Figure 6-443-to-2 Spin-Orbit Coupling Mercury undergoes three sidereal rotations every 2 years. You can see this by following the flag on Mercury (a) from timestep 1 to timestep 15 and then (b) from timestep 15 to timestep 29. The flag points to the right four times during this interval (at times 1, 10, 20, and, finally, at 29). This means that Mercury has rotated 3 times in exactly 2 sidereal Mercurian years. Because a sidereal year is 88 Earth days long, a sidereal day on Mercury is 58.7 Earth days long. During the same interval, however, the Sun is at noon as seen from the flag’s location only twice: at timestep 1 and timestep 29. Therefore, a solar day is 176 Earth days long.