Solar rotation
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Solar rotation can vary because the sun is composed of a gaseous plasma, and therefore may lack a fixed rotation rate. The rate of rotation is observed to be fastest at the equator (latitude φ=0 deg), and to decrease as latitude increases. The rotation rate is usually described by the equation:
where ω is the angular velocity in degrees per day, φ is the latitude and A and B are constants. Note that there are interesting deviations from this simple relationship.
- A= 14.18 deg/day (+/- 0.35)
- B= -2.00 deg/day (+/- 0.48)
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[edit] Sidereal rotation
At the equator 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. When the sun is viewed from the "north" (above the Earth's northern pole) solar rotation is counterclockwise. Sunspots viewed from Earth (its Northern hemisphere) appear to move from left to right across the face of the sun.
[edit] Using sunspots to measure rotation
The rotation constants have been measured by measuring the motion of various features ("tracers") on the solar surface. The first and most widely used tracer are sunspots. Though sunspots had been observed since ancient times, it was only when the telescope came into use that they were observed to turn with the Sun, and thus the period of the solar rotation could be defined. The English scholar Thomas Harriot was probably the first to observe sunspots telescopically as evidenced by a drawing in his notebook dated December 8, 1610, and the first published observations (June 1611) entitled “De Maculis in Sole Observatis, et Apparente earum cum Sole Conversione Narratio” ("Narration on Spots Observed on the Sun and their Apparent Rotation with the Sun") were by Johannes Fabricius who had been systematically observing the spots for a few months and had noted also their movement across the solar disc. This can be considered the first observational evidence of the solar rotation. Christopher Scheiner (“Rosa Ursine sive solis”, book 4, part 2, 1630) was the first to measure the equatorial rotation rate of the Sun and noticed that the rotation at higher latitudes is slower, so he can be considered the discoverer of solar differential rotation.
Each measurement gives a slightly different answer, yielding the above standard deviations (shown as +/-). St. John (1918) was perhaps the first to summarise the published solar rotation rates, and concluded that the differences in series measured in different years can hardly be attributed to personal observation or to local disturbances on the Sun, and are probably due to time variations in the rate of rotation, and Hubrecht (1915) was the first one to find that the two solar hemispheres rotate differently.
[edit] References
, Ed. "Allen's Astrophysical Quantities", 4th Ed, Springer, 1999.
- Javaraiah, J., 2003. Long-Term Variations in the Solar Differential Rotation. Solar Phys., 212 (1): 23-49.
- St. John, C., 1918. The present condition of the problem of solar rotation, Publications of the Astronomical Society of the Pacific, V.30, No. 178, 318-325.
