Kitabı oku: «Collins Night Sky»

COLLINS

NIGHT

SKY

STORM DUNLOP & WIL TIRION

CONTENTS

Cover

Title Page

HOW TO USE THIS BOOK

OBSERVING THE NIGHT SKY

What to use

Starting out

Exploring the night sky

The celestial sphere

Positions in the sky

Date and time

Constellations

The names of stars and other objects

Magnitudes

Photographing the night sky

THE CELESTIAL SPHERE

Winter constellations

Spring constellations

Summer constellations

Autumn constellations

MONTHLY CHARTS

THE MOON

Lunar eclipses

The surface of the moon

North-west quadrant

North-east quadrant

South-west quadrant

South-east quadrant

THE PLANETS

Planetary positions

Minor planets

OBJECTS IN THE SKY

Nacreous and noctilucent clouds

Aurorae

Rockets and satellites

Meteors and the zodiacal light

Comets

Stars

Clusters

Nebulae

Galaxies

CONSTELLATIONS

ADDITIONAL TABLES

FURTHER INFORMATION

GLOSSARY

INDEX

Copyright

About the Publisher

HOW TO USE THIS BOOK

This book is intended for those taking an interest in the sky, wanting to find their way around among the many constellations, and also carry out simple observing with the naked eye, binoculars or small telescopes.

The first section begins with some general advice on OBSERVING THE NIGHT SKY. This is followed EXPLORING THE SKY by information that will help readers to locate objects in the sky, and also to understand the charts, diagrams, and tables of positions that are provided both in this book and elsewhere. This portion includes descriptions on the names of stars and other celestial objects, together with the reasons for their differences in brightness and colour. The section ends with some information about simple photography.

The next section will help beginners to find their way around the sky. This is not as complicated as it might at first appear. Everyone starts by learning to recognize the five main circumpolar constellations, which are visible whenever the sky is clear (THE CELESTIAL SPHERE). The next few pages describe the location and recognition of the constellations that are most clearly visible at different seasons, together with the most notable objects in that part of the sky. Although arranged to start with the winter, readers may choose any season to begin observing. The same applies to the following section (MONTHLY CHARTS), which shows the appearance of the night sky with two charts for each month of the year, looking north and looking south.

Specific information about individual objects follows, beginning with the Moon (THE MOON), and then the planets, including detailed charts showing planetary positions for a period of five years. These are followed by charts showing the periods when some of the brighter minor planets (asteroids) are visible in small instruments.

The next section (OBJECTS IN THE SKY) describes individual phenomena or objects in more detail, ranging from unusual high-altitude clouds that are visible in twilight or in the middle of the night, to distant galaxies.

The final section (CONSTELLATIONS) may be used at any time, but comes into its own when readers are moderately familiar with finding their way around the sky. There are charts and photographs of all the constellations visible to mid-latitude northern-hemisphere observers, together with a description of objects of interest. The positions of meteor radiants are shown, and charts and comparison stars are given for a number of interesting variable stars. Details of sources of further information, and a glossary.

OBSERVING THE NIGHT SKY

WHAT TO USE

If you have only just become interested in astronomy, resist the temptation to rush out and buy a telescope. The majority of cheap telescopes offered in many shops and magazines are generally not worth buying. You do not need any equipment at all to learn to find your way around the sky, identify the brightest objects, and begin observing.

Once you can find your way around, you can consider using some form of optical equipment. Old-fashioned opera glasses are actually quite useful, because they have a low magnification – so you do not become lost – but are not particularly common nowadays. It is probably best to invest in a good pair of binoculars. These are always designated by their magnification and aperture, such as ‘8 × 42’ or ‘7 × 50’, where the first figure indicates the magnification (8 and 7 times, respectively) and the second the aperture – the diameter of the main lens (the objective or object glass) through which light enters the instrument. As a general rule in astronomy, the larger the aperture the better, because more light is being gathered.

