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ОглавлениеIf 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.
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.
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.
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.
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.
Occasionally you may come across the ‘position angle’, which is an accurate method of describing an object with reference to another. It is particularly use in connection with double and multiple stars and, as such, is described here: DOUBLE AND MULTIPLE STARS.
STAR-HOPPING
The easiest method of finding things on the sky is to use the time-honoured method known as ‘star-hopping’. This simply consists of using stars, or patterns of stars, that you know, to help you find less familiar ones. It is the method everyone uses to learn the constellations, and may be applied just as successfully to fainter objects, whether using the naked eye, binoculars, or a telescope.
Storm Dunlop
As this photograph of Perseus shows, the eye tends to travel outwards from the brighter stars by recognizing lines and specific patterns of stars.
Generally, one works from bright, known stars or patterns to the fainter ones. When faced with finding an unknown object from a star chart, start by locating some stars that you know or can find easily on the sky. Use these to extend a line across the sky; to form the base of a triangle; the start of a curve of stars; perhaps even an ‘arrow’ pointing in the right direction; or some similar sort of pattern to take you to another star, pair or group of stars, closer to your destination. You can use this next reference point to repeat the process. It all sounds more complicated than it is in practice, because pattern recognition is so innate in everyone that it soon becomes second nature to apply it to stars and star charts.
One important aspect of astronomy is the question of time. Because observers are scattered across the globe, they lie in different time zones, and an event (such as the eclipse of one of Jupiter’s satellites, for example), may happen in daylight for one observer, and in darkness for another. Then there are the problems caused by the use of summer time (daylight-saving time) in certain countries, especially when (as in the United States) not all parts of a country adopt it.
Chart of time zones
The solution is for all astronomical events and observational reports to be given in Universal Time, which is the same for everyone, everywhere. Universal Time (UT) is the time of the Greenwich meridian, originally known as Greenwich Mean Time (GMT). It runs from 00:00 to 24:00 hours, starting at midnight. It is used for all the astronomical phenomena that are listed in the standard yearbooks and handbooks (such as those listed under ‘Further Reading’). In tables of planetary positions, for example, the Right Ascension and Declination of planets are normally given for 00:00 UT (i.e., midnight on the Greenwich meridian).
LOCAL TIME
Because the periods of day and night that you experience depend upon your position on the Earth, you do need to take local time into account in determining whether an event will be visible (unless you live close to the Greenwich meridian).
The map shows the different time zones around the world, and the amount by which each is in ahead or behind Universal Time. Note that the boundaries of the time zones are generally chosen to coincide with political boundaries, and are highly irregular. For exact calculations, allow a difference of one hour for every 15° east or west of the Greenwich meridian, 4 minutes for every degree, etc.
SUMMER TIME / DAYLIGHT-SAVING TIME
It is necessary to take summer time (daylight-saving time, DST) into account when planning observing sessions. Because most people observe in the evening, the monthly charts in this book are drawn for 22:00 local mean time (10 p.m. LMT), which is 23:00 local summer time (11 p.m. LST). This is a compromise, because at high latitudes in summer twilight persists throughout the night and even at 23:00 summer time it may not be really dark.
NOTE
At the time of writing, Hawaii, Saskatchewan and parts of Indiana and Arizona do not employ daylight-saving time.
In Europe, summer time is in force from the last Sunday in March to the last Sunday in October. The change (forwards in spring, backwards in autumn) takes place at 01:00 GMT (UT) on the dates given. In the United States and Canada, daylight-saving time (DST) starts on the second Sunday of March, and ends on the first Sunday of November.
THE DATE
The other important point to bear in mind is that of the date. For astronomers world-wide, this changes at midnight (24:00 hours) UT. This may cause confusion, because observers in a different time zone, east of the Greenwich meridian, will have already changed their civil date, and for observers to the west, their civil date has yet to alter. The simplest solution is to have a clock that permanently shows Universal Time, preferably a digital one that also shows the date. All serious observers in the Greenwich time zone also do the same, because it avoids any potential confusion over the use of summer time.
