Celestial Equator and Celestial Poles

Recall how a spherical globe of the Earth has a north pole and a south pole.  The celestial sphere also has poles.  Directly above the Earth’s north pole on the celestial sphere lies the north celestial pole (NCP).  Directly above the Earth’s south pole lies the south celestial pole (SCP).

If you were standing at the Earth’s north pole, the north celestial pole would lie at the zenith, the imaginary point directly over your head.  The star Polaris would lie almost directly at this point.  It’s the same story for the south… the south celestial pole (SCP) is directly above the Earth’s south pole.

In the northern hemisphere, a moderately bright star—the North Star, also called Polaris— lies almost exactly at the position of the north celestial pole (NCP).  There is, however, no bright star near the SCP, that is, there is no southern counterpart to Polaris.

The celestial poles and equator lie above their terrestrial counterparts

The celestial poles and equator lie above their terrestrial counterparts

As it is with the poles, so it is with the equator.  Directly above the Earth’s equator lies the celestial equator, a circle which goes all the way around the sky and which divides the northern half of the celestial sphere from the southern half (see image above).

If you were standing at the north pole, the celestial equator would coincide with the horizon.  And if you were standing on the Earth’s equator, the celestial equator would stretch from the east to the west directly overhead.  As seen from the equator, the north and south celestial poles would lie on the northern and southern horizon, respectively.

But how about if you’re standing at some intermediate latitude, between the north pole and the equator?

The horizon, meridian, and cardinal points

The horizon, meridian, and cardinal points

In that case, the north celestial pole (NCP) and the north star would lie at some angle above the northern horizon.  This angle is equal to your latitude.  If you are at the equator, for example, which is 0 degrees latitude, then the NCP (and Polaris) would lie zero degrees above the horizon, that is, on the horizon.  At 10 degrees latitude, Polaris would lie 10 degrees above the horizon.  And in London, England, which has latitude of 51 degrees, Polaris would lie 51 degrees above the horizon.  This is how navigators have determined their latitude for thousands of years… by measuring the angle of Polaris above the horizon.

One more circle… the imaginary great circle that runs from the northern horizon, up through Polaris, through the zenith, then down to the southern horizon is called the meridian (again, see image above).