Text: A "Day" on Mercury 1. Mercury is an extreme example of the difference between the time it takes a planet to rotate on its axis and the length of a day (day/night cycle). 2. Figure 3comes from a early version of William Hartman's Astronomy: The Cosmic Journey. It takes Mercury 59 days to rotate once on its axis and 88 days to revolve around the Sun. 3. Referring to Figure 3: Start at position 0, where it is noon on the mountian peak. In going from position 0 to position 4, the planet has rotated once (it is again facing the bottom of the page), and it has gone 2/3 of the way around the planet. However, the mountain is not facing the Sun, it is not yet noon again. In fact, in another 29 days, Mercury has gone once around the Sun (position 6). Mercury has made an additional 1/2 turn and the mountain is now facing the other direction, it is finally midnight on the mountain! Mercury must go one more complete orbit and rotate 1 1/2 more time before it is noon again on the mountain. A `day' on Mercury is 3 rotations or 2 years long!

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