How should you record the date and time? The best way is to use the standard scientific method, in which the elements are given in descending order of size: Year, Month, Day, Hour, Minute, and Second (and in very precise cases, decimal fractions of a second). Again, to prevent confusion, it is best to record the month as an abbreviation rather than a number. So we might have a date and time given as (say) 2001 Oct. 05, 01:35:07 UT. This is unambiguous, and in many countries (such as Great Britain) is a legally recognized way of specifying the date and time.
If you happen to observe an important event – perhaps a brilliant daytime fireball, for example – when a watch or clock showing Universal Time is not immediately to hand, then write down the observation using the current local time (even summer or daylight-saving time), making sure that you record that fact as well. A correction to UT may be made later. Don’t try to work out the correction there and then, because this takes your attention away from the event in question and is liable to error. It is all too easy to add an hour in ‘correcting’ summer time, for example, instead of subtracting one.
Another method to reckoning the date (the Julian Date) will be described later, when discussing variable stars.
A solar day is slightly longer than a sidereal day, because the Earth must rotate through an additional angle before the Sun again crosses the meridian.
SIDEREAL TIME
We have already noted how the stars visible on any night slowly change throughout the year, new stars rising over the eastern horizon at dusk, as stars that have been visible for some months disappear beneath the western horizon. This change occurs because although our civil time is based on the Earth’s rotation, relative to the Sun, where one day equals 24 hours, relative to the stars, the Earth rotates once in 23 hours, 56 minutes, and 4 seconds. (The difference arises because of the Earth’s motion around the Sun, which means it has to rotate a little further to bring the Sun back into the same position in the sky.)
Steve Massey
Knowing the sidereal time (the RA on the meridian) can be of considerable help in locating faint objects, such as the Helix Nebula (NGC 7293) in Aquarius.
Sidereal time is time as determined by the stars. It is equal to the right ascension of objects that are currently on the meridian. If you know the sidereal time, you can immediately tell which objects are visible in the sky. Sidereal time obviously differs for every location around the planet. Handbooks often tabulate the sidereal time at the Greenwich Meridian for 00:00 UT, and you can obtain your local sidereal time by making the appropriate corrections for the time that has elapsed since 00:00 UT and for your longitude. Many amateur astronomers, however, adjust a clock to run slightly fast, and thus keep sidereal time. Once set, this gives an immediate indication of objects that are on their local meridian.
If Ori is now on the meridian, exactly 24 hours earlier, δ had just passed the meridian and, tomorrow night, 24 hours later, ζ will be just approaching it.
You may occasionally come across the term ‘hour angle’ used to describe the position of an object in the sky. Hour angle is the angle, measured westwards along the equator (i.e., anticlockwise looking down on the North Pole), between the meridian and the hour circle passing through the object in question. Hour angle therefore increases with the passage of time.
SEE ALSO
The celestial sphere is divided into 88 individual constellations. Many of these hark back to antiquity, and various ancient civilizations divided the sky in different ways and had a whole range of myths that were associated with the various groups of stars. The constellation names that we use today largely derive from Greek and Roman sources. Very few bear any resemblance to the objects that they purport to represent.
Northern constellations are often ancient, but many of those for the southern hemisphere were introduced about the middle of the 18th century by the French astronomer de Lacaille. For hundreds of years the boundaries between constellations were extremely fluid, and largely depended on the whim of the person drawing up the descriptions or charts. The boundaries were more or less arbitrary lines around the figures. Stars might be allocated to one constellation by one cartographer, and to the neighbouring constellation by another person. Various map-makers introduced smaller constellations of their own, some of which have survived, but most of which have disappeared.
Royal Astronomical Society
This pictorial depiction of the constellation of Cygnus comes from the second edition of Johannes Bayer’s URANOMETRIA, originally published in 1603.
