Starguide - A Practical Guide to Astro-Photography

The Earth Rotation Angle (ERA) measures the rotation of the Earth from an origin on the celestial equator, the Celestial Intermediate Origin, that has no instantaneous motion along the equator; it was originally referred to as the non-rotating origin. 

ERA replaces Greenwich Apparent Sidereal Time (GAST). The origin on the celestial equator for GAST, called the true equinox, does move, due to the movement of the equator and the ecliptic. The lack of motion of the origin of ERA is considered a significant advantage

ERA is going to replace the older GAST and GMST = Greenwich Mean Sidereal Time

Reference:

[1] Wikipedia Sidereal Time (en)

Created with the Personal Edition of HelpNDoc: Free EPub producer