# Things I have recently learned: The Sun Rotates

While in Oregon in the high desert to chase the eclipse I setup my camera and telescope and took a few quick shots to ensure all was in working order. Note that within the above picture, the day before the eclipsec Sun 20th Aug 2017, there are a few sunspots visible – I’ve circled them. Least you wonder, yes they are sunspots and not dust on the lens, I checked that by moving the setup a bit. If the spots were on the lens then such movement would move the spots to a new location – they stayed in place.

The next day, eclipse day, I’m recording the eclipse itself.

Notice how, apart from the rather obvious start of the eclipse, the sunspots have now moved and also how some new sunspots in the bottom have appeared.

What is going on here?

My initial thought was that these were not sunspots, but instead I did have a lens problem of some sort, so I not only ran my own checks but also double checked with others around me to verify that they were also seeing them. Once confirmed I then pondered over what might be going on here.

Obviously the earth rotates around the sun, but one day’s worth of movement would not be enough to make such a huge difference, and so the only conclusion is that the sun itself is rotating.

If this was indeed true then I will not have been the first to notice this, and so I later googled it and discovered that it was indeed true, the sun really does rotate.

I never appreciated this until I made the observation myself.

## Solar Rotation

Solar rotation varies with latitude because the Sun is composed of a gaseous plasma. The rate of rotation is observed to be fastest at the equator(latitude φ = 0 °), and to decrease as latitude increases. The solar rotation period is 24.47 days at the equator and almost 38 days at the poles.

… but NASA also have a time-lapse clip that illustrates it …

… and here is another that shows just sunspots over a one month period …

## It is not a new discovery

Around the year 1612, Galileo Galilei observed sunspots and calculated the rotation of the Sun.

In 1630, Christoph Scheiner reported that the Sun had different rotational periods at the poles and at the equator. His calculations are more or less in agreement with modern observations.

## It can all get very weird indeed

It is not just the sun, all stars rotate. …

Stars are believed to form as the result of a collapse of a low-temperature cloud of gas and dust. As the cloud collapses, conservation of angular momentum causes any small net rotation of the cloud to increase, forcing the material into a rotating disk. At the dense center of this disk a protostar forms, which gains heat from the gravitational energy of the collapse.

As the collapse continues, the rotation rate can increase to the point where the accreting protostar can break up due to centrifugal force at the equator. Thus the rotation rate must be braked during the first 100,000 years to avoid this scenario. One possible explanation for the braking is the interaction of the protostar’s magnetic field with the stellar wind in magnetic braking. The expanding wind carries away the angular momentum and slows down the rotation rate of the collapsing protostar.[13][14]

A black hole is of course an object with a gravitational field that is so strong not even light can escape. When a black hole is formed from the collapse of a rotating mass then the black hole retains this rotation and so it also rotates and that results in a few mind boggling numbers …

In late 2006, astronomers reported estimates of the spin rates of black holes in The Astrophysical Journal. A black hole in the Milky Way, GRS 1915+105, may rotate 1,150 times per second,[2] approaching the theoretical upper limit….

…The formation of a rotating black hole by a collapsar is thought to be observed as the emission of gamma ray bursts.

## One last quote

Let’s permit Galileo Galilei to have the last work here since he made the observation that the sun rotates back in 1612. Yes, it is perhaps a slightly out-of-context quote, but is still appropriate …

E pur si muove – And yet it moves

OK, in the interests of accuracy, the above is attributed to him but is disputed, so I’ll link to his actual drawings and letters on the topic instead.