Archive

Archive for August 8, 2025

Ephemeris: 08/08/2025 – Mercury’s strange rotation

August 8, 2025 Comments off

This is Ephemeris for Friday, August 8th. Today the Sun will be up for 14 hours and 21 minutes, setting at 8:58, and it will rise tomorrow at 6:38. The Moon, 1 day before full, will set at 6:42 tomorrow morning.

The planet Mercury is the closest planet to the Sun, and for a long time was thought to have synchronous rotation. That is it rotated in the same 88 days it took to orbit the Sun. However, in 1965 using radar from the Arecibo Observatory in Puerto Rico, astronomers learned that the rotation was exactly 2/3 of that 88 days. An interesting relationship of the sidereal rotation of 59 days against the stars, 2/3 of the Mercury’s year. Earth’s solar day, noon to noon is 3 minutes 56 seconds longer than the sidereal day. Mercury’s solar day turns out to be exactly 2 Mercurian years long.

The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.

Addendum

Mercury's 2/3 of a year rotation. 59 days vs 88 day year
Mercury’s 2/3 of a year rotation of 59 days vs 88 day year. Mercury rotates 3 times while it orbits the Sun twice, making its solar day equal two Mercurian years. Each Mercury position is at 90° or ¼ of a rotation. The spots on the Mercury symbol show the rotation of a fixed spot on the planet, blue for the first year, red for the second. Spots 7 and 15 actually overlay 1. Credit: https://solarviews.com/eng/mercury.htm.
Categories: Concepts, Ephemeris Program, Mercury