Carrington and Bartels

Carrington Solar Coordinates

Lord Carrington determined the solar rotation rate by watching low-latitude sunspots back in the 1850s. He defined a fixed solar coordinate system that rotates in a sidereal frame every 25.38 days. The synodic rotation varies a little during the year because of the eccentricity of the Earth's orbit. The mean value is 27.2753 days. See the back of the Atrophyscical Almanac for details.

Carrington coordinates are heliographic and measured in latitude and longitude in that rotating frame. Carrington Time is the rotation and longitude of the point on the Sun that is at the sub-terrestrial point. The first Carrington rotation began at an arbitrary instant on Nov 9, 1853. Rotations are counted from that time with longitude decreasing from 360 to 0 during each rotation as the central meridian point rotates under the Earth.

The Bartels Calendar

Bartels defined his calendar based on observations of daily geomagnetic activity. Over long periods the geomagnetic recurrence rate is very close to 27 days. Bartels' rotations are exactly 27 days long and are counted from Feb 8, 1832.

The two systems were defined independently, but it's not totally coincidental that the rates are nearly the same. It's the Sun's influence on the Earth's magnetosphere through the solar wind that causes geomagnetic activity, after all.

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