The end to this confusion came in 1920 when the International Astronomical Union (IAU) adopted a standard set of constellation names; and again in 1930, when specific constellation boundaries were laid down. The latter were defined as straight lines in right ascension and declination for epoch 1855. This date was selected because that epoch had already been used for defining many southern constellations. If you glance at a modern chart, you will see that, because of precession, the constellation boundaries have drifted away from the modern co-ordinate grid.
SEE ALSO
Although popular names for the constellations are sometimes encountered, astronomers always use the Latin forms, so these are employed throughout this book, as are the appropriate genitive forms and standardized three-letter abbreviations, all of which were formally adopted by the IAU.
ASTERISMS
An asterism is a conspicuous pair or group of stars that is given a specific name, but which does not necessarily form a whole constellation. One example is The Plough – known as the Big Dipper in North America, as well as by many other names – the seven brightest stars in Ursa Major, a large constellation that includes dozens of other stars.
Neil Bone
The most famous asterism of all: the seven stars of ‘The Plough’ or ‘Big Dipper’.
THE ZODIAC
The Zodiac was originally a specific set of twelve constellations with a particular significance for ancient civilizations. They were the constellations in which the Sun, Moon, and major planets were found. As such, they were centred on the ecliptic, the Sun’s apparent path across the sky, and each constellation was regarded as extending for 30° along the ecliptic. Because the majority of the constellations were associated with animals, the whole band was named the Zodiac.
It was once believed that stars and planets exerted a direct influence upon human affairs, so the twelve constellations assumed an astrological significance and were seen as specific ‘signs’. Rational people no longer believe in such superstitions, and precession has shifted the position of the constellations eastwards by some 30-odd degrees, so they no longer agree with their signs. Changes in constellation boundaries mean that some constellations occupy more or less than 30° of the ecliptic, and also that the Sun, Moon and planets may appear in various additional constellations, most notably Cetus, Orion, Sextans, and Ophiuchus. The Zodiac is now defined as a band, stretching 8° on either side of the ecliptic, and regarded as including parts of all these different constellations. Charts of the zodiac are useful for showing planetary positions, as seen later in this book.
ZODIACAL CONSTELLATIONS
Virgo to Aquarius: the zodiac is the strip between the two broken green lines, where the Sun, Moon and planets are found.
Pisces to Leo: the zodiac is the strip between the two broken green lines, where the Sun, Moon and planets are found.
THE NAMES OF STARS AND OTHER OBJECTS
Originally, only some of the brightest stars were given individual names. Some are of Greek or Roman origin (such as Sirius and Polaris), but many were devised by medieval Arab astronomers (Betelgeuse, Deneb, Zubenelgenubi, etc.). For hundreds of years fainter stars tended to be identified as ‘the first in Orion’s club’ or ‘the right foot of Hercules’, and other similar, ambiguous descriptions. The first step in introducing a sensible method of identifying stars was taken by the German cartographer, Johannes Bayer, who published the first true stellar atlas, the Uranometria in 1603.
Bayer designated each of the brightest stars in every individual constellation by a Greek letter, arranged approximately in decreasing order of brightness, so that the brightest was labelled α (alpha), the second brightest β (beta), and so on. When Bayer exhausted the Greek alphabet, he employed lower-case Roman letters (a, b, c ...), and in just a few instances, when he had also exhausted those, he used upper-case Roman letters (A, B, C ... Q). Although we now know that there are exceptions to the correct order, in general all these names have been retained to the present day. Astronomers tend to use the Bayer letters in preference to the proper names. The Greek names are followed by the genitive of the Latin constellation name, so the star Alpheratz, for example, is α Andromedae or, abbreviated, α And. Occasionally, the Greek letters have been supplemented by adding a superscript number, usually where stars are close together. The extreme example of this is in Orion, where, on the western side, there is a chain of stars designated π1 to π6.
