<P> It is possible to construct a diagram with the longitude of sunspots horizontally and time vertically . The longitude is measured by the time of crossing the central meridian and based on the Carrington rotations . In each rotation, plotted under the preceding ones, most sunspots or other phenomena will reappear directly below the same phenomenon on the previous rotation . There may be slight drifts left or right over longer periods of time . </P> <P> The Bartels "musical diagram" or the Condegram spiral plot are other techniques for expressing the approximate 27 - day periodicity of various phenomena originating at the solar surface . </P> <P> The rotation constants have been measured by measuring the motion of various features ("tracers") on the solar surface . The first and most widely used tracers are sunspots . Though sunspots had been observed since ancient times, it was only when the telescope came into use that they were observed to turn with the Sun, and thus the period of the solar rotation could be defined . The English scholar Thomas Harriot was probably the first to observe sunspots telescopically as evidenced by a drawing in his notebook dated December 8, 1610, and the first published observations (June 1611) entitled "De Maculis in Sole Observatis, et Apparente earum cum Sole Conversione Narratio" ("Narration on Spots Observed on the Sun and their Apparent Rotation with the Sun") were by Johannes Fabricius who had been systematically observing the spots for a few months and had noted also their movement across the solar disc . This can be considered the first observational evidence of the solar rotation . Christopher Scheiner ("Rosa Ursine sive solis", book 4, part 2, 1630) was the first to measure the equatorial rotation rate of the Sun and noticed that the rotation at higher latitudes is slower, so he can be considered the discoverer of solar differential rotation . </P> <P> Each measurement gives a slightly different answer, yielding the above standard deviations (shown as + / -). St. John (1918) was perhaps the first to summarise the published solar rotation rates, and concluded that the differences in series measured in different years can hardly be attributed to personal observation or to local disturbances on the Sun, and are probably due to time variations in the rate of rotation, and Hubrecht (1915) was the first one to find that the two solar hemispheres rotate differently . A study of magnetograph data showed a synodic period in agreement with other studies of 26.24 days at the equator and almost 38 days at the poles . </P>

When was it discovered that the sun rotates