Nature of the Universe Chapter 2 Motions of Heavenly Bodies

In this chapter, we will discuss the apparent motion of the heavenly bodies. We will understand why the Sun, the Moon and the planets "move," but stars do not.

Motion of Stars
Everyone knows that the Sun rises from the east and sets in the west. Less well known is that almost everything on the sky, including the Moon, planets and most of the stars, also rises from the east and sets in the west. This is the major movement of objects on the sky and it is due to the rotation of the Earth.

We could imagine that the Earth is at the center of a large sphere, called the celestial sphere; and the Sun, stars, etc. are located on the sphere. Because the Earth is rotating from the west to the east, everything on the celestial sphere will apparently move from the east to the west. This is why the Sun rises from the east.

From the picture, we can see that those stars near the north celestial pole never set. We call them circumpolar stars. One of the circumpolar star, called Polaris, is special because it is very near the north celestial pole. Thus, it appears to be stationary.


Here are three simulations of what can be seen in the northern hemisphere. The first one is pointing to the north. (Note that Polaris does move a little bit.) The second and the third show the motions of other stars at south and the east respectively.

Motion of the Sun
Stars do not move on the celestial sphere. They are fixed. Thus, if we throw away the rotation of the Earth, stars are stationary. (Note: The truth is that some stars do move on the celestial sphere. We call this the proper motion of stars. Usually, proper motion of a star is very small and can only be detected if we observe the star for decades.)

There is one important exception however. The Sun is also a star, but the Sun does move on the celestial sphere because the Earth revolves around it. It moves from west to east, and completes a full circle in a year. The path that the Sun traces out on the celestial sphere is called the ecliptic and the twelve constellations that the Sun goes through are the zodiac. (Note: Ecliptic does also go through the constellation Ophiuchus, but due to historical reason, it is not included in the zodiac.) These are the origin of zodiac in astrology. Contrary to common belief, the Sun does not spend equal time on each ecliptic constellation.
Question: What is a day?
Answer: Most people will define a day as the time the Sun comes back to the same position relative to the ground, for example, from one midday to another. This is exactly how the sundial works. This defines the solar day. There is another less common definition. We call it a day if other stars come back to the same positions. This is the sidereal day. Due to the revolution of the Earth, a solar day is longer than a sidereal day. Approximately, a year has 365 solar days but 366 sidereal days. Do you know why?