Solar Radius

The determination of the solar radius is fairly easy once the solar distance (the astronomical unit) is determined.

We simply measure the angular size of the Sun and then use trigonometry to show that

tangent( angle ) = ( radius of the Sun ) / ( astronomical unit )

or

radius of the Sun = r = D x tangent( R )

In this way we infer that the radius of the Sun is equal to 696,000 km.

Finding the radii of other stars is very difficult. One cannot directly measure the angular sizes of stars in the above manner except for the largest most nearby stars using a technique known as speckle interferometry. The most important way of finding the radii of distant stars is to binary (eclipsing) star systems. We return to this issue later in the quarter.