Because the sun is made of gas, it does not rotate as a unified body, but instead different parts of it rotate at different speeds. Richard Christopher Carrington recorded observations of sunspots over a period of time, which revealed that they travelled in inconsistent ways. Due to his discoveries, he lent his name to 'Carringtonian rotation', a system for comparing locations on the sun over time, based on his calculation that the sun rotates every 25.38 days, which from Earth we see as 27.2753 days.
Editors. "Differential nature of solar rotation discovered by Richard Carrington". The Literary Encyclopedia. First published 30 August 2013
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6326, accessed 18 December 2014.]