Sun Right Ascension

Wikipedia defines Right ascension (RA) as 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.  When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system.

RA has a linear relationship to the day of the year.

If we make t represent the day of the year where 1 is 1st Jan , 80 is 21st March , and 366 is 31st December we can calculate RA by the following linear equations:

From 1 to 79 (01-Jan to 20-Mar):

RA = 0.9977t + 281.8

From 80 to 365/366 (21-Mar to 31-Dec) :

RA = 0.97292t  – 78.40042

But if we start the year(t = 1) in 21st Mar as the ancient Babylon calendar we will get only one equation:

RA = 0.9873 t  – 3.0797

RA can be expressed in degrees or in hours. 1 hour = 15 degrees.

More accurate calculations can be done with Julian day and other orbital elements.

There a lot of algorithms to do that.

Following is the graph of Sun Right Ascension for the whole year using the algorithm:

Right Ascension Graph

Note : Morris charts script has been used to draw the graph.

Leave a Reply