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
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.
The calendars are related as follows:
CT 1936:249 696711
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