Download THE CELESTIAL SPHERE

THE CELESTIAL SPHERE
The stars are at a very large distance from us. So the relative movement
between them is of no consequence to day-to-day observations. We therefore
imagine the stars to remain fixed on a sphere of very large radius with the earth
at its centre. We call this sphere the celestial sphere.
At any point of time we can only see the upper half of the celestial sphere. The
point on this dome right above our head is known as zenith. The lower boundary
of this dome is the horizon. As the surface is 2-dimensional, to specify the
position of any star on this dome we need two coordinates. We usually use a
spherical polar coordinate system and ignore ‘r’. One system is the alt-azimuth
system. The shortest line (great circle) is drawn from the zenith joining the star
right down to the horizon. The angle the star makes above the horizon measured
along this line is known as altitude. The position of this line with respect to some
fixed reference is known as azimuth. Usually the meridian line (the line joining
the zenith and the pole star) is taken as the reference and angle of meridian is
measured westward from it.
Due to rotation of the earth, the entire celestial sphere appears to rotate around
the earth, once in about 24 hours. There are two points on the sphere, which do
not move – they are the celestial north and south poles. If we extend the axis
of the earth to meet the celestial sphere, it would cut the latter at these two
points. Very close to the celestial North Pole (for all practical purposes, right at
the celestial north pole) lies the famous Pole star or North Star (systematic name:
α-Ursae Minoris.). This star remains fixed in its position. At any latitude λ in the
northern hemisphere, the altitude of the pole star is λ above the horizon. All the
other stars seem to rotate around the North Star in circles in the anticlockwise
sense (think why?). The further the (angular) distance of the star from the pole
star, the greater its radius of the traced circle.
When the angular distance increases to 90º, the traced circle becomes a great
circle – the celestial equator. If we extend the plane containing the equator in
space, the circle formed by its intersection with the celestial sphere is known as
the celestial equator. As the angular distance exceeds 90º, the circles traced
become smaller and smaller.
At any latitude in the Northern Hemisphere there will some stars that are always
above the horizon (in fact all stars less than λ from the pole). If we draw a line
from the North Star through where we are standing to intersect the celestial
sphere in the south at λ below the horizon, we will get the celestial South Pole.
There will be some stars around the South Pole that will always be below the
horizon. The rotation in 24 hrs is 360º. Simple calculation shows that in 1 hr it is
15º and it is 1º in 4 minutes.
The sun moves along with the celestial sphere but together with that it shifts
every day a bit to the east with respect to the celestial sphere. This is due to the
westward revolution of the earth. As the period is 365 days, the sun moves
360º/365 = 1º (approx) in one day. Now, the time we follow is based in the rising
and setting of the sun (solar time). Any star moves 1º to the west with respect to
the sun in one day and hence rises 4 minutes earlier. After 1 month it rises 2
hours earlier. So we see different stars during different seasons. The path taken
by the sun the celestial sphere is known as the line of ecliptic to which we will
come later.
Contributed by Archisman Ghosh