Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
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Mercurian day

An amazing fact about Mercury is that its sidereal period is exactly 2/3 of its year (i.e. period of revolution about Sun).

It takes Mercury 88 days to complete one orbit around Sun. This is called its period of revolution. (Note: The unit day refers to Earth day which is equal to 24 hours.) It takes Mercury exactly 2/3 of this time to rotate once with respect to the background stars.

How long is a Mercurian day? That is, if you lived on Mercury, how much time would elapse between local noon on one day and local noon on another day.

We'll answer this question by simple reasoning. The simulation below will help. Click once on the simulation to begin.

Rotation of Mercury. This video shows Mercury as it rotates about its axis and revolves about Sun.



Now, let's see if you can apply the same reasoning used in the video in order to sketch Mercury at different locations in its orbit and show why a solar day on Mercury is equal to two of its years.

Download this handout titled What is a day on Mercury and Venus?

The handout will guide you through the reasoning used in the video for mercury. Then, you will be asked to extend the same reasoning to studying the motion of Venus. This will test your analytical reasoning skills, but it's fun!






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