Right ascension

Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, which is the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is currently located in the constellation Pisces.

Any angular unit could have been chosen for right ascension, but it is customarily measured in hours (h), minutes (m), and seconds (s), with 24h being equivalent to a full circle.

[5] Because right ascensions are measured in hours (of rotation of the Earth), they can be used to time the positions of objects in the sky.

Sidereal hour angle, used in celestial navigation, is similar to right ascension but increases westward rather than eastward.

This movement, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly.

[8] The concept of right ascension has been known at least as far back as Hipparchus who measured stars in equatorial coordinates in the 2nd century BC.

But Hipparchus and his successors made their star catalogs in ecliptic coordinates, and the use of RA was limited to special cases.

Equatorial mounts could then be accurately pointed at objects with known right ascension and declination by the use of setting circles.

Right ascension and declination as seen on the inside of the celestial sphere . The primary direction of the system is the March equinox , the ascending node of the ecliptic (red) on the celestial equator (blue). Right ascension is measured eastward up to 24 h along the celestial equator from the primary direction.
Right ascension (blue) and declination (green) as seen from outside the celestial sphere
Various hour angles are depicted here. The symbol ♈︎ marks the March equinox direction.
Assuming the day of the year is the March equinox: the Sun lies toward the grey arrow, the star marked by a green arrow will appear to rise somewhere in the east about midnight (the Earth drawn from "above" turns anticlockwise). After the observer reaches the green arrow, dawn will over-power (see blue sky Rayleigh scattering ) the star's light for about six hours, before it sets on the western horizon. The Right ascension of the star is about 18 h . 18 h means it is a March early-hours star and in blue sky in the morning. If 12 h RA, the star would be a March all-night star as opposite the March equinox. If 6 h RA the star would be a March late-hours star, at its high (meridian) at dusk.
How right ascension got its name. Ancient astronomy was very concerned with the rise and set of celestial objects. The ascension was the point on the celestial equator (red) which rose or set at the same time as an object (green) on the celestial sphere . As seen from the equator, both were on a great circle from pole to pole (left, sphaera recta or right sphere). From almost anywhere else, they were not (center, sphaera obliqua or oblique sphere). At the poles, objects did not rise or set (right, sphaera parallela or parallel sphere). An object's right ascension was its ascension on a right sphere. [ 9 ]
The entire sky, divided into two halves. Right ascension (blue) begins at the March equinox (at right, at the intersection of the ecliptic (red) and the equator (green)) and increases eastward (towards the left). The lines of right ascension (blue) from pole to pole divide the sky into 24 hours, each equivalent to 15°.