Right ascension

Definition

Right ascension (RA) is the angular distance measured eastward along the celestial equator from the vernal equinox. It is analogous to longitude on Earth but measured in hours, minutes, and seconds.

5 Must Know Facts For Your Next Test

1. Right ascension is measured in hours, minutes, and seconds rather than degrees.
2. One hour of right ascension corresponds to 15 degrees of angular distance.
3. Right ascension coordinates are used in conjunction with declination to locate objects in the sky.
4. The zero point for right ascension is the vernal equinox, also known as the First Point of Aries.
5. RA increases eastward from the vernal equinox up to 24 hours.

Review Questions

• How is right ascension similar to and different from longitude?
• What coordinate system uses right ascension and declination?
• What is the starting point for measuring right ascension?

Related terms

Declination:

The angular distance of a celestial object north or south of the celestial equator.

Celestial Equator:

An imaginary extension of Earth's equator onto the celestial sphere.

Vernal Equinox:

The point in the sky where the Sun crosses the celestial equator moving northward; it marks zero right ascension.