Editing Differential rotation (section)

== Measurement ==
There are many ways to measure and calculate differential rotation in stars to see if different latitudes have different angular velocities. The most obvious is tracking spots on the stellar surface.

By doing [[Helioseismology|helioseismological]] measurements of solar "p-modes" it is possible to deduce the differential rotation. The Sun has very many acoustic modes that oscillate in the interior simultaneously, and the inversion of their frequencies can yield the rotation of the solar interior. This varies with both depth and (especially) latitude.

The broadened shapes of absorption lines in the optical spectrum depend on v<sub>rot</sub>sin(i), where ''i'' is the angle between the line of sight and the rotation axis, permitting the study of the rotational velocity's line-of-sight component v<sub>rot</sub>. This is calculated from [[Fourier transforms]] of the line shapes, using equation (2) below for v<sub>rot</sub> at the equator and poles. See also plot 2.
Solar differential rotation is also seen in magnetograms, images showing the strength and location of solar magnetic fields.

It may be possible to measure the differential of stars that regularly emit flares of radio emission.  Using 7 years of observations of the M9 [[Ultra-cool dwarf|ultracool dwarf]] TVLM 513-46546, astronomers were able to measure subtle changes in the arrival times of the radio waves.  These measurements demonstrate that the radio waves can arrive 1–2 seconds sooner or later in a systematic fashion over a number of years.  On the Sun, [[Sunspot|active regions]] are common sources of radio flares.  The researchers concluded that this effect was best explained by active regions emerging and disappearing at different latitudes, such as occurs during the solar [[Solar cycle|sunspot cycle]].<ref>{{cite journal|last1=Wolszczan|first1=A.|last2=Route|first2=M.|title=Timing Analysis of the Periodic Radio and Optical Brightness Variations of the Ultracool Dwarf, TVLM 513-46546|journal=The Astrophysical Journal|date=10 June 2014|volume=788|issue=1 |page=23|doi=10.1088/0004-637X/788/1/23|arxiv=1404.4682|bibcode=2014ApJ...788...23W|s2cid=119114679 }}</ref>