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Right ascension is measured eastward up to 24 h along the celestial equator from the primary direction. Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the Earth. [1]
In this case, the longitude is also called the right ascension of the ascending node (RAAN). The angle is measured eastwards (or, as seen from the north, counterclockwise) from the FPA to the node. [2] [3] An alternative is the local time of the ascending node (LTAN), based on the local mean time at which the spacecraft crosses the equator.
Right ascension is usually measured in sidereal hours, minutes and seconds instead of degrees, a result of the method of measuring right ascensions by timing the passage of objects across the meridian as the Earth rotates. There are 360° / 24 h = 15° in one hour of right ascension, and 24 h of right ascension around the entire ...
Like right ascension in the equatorial coordinate system, the primary direction (0° ecliptic longitude) points from the Earth towards the Sun at the March equinox. Because it is a right-handed system, ecliptic longitude is measured positive eastwards in the fundamental plane (the ecliptic) from 0° to 360°.
In astronomy, the March equinox is the zero point of sidereal time and, consequently, the right ascension and ecliptic longitude. [12] It also serves as a reference for calendars and celebrations in many cultures and religions.
An object's right ascension and the local sidereal time can be used to determine the time of its culmination (see hour angle). The term meridian comes from the Latin meridies , which means both "midday" and "south", as the celestial equator appears to tilt southward from the Northern Hemisphere .
This means that the Earth's rotational North pole does not point quite at the J2000 celestial pole at the epoch J2000.0; the true pole of epoch nutates away from the mean one. The same differences pertain to the equinox. [9] The "J" in the prefix indicates that it is a Julian equinox or epoch rather than a Besselian equinox or epoch.
For this reason, to simplify the description of Earth's orientation in astronomy and geodesy, it was conventional to chart the positions of the stars in the sky according to right ascension and declination, which are based on a frame of reference that follows Earth's precession, and to keep track of Earth's rotation, through sidereal time ...