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The Winter 2010 issue of The Earth Scientist includes a variety of educational resources, ranging from astronomy to glaciers. Check out the other publications and classroom materials in our online store.
One day on Mercury equals 176 earth-days. This drawing follows the position of a single point, in yellow, as Mercury spins (day to night) and as Mercury travels around the Sun (year). The yellow point faces the Sun only once every 176 earth-days or two Mercury years.
Image from: NASA

Mercury's Orbital Resonance

It takes Mercury about 59 Earth days to spin once on its axis (the rotation period), and about 88 Earth days to complete one orbit about the Sun. However, the length of the day on Mercury (sunrise to sunrise) is 176 Earth days. How can this happen? The figure shows the path of Mercury about the Sun with a mark indicating the same spot on the surface of the planet at different times in the orbit. A point initially pointing toward the Sun will point in the same direction after one rotation (59 days or 2/3 of the orbital period), but that point will no longer be directed toward the Sun. It takes three rotations of the planet during two orbits of the planet about the Sun, or 88 x 2=176 days, for the mark to get back to the same position.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF