Another method uses the flux of energy reaching the Earth and the inverse square law. The upper layers of the photosphere are cooler and less dense than the deeper layters, so you see absorption lines in the solar spectrum. Which element absorption lines are present and their strength depends sensitively on the temperature . There are also movies of the granulation available at his web site. Galileo discovered that the Sun's surface is sprinkled with small dark regions called sunspots .
Sunspots are cooler regions on the photosphere.
They can last a few days to a few months. Galileo used the longer-lasting sunspots to map the rotation patterns of the Sun. Because the Sun is gaseous, not all parts of it rotate at the the same rate. Animation begins with sunspots at different latitudes lined up. Hundreds of years of observing the sunspots on the Sun shows that the number of sunspots varies in a cycle with an average period of 11 years.