Earth is a Planet: Backyard Astronomy: Navigating the Celestial Sphere

In my first post, I briefly alluded to the celestial sphere - the imaginary sphere on which the stars appear to be fixed. Just like how cartographers use latitude and longitude to define all points on Earth's surface, astronomers use coordinates of declination (DEC) and right ascension (RA) to map the night sky for observation. The priority is to have a coordinate system for stars and constellations that is universal for any geographic location at any time of year; the north star, for example (a.k.a. Polaris, which is on the handle of the Little Dipper constellation) is near the north celestial pole at a declination of approximately 90°. The celestial equator is simply a projection of Earth's equator into space, and defines 0° declination. This means that for an observer on Earth, the declination of the zenith (the point directly overhead) is equal to their latitude. Given that the equator is defined based on the planet's rotation, it is not to be confused with the ecliptic, which is the plane of Earth's orbit around the Sun. Recalling the previous Backyard Astronomy post, the ecliptic and the celestial equator are separated by an angle of about 23.4° due to the tilt of the planet's spin axis. Like Earth, the celestial sphere has north and south poles at DEC = ±90°.

A depiction of the basic coordinates on the celestial sphere.
(Click to Expand)

To fully understand right ascension, it is important to grasp the distinction between solar time and sidereal time. Most of us recognize the length of a solar day, which is approximately 24 hours and is defined based on the position of the Sun in the sky; one solar day is the length of time it takes the Sun to make a single revolution around the sky before returning to a given point. When mapping distant stars on the celestial sphere, however, it is important to consider the effect of Earth's orbit around the Sun. From an observer's perspective on Earth, the celestial sphere makes one revolution four minutes more quickly than the Sun. Because of this effect, one sidereal day is actually about 23 hours and 56 minutes in length. This means that it takes slightly less than 24 hours for a given constellation like the Big Dipper to circle the sky and return to its original position. As a result, the celestial sphere appears slightly shifted from one midnight to the next. This is the reason why we see different constellations in the night sky from season to season. When defining the coordinates of right ascension, astronomers use units of hours (h) instead of degrees (°), with 24h defining one 360° circle. For a given point in the sky, the right ascension shifts by approximately 1h each hour. With Earth's rotation under consideration, as well as the distinction between solar and sidereal time, the prime meridian of the celestial sphere (RA = 0h) is defined as the projection of Earth's prime meridian into space at noon during the spring equinox. For an observer at any location, the right ascension of the zenith at a given time would shift by four minutes from night to night.