Binoculars have one disadvantage: if too large or heavy they are difficult to keep steady. You can buy or make suitable mountings for binoculars, but these tend to be cumbersome and make life difficult for beginners. Avoid very high magnifications, because these restrict the field of view and amplify every minor movement. (The modern image-stabilized binoculars are marvellous to use, but extremely expensive.) Generally, magnifications of 7×, 8×, and 10× are best, while one of 12× becomes difficult to hold steady. Some simple tricks that help with the problem of steadiness are sitting in a garden chair that has arms to support the elbows; resting the binoculars on top of a broom, a wall, or something similar; and tying a length of string or light chain to the binoculars, and standing on one end while gently pushing upwards to put the string or chain in tension. When trying to observe objects right overhead, it is best to lie flat on the ground on a suitable mattress.

You must also avoid too low a magnification. In the dark, the pupil of the eye expands to a maximum diameter of about 7 mm – slightly less for older people. The light leaving any optical instrument does so as a circular beam, the diameter of which equals the aperture of the instrument divided by the magnification. Put in simple terms, the diameter of the exit pupil should not be larger than the maximum diameter of the pupil of the observer’s eye. With 7 × 50 binoculars, the exit pupil is about 7 mm, which is about the maximum for useful work. (Technically this is simplification and other factors affect an instrument’s performance, but it is a useful rule-of-thumb.)

Monoculars, many of which essentially consist of half of a pair of binoculars, are also useful. The types used by birdwatchers often come with a tripod, but observation of objects high in the sky may be difficult. The magnification on many monoculars, however, is often too high for most astronomical purposes.

If you become sufficiently interested in astronomy to want to observe fainter objects, there are many different types of telescope that you can buy (or even build). Details about choosing between the different types are given in some of the publications mentioned under ‘Further Information’. Although some amateurs have highly sophisticated equipment, including computer-controlled telescopes and electronic (CCD) cameras, it is worth remembering that serious scientific work is carried out by many observers with nothing more than a pair of 7 × 50 binoculars.

Measure the circular exit pupil by holding the binoculars in front of an evenly illuminated surface (such as the blue sky). If the exit pupil has flattened sides, the prisms inside the binoculars are cutting off some of the light.

View a horizontal line on a distant object, and move the binoculars away from your eyes until you see twin images. They should be perfectly aligned (TOP). If one is slightly higher than the other (CENTRE), your eyes may be able to correct the difference, but if they are tilted (BOTTOM), eyestrain will result.

STARTING OUT

There are a few points to remember when you start observing. You need to find somewhere fairly close by where it is dark and you have a clear view of the sky. (When you become more experienced, you may be happy to travel considerable distances to suitable observing sites, but initially you need somewhere that is convenient.) Because of the problems of light pollution it may be difficult to avoid a general glow of light in the sky, but try to find somewhere that is at least out of any direct lighting. You may need to use more than one position to see the whole sky. Ideally, find somewhere that is also sheltered from the wind.

Choose the darkest position possible, because this will help your eyes to become dark adapted. Unlike the expansion in size of your pupils which happens almost instantaneously when you move into a dark area, dark adaptation depends upon the concentration of a particular pigment in the retinas of your eyes. This accumulates after a period in the dark and may take 15–20 minutes (or even longer if you are tired), and enables you to see faint objects. Although it varies between individuals, most people are actually able to see quite well – enough to move about without falling over things – by starlight alone. It help greatly if garden paths, patios, or similar areas are light in colour, especially because dark-adapted vision is colourless, so everything just appears as a shade of grey.

Stephen Pitt

The Moon, moving from west to east (right to left) is just leaving total eclipse.

Unfortunately bright light destroys dark adaption, so astronomers use dim red lights to illuminate their charts, books, and notebooks, because this colour has the smallest effect. If you do not have a dim red light, cover a torch (flashlight) with red plastic or cellophane. For some types of observation it is not essential to be able to see what one is writing – if it consists of figures, letters, words or times, perhaps, rather than drawings – so some astronomers develop the habit of writing in the dark, using a dark pencil and drawing a thick line under each observation. They then transcribe their notes later – exactly, with no changes whatsoever, but more neatly – into their observing notebooks.

Stephen Pitt

Two Leonid meteors frame Orion. Jupiter is the bright, over-exposed object in Gemini, and the tiny red dot in the centre is the Rosette Nebula in Monoceros.

