Since the sun is composed of a gaseous plasma, it does not have a fixed rotation rate. The rate of rotation is fastest at the equator (Latitude φ=0 deg) and decreases as latitude increases. The rotation rate is well described by the equation
where ω is the angular velocity in degrees per day, φ is the latitude and A and B are constants.
- A=14.1844 deg/day (+/- 0.35)
- B=-2.00 deg/day (+/- 0.48)
At the equator then, the solar rotation period is 25.38 days. This is called the sidereal rotation period,
and should not be confused with the synodic rotation period of 27.2753 days, which is the time for a fixed feature on the sun to rotate to the same apparent position as viewed from earth.The synodic period is longer because the sun must rotate
for a sidereal period plus an extra amount due to the orbital motion of the earth around the sun.
The rotation constants have been measured by measuring the motion of various features on the solar surface. Each
measurement gives a slightly different answer, yielding the above standard deviations (shown as +/-).
References
- Arthur N. Cox, Ed. "Allen's Astrophysical Quantities", 4th Ed, Springer, 1999.