Figure 9-9 The Sun’s Rotation This series of photographs taken in 1999 shows the same sunspot group over one-third of a solar rotation. Note how the sunspot groups changed over this time. By observing a group of sunspots from one day to the next in this same manner, Galileo found that the Sun rotates once in about 4 weeks. Sunspot activity also reveals the Sun’s differential rotation: The equatorial regions rotate faster than the polar regions.