Right Ascension

Astronomers use a system to locate objects in the night sky that is similar to the latitude-longitude system we use on the surface of the Earth. It is as if the latitude-longitude lines on Earth were projected up onto the sky. The names for these lines are different, though. The “latitude”, or distance above or below the celestial equator is called declination. The “longitude” position is called right ascension(RA). This is the position that will be important to us in determining the Star of Your Birth. The zero-point of RA is taken as the point where the Sun reaches the Vernal Equinox, crossing the celestial equator. RA is measured not in degrees, as longitude is, but in hours, minutes, and seconds, along the celestial equator. As the Earth rotates, the RA lines appear to move past us in the sky. If a star at RA 1 hour is directly above us now, it will take about an hour until a star at RA 2h comes directly above.

As the Earth takes its yearly journey around the sun, the sun appears to move day-to-day in a path across the sky called the ecliptic. It increases in RA every day, but the rate varies throughout the year. The sun passes through 3.5 to 4.25 minutes of RA each day during the year.

Because of precession, the “zero” point of RA moves through the sky over the years. RA coordinates are given in 1950 or 2000 reference points to correct for this.


The location system of right ascension and declination used to describe celestial positions.
By haade (own work redrawn from en wikimedia) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons

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