Today, I want to talk about the equatorial coordinate system. Start with this: What if we could project Earth’s coordinates of longitude and latitude into the sky? Latitude wouldn’t be a problem, but longitude would change continuously because of Earth’s rotation. Somehow, we need to attach longitude to the sky.
If we project Earth’s equator and poles onto the celestial sphere, we get the celestial equator and the north and the south celestial poles. Circles through both poles are perpendicular to the celestial equator. To find the position of an object in the sky, just imagine a circle passing through the poles and the object. This is the star’s hour circle, and it corresponds to a meridian of longitude on Earth.
Right ascension and declination
The first coordinate in the equatorial system is declination, the angle between the object and the celestial equator (measured along the object’s hour circle). It varies from 0° for an object on the celestial equator to 90° north or south. We measure it in degrees, minutes (1/60 of a degree), and seconds (1/60 minute). So, declination is easy. The other coordinate, right ascension, isn’t.
To set the zero point of right ascension, astronomers use the intersection of Earth’s equator and its orbital plane,the ecliptic. This is called the March (or vernal) equinox, but you might see it called the First Point of Aries. As Earth orbits the Sun, the Sun appears to move through this point each year around March 21, crossing the celestial equator moving from south to north.
So, when we measure the angle between the vernal equinox and the hour circle of the object, that’s its right ascension. It’s measured in hours, minutes, and seconds. Yes, hours, not degrees like declination. Sometimes, astronomers do things just to be different. Because a day has 24 hours, the circle of right ascension also has 24 hours, each hour corresponding to 15°. Right ascension is measured from west to east starting at the vernal equinox, which becomes the starting point, 0 hours.
Things are moving
Regarding the right ascensions and declinations of celestial objects for the purposes of amateur astronomy, some coordinates change and some do not (except over very long periods of time). Objects whose coordinates change include the Sun, Moon, planets, asteroids and comets. Objects whose coordinates may be said not to change include stars, nebulae, clusters and galaxies.
Because of small changes in Earth’s rotation axis, mainly caused by the gravitational pulls of the Sun and Moon, the vernal equinox changes its position slowly, so the equatorial coordinate system is also slowly changing with time. To account for this, astronomers specify an epoch (a moment of time) to which the coordinate system is referenced. Current sources use epoch 2000.0, the beginning of the year 2000 a.d.
Finally, here’s something connecting latitude and a star’s declination. In the Northern Hemisphere, if a star’s declination is greater than 90° degrees minus your latitude, that star will never set. We call it a circumpolar star. In the Southern Hemisphere, if a star’s declination is less than the latitude minus 90°, the star will be circumpolar.