With advances in astronomy – and particularly with the advent of the telescope – it became important to give designations to many fainter stars. The British astronomer, John Flamsteed was the first to carry this out systematically, and his catalogue of 1725 gave numbers to all the stars in a constellation in increasing right ascension – i.e., from west to east – including stars that Bayer had labelled. These Flamsteed numbers continue to be used, but to prevent confusion, the older, Bayer letters are generally given where applicable. For example, α And is also Flamsteed 21 Andromedae, usually written ‘Fl 21’ or ‘21 And’. If a number appears on a chart, in the absence of any information to the contrary, it may be assumed to be a Flamsteed number.
With the advent of photography, tens and hundreds of thousands of stars had to be catalogued, and so all later catalogues use a numerical format. Such numbers are rarely required when dealing with the majority of objects visible with the naked eye or small instruments.
Variable stars have a complex system of names, but the brightest in any constellation will generally have a single, upper-case, Roman letter designation in the range R to Z – the letters Bayer did not use. Other variables will have a double letter designation (e.g., CH Cygni), or be known by the letter ‘V’ followed by a number (e.g., V465 Cassiopeiae). The Bayer, Greek-letter designations are retained for those stars that have subsequently proved to be variable, for consistency with older records.
MESSIER, NGC & IC NUMBERS
Non-stellar objects frequently have names consisting of the letter ‘M’ followed by a number in the range 1–110. This stands for the designation in the catalogue of non-stellar objects prepared by Charles Messier in the late 18th century to help in the search for comets. Messier catalogued 103 objects, M104 to M110 being added by later observers.
Similar numbered catalogues prepared in the 19th century were the New General Catalogue (NGC) and the Index Catalogue (IC), and these abbreviations are frequently encountered. These catalogues included objects in the Messier list, so alternative designations are sometimes encountered. The great Andromeda Galaxy, M31, for example, is also NGC 224.
The brightness of a star, planet, or satellite is known as its magnitude. Originally, when this system was first devised, the brightest stars were judged to be of first magnitude, slightly fainter ones as second magnitude, and so on, down to sixth magnitude, which were the faintest visible to the naked eye.
This very crude system survived until the nineteenth century, when it was set on a firm scientific basis. A first magnitude star was defined as being 100 times as bright as a sixth magnitude star, leading to a precise mathematical relationship between magnitudes. A first-magnitude star is 2.512 times as bright as a second-magnitude star. This apparently rather odd relationship – a logarithmic ratio – actually closely matches the way in which the eye perceives brightness.
There was no problem in extending the scale to the innumerable fainter stars that could be seen with telescopes, but it was found that a few stars (and certain planets at particular times) were brighter than the star that was selected as the zero point of the scale. So the scale was extended to negative magnitudes. The brightest star in the sky, Sirius, in Orion, is of magnitude -1.44, Jupiter may reach magnitude -2.5, Venus magnitude -4.6, while the Full Moon is about -13.0.
The sizes of stars on charts are carefully related to their actual magnitudes.
The faintest star that is visible with the naked eye, or with a particular instrument is known as the limiting magnitude. Under perfect conditions, with an absolutely black, pollution-free sky, the limiting magnitude for the naked eye is about magnitude 6.5. The colour of a particular star can affect the way in which it is detected by the eye, and this will be discussed later.
Stars of different magnitudes are shown on most star charts as dots of differing sizes: the larger the dot, the brighter the star. If care is taken in choosing the correct dot sizes, most people find it relatively easy to relate stars on a chart to stars of different brightness in the night sky. Depending on the scale of the charts and their intended purpose, their limiting magnitudes will vary. The individual constellation charts given later in this book reach magnitude 6.5 and thus show all the stars visible to the naked eye. Individual finder charts for certain specific objects have fainter limiting magnitudes.
SEE ALSO
LUMINOSITIES
The magnitudes that we have discussed are the apparent magnitudes (m), that is, the magnitudes as they appear in the sky, taking no account of the stars’ very differing distances.
A telescopic view of Jupiter, the second brightest planet, often reaching magnitude –2.5.
