DAILY MOTIONS

The daily ( diurnal) motion of the celestial objects is caused by the rotation of the Earth on its axis. We see the stars rise in the east, set in the west, and then rise again in the east every 23 hours and 56 minutes (the sidereal day). This is fine, but let us now be a little more precise in our description.

We argued that the stars appear to be fixed to the Celestial Sphere and since we are interested only in relative motion, we can address this issue by imagining that the Earth is stationary and that the Celestial sphere is turning. The resultant motions are then rather simple. Because the stars do not move on the Celestial Sphere, they maintain constant distances from the rotation axis and so they simply trace out circles on the sky centered on the projection of the axis of rotation of the Earth onto the Celestial Sphere (that is, the North [or South] Celestial Pole).

These daily paths (the circles) are referred to as diurnal circles

The daily motions of all celestial objects (the stars, the Sun, the Moon, and the planets) follow this pattern. Note that the objects move from east-to-west across the sky which means that the Earth spins from west-to-east. Or, as viewed from the NCP, the Earth rotates in the CCW-sense.

How does the appearance of the sky change as we move around in latitude on the Earth?

Horizon

We must first define the horizon. Because we cannot look through the Earth, we can only see that part of the sky which is above the ground, that is, above the horizon.

The point directly above the head of the observer is referred to as the zenith. The point directly below the feet of the observer is referred to as the nadir. The horizon is 90 degrees down from the zenith. Note that every observer has his or her own horizon; it is a purely locally defined system. Everyone in this room has roughly the same zenith and horizon, but they are not precisely the same. In addition, the zenith and horizon for an observer change as he or she moves (in latitude) along the surface of the Earth. For completeness note that the meridian is defined as the circle which passes through the zenith, the NCP, and SCP.

The sky at different latitudes

North Pole (latitude=90 degrees):

the zenith and the NCP coincide and the horizon coincides with the Celestial Equator. The diurnal circles are parallel to the horizon. Star don't rise or set ( all stars are circumpolar).

Equator (latitude=0 degrees):

the NCP sits on the North point of the horizon while the Celestial Equator passes through the east point on the horizon, the zenith and west point on the horizon. The diurnal circles are roughly perpendicular to the horizon near the east and west points on the horizon. All stars rise and set.

Note that the altitude of the NCP above the horizon is 90 degrees at the North Pole and 0 degrees at the equator. A general result which is easily shown is that the altitude of the NCP = the latitude of the observer

In the northern hemisphere since Polaris, the North Star, sits right near the NCP, the altitude of the North star tells you your latitude on the Earth.

Comments--What is the altitude of the Celestial equator?