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Bartels Calendar:
Julius Bartels defined his calendar based on observations of daily geomagnetic activity (J. Bartels, Terrestrial Magnetism and Atmospheric Electricity, 39 (3), p.201). 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 serial number assigned to 27-day recurrence periods of solar and
geophysical parameters is called Bartels rotation number. With 25.38 days
(R. C. Carrington, Observations of the Spots on the Sun, 1863,
p.221), the synodic equatorial rotation rate of the Sun is very close
to 27 days. Rotation 1 Day 1 in this sequence was assigned arbitrarily by Julius Bartels
to February 8, 1832.
On long timescales recurrent events, like coronal holes, become visible in subsequent Bartels rotation plots.
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(These plots are usually updated every morning. Last update: )
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