If we know the distance to a star, we can calculate the magnitude that it would have at some specific distance and this is chosen to be 10 parsecs (32.616 light-years). Magnitudes calculated for this standard distance are known as absolute magnitudes (M) and are a measure of the actual luminosity of stars. These luminosities are found to cover an extremely wide range, from thousands of times the luminosity of the Sun, to thousandths of its luminosity.
Although apparent magnitudes are often quoted for non-stellar objects, such as planets, comets, nebulae, and galaxies, they need to be treated with some caution, because they do not apply to point sources. The magnitude is taken over the whole extent of the object, so it is far more difficult to see a 7th-magnitude comet or nebula than a 7th-magnitude star.
Ed Grafton
Steve Massey
The magnitudes quoted for extragalactic objects such as M88 (in Coma Berenices) are often deceptive, because they include the light from the outer regions, which are difficult to detect.
It is not difficult to take photographs of the night sky, and most of those shown in this book have been obtained with relatively simple equipment. Because photographs of constellations usually bear a close resemblance to the sky, many people find them easier to understand than maps and charts, especially when they are first beginning to recognize the constellations.
Nearly all cameras are digital nowadays, but there are advantages in using film, notably in the reproduction of the colours of stars and certain objects, such as the Orion Nebula, where the distinctive pink tint visible in the (film) photographs opposite and MONTHLY CHARTS is lost by most digital cameras. Film cameras with a ‘B’ setting also provide a basically unlimited length of exposure for recording star trails or very faint objects. Most digital cameras (even digital SLRs) often have a maximum duration of a few minutes – rarely more than 30 minutes – so long, unbroken star trails are impossible to achieve. Film also has the advantage of being less sensitive to light pollution during long exposures.
Digital imaging has advantages in that it is possible to examine an image immediately after it has been obtained to check framing or whether the image shows the desired amount of detail; several images may be ‘stacked’ to reveal fainter objects and features; and computer-processing is able to remove background colour casts and reduce the effects of light pollution.
Tunc Tezel
A powerful telephoto lens or a telescope is required to obtain large images as in this photograph of the crescent Moon and Venus.
In short, either film or digital imaging may be used. At the time of writing, Ektachrome, an excellent transparency film for constellation images and star trails, is still available, and is recommended. For either type of camera you will, of course, require a stable tripod, and a suitable mechanical or electrical/electronic remote shutter release, with a locking mechanism to allow long exposures.
With 35-mm film cameras, a standard 50-mm lens covers a field of about 35° ∞ 47°, a useful area to include the whole of Orion and similar-sized constellations. The standard lenses on many digital cameras cover a similar field. Such a lens is often suitable for photographs of planetary conjunctions taken at twilight, when you will probably want to include some foreground objects to provide a more attractive picture. Remember, however, that images of the Moon and planets are extremely small with all ordinary lenses (with a 50-mm lens, the Moon occupies approximately one-hundredth of the width of a 35-mm frame). For large images, you need to use a telescope, driven to follow the object in the sky.
Zoom lenses allow the area covered to be altered but always involve some loss of light when compared with fixed focal-length lenses. A wide aperture (f/2.8 or better) is desirable to gather plenty of light. Focussing may be a problem with digital cameras, because most rely upon automatic focus, so manual focussing should be chosen if available.
If using film, it is important to specify it should be returned to you uncut, because automated machinery will sometimes cut through the centre of each frame. Standard ideas of film speeds do not fully apply to astronomical subjects, but films in the range ISO 100–400 will generally give good results, and similar ISO-equivalents should be used with digital cameras.
With a fixed tripod and camera position, the Earth’s rotation causes everything to record as a trail. With short exposures the trailing may be too small to see. Experiment with exposures of 10, 15, 20 and 30 seconds. The apparent rotation of the sky is fastest near the equator, so longer exposures may be used near the poles. Long star trails look dramatic, but they make it difficult to recognize the constellations that have been photographed.
Storm Dunlop
A trailed photograph of Orion taken with a standard 50-mm lens, a fixed 35-mm camera, and an exposure of 5 minutes.
