Carrington and Bartels Calendars

Carrington Solar Coordinates

Richard C. Carrington determined the solar rotation rate by watching low-latitude sunspots in the 1850s. He defined a fixed solar coordinate system that rotates in a sidereal frame exactly once every 25.38 days (Carrington, Observations of the Spots on the Sun, 1863, p 221, 244). The synodic rotation rate varies a little during the year because of the eccentricity of the Earth's orbit; the mean synodic value is about 27.2753 days. See the back of an Astronomical Almanac for details.

Carrington coordinates are heliographic, i.e. Sun-centered, and measure latitude and longitude in the rotating frame described above. Carrington Time is the rotation number and longitude of the point on the Sun that is at the sub-terrestrial point. Sorting out time, Carrington Time, and Carrington longitude can sometimes be problematic.

Carrington Rotation 1 began at a seemingly arbitrary instant late on Nov 9, 1853, when Carrington began his Greenwich photo-heliographic series. Rotations are counted from that time with the central meridian longitude decreasing from 360 to 0 during each rotation as the central meridian point rotates under the Earth. Actually the canonical zero meridian used today is the one that passed through the ascending node of the solar equator on the ecliptic at Greenwich mean noon on January 1, 1854 (Julian Day 239 8220.0). Interestingly, Carrington's own zero meridian was defined to have passed the ascending node 12 hours learlier, at midnight. (Source: American Ephemeris and Nautical Almanac, 1970).

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.