It is also important to keep warm and dry. Depending on where you are, it may become cold at night even in summer, so warm clothing is always important. Roughly a quarter of body heat is lost through the head, so wear a proper hat. Make sure you have adequate footwear and, if it is likely to be very cold, wear two pairs of socks and have proper gloves or mitts. Try to avoid standing on wet grass and, if you do have to lie down to make observations high overhead, use a groundsheet and a thick piece of foam. An adjustable garden chair with arms is extremely helpful, but even here try to avoid sitting over wet grass. In some climates, it may be warmer at night, but you may then have to think about wearing clothing that protects you against insect bites, or using insect repellent.

Finally, it is useful to keep a note of your observations. Use a notebook with fixed leaves, rather than a loose-leaf type. Keeping records need not be a chore. At first you may have little to record, but even a simple note of your observing sessions can prove very useful. Make a habit of recording the date and time of any observation. This is particularly important when you see anything unusual – such as an aurora or extremely bright meteor – and we will discuss dates and times shortly.

EXPLORING THE SKY

It is not difficult to start finding your way around the sky, but there are a few details and some specific terms that need to be mentioned first. Initially, it helps to have some idea of the location of north from your observing position. (We shall see shortly how this may be determined accurately from the stars themselves.)

Everything in the sky appears to lie on a gigantic sphere (the celestial sphere), centred on the observer. From our position on Earth, we can, of course, see only half of this at any one time, because half is below our horizon. Although in practice the actual horizon is irregular, and quite large parts of the sky may be hidden by mountains, hills, trees, buildings or other objects, the astronomical horizon is assumed to be a perfectly even boundary, like a sea horizon. It forms the basis of one method of describing positions in the sky, using co-ordinates known as altitude and azimuth.

Altitude is the elevation of an object, in degrees, above the horizon, ranging from 0° (object on the horizon), to 90° (object directly overhead). Note that objects may have negative altitude, i.e., be below the horizon. The second co-ordinate, azimuth, is measured from 0–360 degrees, clockwise, from the north point of the horizon. Due north is thus 0° (and 360°), east 90°, south 180°, and west 270°. (Note that some older books use a different definition of azimuth, but the one just described is the form generally used today.)

Important terms for positions in the sky, relative to the observer at the centre.

The point directly above the observer’s head is known as the zenith (altitude 90°) and is frequently used in astronomy. The corresponding point directly below the observer’s feet is known as the nadir.

An important line in the sky is the meridian, which runs around the sky from the north point, through the zenith, the south point, the nadir, and back to the north point. From the surface of the Earth, only half of the meridian is visible at any one time, of course. Astronomers use the term ‘transit’ for when an object crosses the meridian in the south, when it is also said to ‘culminate’ (reach its highest altitude).

Because of the Earth’s rotation from west to east, the celestial sphere seems to rotate round the Earth once a day from east to west. Everyone is used to seeing the Sun (and Moon) rise in the east and set in the west, but it still comes as a surprise to some people that the stars and planets do the exactly same.

The celestial sphere appears to rotate around an invisible axis, running from the north celestial pole, through the centre of the Earth, to the south celestial pole. The location of the celestial poles relative to an observer depends upon the latter’s position on Earth, more specifically, on their latitude. At the North Pole, the North Celestial Pole is directly overhead (at the zenith); at the Equator, both celestial poles lie (theoretically) on the horizon; and at the South Pole, it is the South Celestial Pole that is at the zenith, with the North Celestial Pole at the nadir. The altitude of the celestial pole is exactly the same as the observer’s latitude. At 40°N, for example, the North Celestial Pole has a altitude of 40°, and an azimuth of 0°. Similar considerations apply in the southern hemisphere.

The altitude of Polaris above the northern horizon is equal to the observer’s latitude.

This has an important effect. An area of sky around the celestial pole, with a radius equal to the observer’s latitude, is always above the horizon. Stars in this region are circumpolar: they are visible whenever it is dark. Identifying the constellations in this area is therefore easy, and an ideal way of starting to learn your way around the sky.

THE CELESTIAL SPHERE

Although it would be possible to locate anything in the sky by referring to its altitude and azimuth, this is not particularly practical for most observers. Both co-ordinates alter throughout the night as an object moves across the sky and, in any case, they are different for every observer. Computer-controlled telescopes (including the largest telescopes on Earth) do use altitude and azimuth, but the positions of objects on the celestial sphere, on charts and atlases, and in catalogues are always specified by a different pair of co-ordinates.

How the celestial co-ordinates Right Ascension and Declination are determined.

