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Measurement of the Sun's Diameter by Timing its Motion

SPU-21 Fall 2015

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How fast does the Sun move across the sky? It appears to move all the way around the Earth (360 degrees) in 24 hours. If we measure how long (minutes and seconds) the Sun takes to move its own diameter along the sundial in our lab, we can measure its angular diameter, in degrees. And from this angle and the known distance to the Sun, the Astronomical Unit (AU), which is approximately 1 AU = 1.5 x 108 km, we can derive the solar diamete, in km.

In the lab you will see a lens attached to the window. The lens is used to focus the Sun's light and make a sharper image on the easel. This is our "sundial".

What you'll need: