Right Ascension – Vernal Equinox Point
The second coordinate in the celestial equatorial system is right ascension. It is analogous to (but not the same as) longitude. Much has Greenwich is the arbitrary zero point for longitude, right ascension also has a zero reference point. That point is the Vernal Equinox Point. Why that point is a convenient choice is discussed in the next section of the module (Seasons and the Ecliptic).
Because the earth rotates, from the perspective of the earth, the celestial sphere rotates once about every 24 (sidereal) hours. Right ascension, consequently, is measured in (sidereal) hours, 0h to 24h east from the Vernal Equinox Point. That is, east is the direction of increasing right ascension. The half-circle with right ascension 0h is called the 0 hour circle. As a circle is 360°, 1 hour right ascension = 15°