The Celestial Sphere
To an observer on the ground, the stars move across the sky at a rate of 15 degrees an hour. This is because of Earth's rotation on its axis once (360 degrees) every 24 hours.
These are stars that never dip below the poles over the year as they move around them. The circumpolar stars may vary in position according to their latitude. For example, the Big Dipper asterism may be seen from mid-northern latitudes as being circumpolar but in the southern latitudes, it cannot be seen as the latitude is too far north to be seen from the south.
Centaurus and Crux
However, the Southern Cross (Crux) may be seen from mid-southern latitudes to be circumpolar. In Singapore, no circumpolar stars are able to be seen as Singapore is too close to the equator and thus all the stars would set at some point in the year.
Different Seasons, Different Seasons
Each night the stars appear to rise 4 minutes earlier (3 minutes 56 seconds in actual fact) as Earth rotates around the sun.
This is a method used by Astronomers to locate stars on the celestial sphere.
Declination and Right Ascension
The declination is the equivalent of latitude on Earth, North or South of the Celestial Equator. Right ascension is the angle along the Celestial Equator from the First Point of Aries (Point on Celestial Equator along the Zodiac constellation Aries.
In the picture below, the constellation Aries is on the right, the constellation on the left is Andromeda. Starry Night Backyard Simulation.
Therefore, a star with the right ascension of 9 hours and 20 minutes reaches its highest point in the sky 9 hours and 20 minutes after the First Point of Aries reaches its highest point in the sky.
Celestial geography | Celestial compass| Celestial ecliptic | Celestial precession | Celestial sphere