RASC Calgary Centre - Spinning Sun
Spinning Sun
by Jason Nishyama
Page last updated November 5, 2018
The spinning Sun
For the summer season of short to non-existent twilight, here's a project about the Sun. For
this project you'll need a telescope with an appropriate solar filter. The solar filter is very
important!
The point of this project is to work out the rotation rate of the Sun. This can be done by simply
observing the transit of a sunspot group across the Sun over a period of time. Over the summer
observe the Sun daily and record the positions of the various sunspot groups. Count the number of
days it takes any group to cross from one side of the Sun to the other.
Multiplying the number of days it takes any particular sunspot group to cross the Sun gives the
synodic rotation period of the Sun. This is the amount of time it takes a part of the Sun to
rotate from pointing at the Earth to pointing at the Earth again. This is different from the
sidereal rotational period of the Sun which is the time it takes to make one revolution of the
Sun. This is because as the Sun is rotating, the Earth is also continuing on in its orbit of the
Sun so that it takes a little longer for the point on the Sun to catch up with the observer on the
Earth. This is the same reason that the Earth's sidereal day is a few minutes shorter than the
solar day.
Now since there's a couple of months of summer, I'm not going to go through the math to work out
the difference between the measured synodic period of the Sun and its actual sidereal rotation
rate. I will leave that for you to work out. I will give you a hint though, if you look at the
time it takes the Earth to orbit the Sun and compare it to the number of degrees in a circle, you
can work out how far around its orbit the Earth goes in one day. From this you should be able to
work out the sidereal period.
So have a fun summer of solar observing!
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