These are right ascension and declination, which correspond to longitude and latitude, respectively, used to specify positions on Earth. Let us start with declination. We have already seen that the Earth’s axis points to the North and South Celestial Poles, which directly correspond to the Earth’s North and South Poles. Similarly, the celestial equator lies in the same plane as the Earth’s equator, and divides the sky into northern and southern hemispheres. Declination is measured north and south of the celestial equator (which has a declination of 0°), positive towards the north, and negative towards the south. So the South Celestial Pole has a declination of -90°, for example.

Right ascension is slightly different from longitude in that it is reckoned in hours, minutes and seconds of time (rather than in degrees). One hour equals 15°, so right ascension runs from 0h to 24h (0–360°). But just as declination has a zero point (the celestial equator), one is required by right ascension. Longitude on Earth is measured from Longitude 0°, which is actually defined as the optical axis of the Airey Transit Telescope at the Old Royal Greenwich Observatory. The zero point for right ascension is the point at which the Sun, travelling along its apparent path in space (the ecliptic), crosses the celestial equator from south to north. This point is known as the vernal (spring) equinox, otherwise known as the First Point of Aries.

From this point (0h), right ascension is measured eastwards along the celestial equator (anticlockwise looking down on the North Pole). As the Earth rotates, the right ascension on the meridian (or in any other fixed direction) continuously increases. As we shall see shortly, however, after 24 hours as shown by our terrestrial clocks, the right ascension on the meridian will not be precisely the same as the previous day, but will have increased by approximately 3m 56s. The line of right ascension passing through an object is known as the hour circle.

For various dynamical reasons, the Earth’s axis is not fixed in space, but undergoes a series of motions. The most important of these is known as precession, which causes the axis to describe a cone in space over a period of about 25,800 years. This causes the celestial poles, the celestial equator and thus the position of the vernal equinox, to change over time. It is obviously impractical for quoted positions on the sky to be constantly changing, so catalogues and charts give positions at a specific point in time, known as an epoch, usually revised at 50-year intervals. At present the epoch used is 2000.0, so the position of Betelgeuse (α Orionis) the bright red star in Orion, given in full, would be: RA = 05h 55m 14s, Dec = +07° 24’ 26” (2000.0). All the charts in this book and most others published in recent years are for epoch 2000.0. You may encounter some older charts for epoch 1950.0 or even earlier dates. For naked-eye, binocular, and small-telescope observation the differences between two epochs are largely irrelevant, but they do become important when aligning and using moderate-sized telescopes and computer-controlled equipment. Similarly, seconds of time and seconds of arc are omitted from the positions given here, because such precision is unnecessary for simple observations.

Because of precession, the vernal equinox has migrated from Aries into Pisces, and is moving southwest towards Aquarius.

POSITIONS IN THE SKY

Describing the directions of celestial objects may cause confusion unless one is careful. We must be certain whether we are talking about the position of an object relative to the horizon and the standard compass points, or whether we are referring to its position on the celestial sphere. Generally if a celestial object as said to lie north-west (for example) of a particular star it is taken to mean that the directions are those that apply on the celestial sphere.

Because we are looking at the ‘inside’ of the celestial sphere, care is needed. Looking south in the northern hemisphere, west is always to the right (and east to the left), as might be expected. Looking towards the northern sky, however, we have to be a bit more cautious. The direction of ‘west’ changes depending on where the objects lie relative to the North Celestial Pole. Going west you circle the Pole in an anticlockwise direction – to the right below it, and to the left above it.

Using compass directions to describe positions on the sky becomes difficult in circumpolar areas, so the terms ‘preceding’ and ‘following’ are used instead.

Partly to avoid this confusion, astronomers also use the unambiguous terms ‘preceding’ and ‘following’ when describing the relative positions of objects. These are related to an object’s right ascension. It two stars have the same declination, the star that rises (and sets) first, precedes the other. In general, except perhaps near 0h (24h) RA, it has a lower value of right ascension. Looking south in the northern hemisphere, it lies to the right. The opposite criteria obviously apply to ‘following’. The two terms are nearly always combined with the directions north and south, to divide the sky around the reference object into four quadrants: north preceding, north following, south following, and south preceding, usually abbreviated ‘Np’, ‘Nf’, ‘Sf’, and ‘Sp’, respectively.