Equatorial coordinates are the most common system of celestial coordinates used by astronomers. Because it takes as a reference plane the plane of the celestial equator (an extension into space of Earth's equatorial plane), it is the sky equivalent of longitude and latitude. Celestial longitude is called right ascension (R.A.) and is measured eastward along the celestial equator, in hours, minutes, and seconds of sidereal time, or, alternatively, in degrees, starting from 0h (or 0°) at the vernal equinox. Celestial latitude is known as declination (Dec.) and is the angular distance of a body north or south of the celestial equator, reckoned positive when north and negative when south. Sometimes, instead of R.A. and Dec., hour angle and polar distance are used.