, since the Earth itself orbits the Sun, the Sun also appears to move eastwards across the celestial sphere, though at the much slower rate of 1 per day. Thus a waxing Moon moves away from the Sun at an...

## Where are the stars

It's tempting to believe that when we look at the night sky we are gazing into an infinite abyss. In fact we see only darkness, which is an altogether different thing. Darkness on its own is merely an undefined void, perceived by eyes that have nothing to focus upon. You can experience darkness simply by closing your eyes. What makes the void of the sky tangible are the stars scattered through it, which act as beacons that give us some sense of the vastness we are contemplating. It has been...

## Explaining the rainbow

A complete explanation of the rainbow, couched in the mathematics of the wave theory of light, is not for the fainthearted. Fortunately, it is possible to account for most of its principal features in terms of reflection and refraction of light, without invoking the wave nature of light. People realised long ago that, since rainbows are always seen on the opposite side of the sky from the Sun, they are due to reflected sunlight. But if raindrops reflected light as efficiently as mirrors,...

## Star brightness

To the naked eye, the most distinctive feature of a star is its brightness, so it is not surprising that when, in the second century b.c., the Greek astronomer Hipparchus compiled the very first catalogue of stars he classified them according to how bright they appeared. Little is known for certain about his methods, though it has been suggested that he used the fact that stars become visible in stages in the gathering darkness after sunset. Stars that are seen soonest after sunset were classed...

## The Sun at the horizon

None of the effects mentioned so far rely on the refraction that occurs as the Sun's light passes through the atmosphere. The degree of refraction increases as this light approaches the Earth's surface because of the increasing density of the atmosphere. The resulting change in direction of the rays is minute, though measurable, and is most marked in the rays that strike the atmosphere obliquely, i.e. in those from the rising and setting Sun. Consequently, the most noticeable effects of...

## The Moon in daylight

In my experience, most people are surprised when they see the Moon in the middle of the day. They believe that this is unusual because they assume that it is visible only at night. Yet this isn't the case unlike all other celestial bodies like the Sun, the stars, or the planets, the Moon is sometimes visible at night, and sometimes during the day. In fact, it is the only celestial body that is regularly visible with little effort both day and night. Furthermore, it is above the horizon at the...

## Sources and notes

Sources of references are listed alphabetically here for each section in the book. These include specific sources of quotations reproduced in the text, and more general sources of reference used during writing, and references that may be followed up by readers who wish to pursue a topic in more depth. References to articles in journals give the author's surname followed by the year of publication, title of the article, name of the journal, its volume number and page numbers on which the article...

## Horizontal rainbows

We expect to have to look up at the sky to see a rainbow. However, rainbows are sometimes seen on horizontal surfaces, such as grass lawns and lakes, when small drops either rest upon the surface, or are suspended a short distance above it. A horizontal rainbow is produced in the same way as a rainbow in the sky a combination of reflection and refraction. A striking feature of this type of bow is its shape. This depends on the height of the Sun above the horizon. When the Sun is directly...