Calculating the Sun's Angular Velocity
What is Angular Velocity?
Angular velocity is how fast something
travels in a circle (or on a sphere): it is the angle by which
an object spins in a certain time -- its rotation rate.
Imagine a clock (the old-fashioned kind with hands). The minute
hand goes around the clock, a circle -- 360 degrees, in 60 minutes.
To find its angular velocity, or rotation rate, you need to
divide the number of degrees by the number of minutes:
360 degrees / 60 minutes = 6 degrees per minute
So, the minute hand's angular velocity, or rate of rotation, is 6 degrees per minute.
How Can I Determine the Sun's Angular Velocity?
We can use the movement of a sunspot to determine the Sun's rate of rotation,
just like we used the minute hand of a clock.
To do this accurately, you will have to use a transparent template for
your sketches of the Sun's disk. The
template
has marked on it the degrees of longitude on a sphere.
From your sketches, pick the sunspot group which is the most long-lived (that is,
which appears in most of your sketches). Using your template,
figure out how many degrees of Sun the spot group traveled across, and
how long it took to move that far.
Sun's Angular velocity = degrees the spot has covered / time it took for the spot to travel
Sun's rotation rate = 360 degrees / angular velocity
Going Further
If the Sun has been particularly active and you have sunspots sketched
at different latitutes on the Sun's surface, compute the angular velocity
for different sunspot groups which appear at varying latitudes.
Do you get the same solar rotation rate for each set? If not, why not?
Want more information on the Sun's rotation?
Return To Sunspot Rotation Activity
