The Celestial Sphere
ATLAS OF THE UNIVERSE
First Point of Aries
(0h or 24h RA, 0 ̊ Dec)South celestial pole
(–90 ̊ Dec)North celestial pole
(+90 ̊ Dec)Ecliptic
(Earth’s orbital plane)Declination
(Dec)Right Ascension
(RA)Celestial equatorDeclination circleRight Ascension circleEquatorA
ncient peoples believed the sky to be solid, with the
stars fixed on to an invisible crystal sphere. This is a con-
venient fiction, so let’s assume that the celestial sphere real-
ly exists, making one revolution round the Earth in just less
than 24 hours and carrying all the celestial bodies with it.
The north pole of the sky is simply the point on the
celestial sphere which lies in the direction of the Earth’s
axis; it is marked within one degree by the second-
magnitude star Polaris, in the Little Bear (Ursa Minor). Of
course there is also a south celestial pole, but unfortunately
there is no bright star anywhere near it, and we have to
make do with the obscure Sigma Octantis, which is none too
easy to see with the naked eye even under good conditions.
Just as the Earth’s equator divides the world into two
hemispheres, so the celestial equator divides the sky into
two hemispheres – north and south. The celestial equator
is defined as the projection of the Earth’s equator on to the
celestial sphere as shown in the diagram above.
To define a position on Earth, we need to know two
things – one’s latitude, and one’s longitude. Latitude is the
angular distance north or south of the equator, as measured
from the centre of the globe; for example, the latitude of
London is approximately 51 degrees N, that of Sydney 34
degrees S. The latitude of the north pole is 90 degrees N,
that of the south pole 90 degrees S. The sky equivalent
of latitude is known as declination, and is reckoned in
precisely the same way; thus the declination of Betelgeux
in Orion is 7 degrees 24 minutes N, that of Sirius 16
degrees 43 minutes S. (Northern values are given as or
positive, southern values as or negative.)
All this is quite straightforward, but when we consider
the celestial equivalent of longitude, matters are less simple.
On Earth, longitude is defined as the angular distance of
the site east or west of a particular scientific instrument,
the Airy Transit Circle, in Greenwich Observatory.Greenwich was selected as the zero for longitude over a
century ago, when international agreement was much easier
to obtain; there were very few dissenters apart from France.
We need a ‘celestial Greenwich’, and there is only one
obvious candidate: the vernal equinox, or First Point of
Aries. To explain this, it is necessary to say something about
the way in which the Sun seems to move across the sky.
Because the Earth goes round the Sun in a period of
one year (just over 365 days), the Sun appears to travel
right round the sky in the same period. The apparent yearly
path of the Sun against the stars is known as the ecliptic,
and passes through the 12 constellations of the Zodiac
(plus a small part of a 13th constellation, Ophiuchus, the
Serpent-bearer). The Earth’s equator is tilted to the orbital
plane by 23^1 ⁄ 2 degrees, and so the angle between the
ecliptic and the celestial equator is also 23^1 ⁄ 2 degrees.
Each year, the Sun crosses the equator twice. On or about
22 March – the date is not quite constant, owing to the
vagaries of our calendar – the Sun reaches the equator,
travelling from south to north; its declination is then
0 degrees, and it has reached the Vernal Equinox or First
Point of Aries, which is again unmarked by any bright
star. The Sun then spends six months in the northern hemi-
sphere of the sky. About 22 September it again reaches
the equator, this time moving from north to south; it has
reached the autumnal equinox or First Point of Libra, and
spends the next six months in the southern hemisphere.
The celestial equivalent of longitude is termed right
ascension. Rather confusingly, it is measured not in
degrees, but in units of time. As the Earth spins, each point
in the sky must reach its highest point above the horizon
once every 24 hours; this is termed culmination. The right
ascension of a star is simply the time which elapses
between the culmination of the First Point of Aries, and
the culmination of the star. Betelgeux culminates 5 hours
53 minutes after the First Point has done so; therefore its
right ascension is 5h 53m.
Oddly enough, the First Point is no longer in the con-
stellation of Aries, the Ram; it has moved into the adjacent
constellation of Pisces, the Fishes. This is because of the
phenomenon of precession. The Earth is not a perfect
sphere; the equator bulges out slightly. The Sun and Moon
pull on this bulge, and the effect is to make the Earth’s
axis wobble slightly in the manner of a gyroscope which
is running down and has started to topple. But whereas
a gyroscope swings round in a few seconds, the Earth’s
axis takes 25,800 years to describe a small circle in the
sky. Thousands of years ago, when the Egyptians were
building their Pyramids, the north celestial pole lay not
close to Polaris, but near a much fainter star, Thuban in the
constellation of the Dragon; in 12,000 years’ time we will
have a really brilliant pole star, Vega in Lyra (the Lyre).
For the moment, let us suppose that Polaris, our
present pole star, lies exactly at the pole instead of being
rather less than one degree away (its exact declination
is 89 degrees 15 minutes 51 seconds). To an observer
standing at the North Pole of the Earth, Polaris will have
an altitude of 90 degrees; in other words it will lie at the
zenith or overhead point. From the Earth’s equator the alti-
tude of Polaris will be 0 degrees; it will be on the horizon,
while from southern latitudes it will never be seen at all.
When observed from the northern hemisphere, the altitude
of Polaris is always the same as the latitude of the observer.
Thus from London, latitude 51 degrees N, Polaris will be
51 degrees above the horizon. From Sydney, the altitude
of the dim Sigma Octantis will be 34 degrees.
A star which never sets, but merely goes round and
round the pole without dipping below the horizon, is said▲ The celestial sphere.
For some purposes it is still
convenient to assume that
the sky really is solid, and
that the celestial sphere is
concentric with the surface
of the Earth. We can then
mark out the celestial poles,
which are defined by the
projection of the Earth’s axis
on to the celestial sphere; the
north pole is marked closely
by the bright star Polaris in
Ursa Minor, but there is no
bright south pole star, and
the nearest candidate is
the dim Sigma Octantis.
Similarly, the celestial
equator is the projection
of the Earth’s equator on
to the celestial sphere; it
divides the sky into two
hemispheres. Declination
is the angular distance of
a body from the celestial
equator, reckoned from the
centre of the Earth (or the
centre of the celestial sphere,
which is the same thing);
it therefore corresponds
to terrestrial latitude.E152- 191 UNIVERSE UK 2003mb 7/4/03 5:47 pm Page 168