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Measurement of the Sun’s Rotational Period using Sunspots

SPU-21 Fall 2014

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The motion of the Sun in the sky tells us something about the rotation of the Earth, and the Earth's orbit around the Sun. An important motion of the Sun itself is its rotation about its own axis. This can be measured by using sunspots as tracers of that rotation, as Galileo first did in 1612.

Why is rotation period important? For one thing, it is believed that the solar rotation ultimately causes the creation of sunspots themselves. Other stars (with similar, or cooler, temperature) that rotate more rapidly than the Sun can have more and bigger spots. A primary goal for this lab is to determine the period of rotation of the Sun - that is, how many days it takes to rotate once around its own axis. A second goal of our observations is to determine the direction of the Sun's rotation.

If we take data over a period of a few days to a few weeks, we can determine how much the sunspots move and determine its apparent motion across the face of the Sun. If we make the assumption that the spots are carried across the Sun by rotation, we can then infer the period with which the Sun rotates. And, if we know which way the Sun is in place of the Earth's orbit, we can tell the direction of rotation.

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