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.