THE STARS
to be circumpolar. To decide which stars are circumpolar
and which are not, simply subtract the latitude of the ob-
serving site from 90. In the case of London, 90 51 39;
it follows that any star north of declination 39° will never
set, and any star south of declination 39° will never rise.
Thus constellations such as Ursa Major (the Great Bear)
and Cassiopeia are circumpolar from anywhere in the
British Isles, but not from the southern Mediterranean.
Another useful example concerns the Southern Cross,
which is as familiar to Australians and New Zealanders as
the Great Bear is to Britons. The declination of Acrux, the
brightest star in the Cross, is 63 degrees. 90 63 27,
so that Acrux can never be seen from any part of Europe,
though it does rise in Hawaii, where the latitude is 20
degrees N. To have a reasonable view of the Cross, there
is no need to travel as far south as the equator.
Incidentally, it was this sort of calculation which gave
an early proof that the Earth is round. Canopus, the second
brightest star in the sky, has a declination of 53 degrees;
therefore it can be seen from Alexandria (latitude 31
degrees N) but not from Athens (38 degrees N), where it
grazes the horizon. The Greeks knew this, and realized
that such a situation could arise only if the Earth is a globe
rather than a flat plane. From Wellington, in New Zealand,
Canopus is circumpolar, so that it can always be seen
whenever the sky there is sufficiently dark and clear.▼ Star trails. This
photograph, with an
exposure of two hours,
shows the stars near the
south celestial pole (the
picture was taken from
New Zealand). The pole
itself is at the bottom
of the picture. Circumpolar and
non-circumpolar stars.
In this diagram Ursa Major
is shown, together with
Arcturus in Boötes; it is
assumed that the observer’s
latitude is that of England.
Ursa Major is so close to
the north celestial pole that
it never sets, but Arcturus
drops below the horizon
for part of its diurnal circuit.
Ursa Major is therefore
circumpolar from England,
while Arcturus is not. Precession. The precession
circle, 47° in diameter,
showing the shift in position
of the north celestial pole
around the pole of the
ecliptic (A). In Egyptian
times (c. 3000 BC) the polar
point lay near Thuban or
Alpha Draconis; it is now
near Polaris in Ursa Minor
(declination 89° 15’); in
AD 12,000 it will be near
Vega. The south celestial
pole describes an analogous
precession circle.E152-191 UNIVERSE UK 2003mb 7/4/03 5:47 pm Page 169