Often asked questions:
Why do the stars seem to rise earlier each night?
Why do the stars that we see in the night sky change from season to season?
There are two separate reasons for these phenomena, Rotation and Revolution.
I.e. The Earth rotates about its axis while it revolves around the Sun.
The Earth rotates from west to east about its axis of rotation which is a line joining the celestial poles and if this axis is produced far enough, it would cut the celestial sphere at a point marked by the North Star (Polaris) as shown in the diagram. Facing north from the Earth, the Pole Star appears stationary, and the other stars appear to rotate from east to west around the Pole Star although in fact the positions of the stars are fixed and it is the Earth which is rotating from west to east.
The time taken for a star to complete a circuit around the Pole Star is called a star’s day or sidereal day. If the sidereal day were to be exactly 24 hours, as the Mean Solar Day is, then the stars would rise and set at the same times every day. However, the Earth completes each rotation about its axis in 23 hours, 56 minutes and 4 seconds so the stars will take the same amount of time to circuit the Pole Star and that is the length of the sidereal day. Therefore, if a star rises in the east at a certain time on a certain day, it will next do so 23 hours, 56 minutes and 4 seconds later. In other words, the star in question will rise 3 minutes and 56 seconds earlier each day (usually rounded off to 4 minutes).
For example, Say that Arcturus (the brightest star in the northern celestial hemisphere) rises at 18.00 mean time on a certain day; we know that it will rise again 23 hrs. and 56 mins. later so we can easily calculate that it will rise at 17.56 mean time the next day (4 minutes earlier).
Circumpolar Stars. Depending on the latitude of the observer, some stars will never rise or set because they are always be above the horizon, these are known as circumpolar stars.
Example. The diagram below shows the constellations Ursa Major (Great Bear) and Cassiopeia which are both circumpolar to observers throughout the northern hemisphere and down to 20o South in the southern hemisphere. As Ursa Major revolves about the Pole Star so do the five stars of Cassiopeia.
The next article in this series will explain Revolution and this will be followed by articles that will discuss the stars and constellations that can be seen as the night sky changes from season to season.
Books of the Astro Navigation Demystified Series: