A serial numbering of 27-day periods of rotations of the sun as seen from the earth, with Bartels' Rotation Number 1 beginning on February 8, 1832. The rotation that began January 8, 2018 is Bartels' Rotation Number 2516.
The Number is named for Julius Bartels (1899 – 1964)¹
Not surprisingly, the time it takes the sun to make one rotation is not an whole number of times it takes the earth to rotate, not 27 or any other integer. In addition, the sun is not a solid body, and rotates at different speeds at different latitudes, being fastest at the equator. One rotation of the sun at the equator takes about 24.47 days. The surface about half way to the poles makes one rotation in about 26 days.² Those are periods of rotation one would observe viewing the sun from a point in space that was fixed in relation to the sun.
However, we observe the sun from a moving platform, the earth. Even if the sun did not rotate at all, we would see a different face every day, and at the end of a year would see the face we first saw a year ago. Because of this effect we observe a period of rotation of 26.24 days, which is called the synodic period of rotation, while 24.47 days is the sidereal period of rotation.
1. J. Bartels.
Twenty-Seven Day Recurrences in Terrestrial-Magnetic and Solar Activity, 1923 1933.
Terrestrial Magnetism and Atmospheric Electricity, vol. 39, no. 3, pages 201–202a (September 1934).
2. See the NASA graphic at https://www.nasa.gov/mission_pages/sunearth/science/solar-rotation.html
Caltech provides a downloadable pdf of the dates of the cycles from 2004 to 2023 at:
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Last revised: 17 February 2018.