In the magazine "Ciel & Espace", it is stated that crater ridge next to Shackleton crater is an interesting landing site for future missions because it is lit up 90% of the time.

I get how an area can be in the shade 90% of the time, but how can an area on the surface of a planetary body be lit up by the Sun 90% of the time?


A body that is tidally locked to the sun has the same orbital and rotational periods, and would have the same side facing the sun at all times - one hemisphere would be in sunlight 100% of the time, and one hemisphere would be in darkness 100% of the time. Precession or an axial "wobble" could make it so that the body isn't perfectly locked, and has some areas that fall into light or darkness a certain fraction of the time, perhaps 90% of the time.

That said, the earth's moon is tidally locked to the earth, but not the Sun. The same side of the moon always faces the earth, but that doesn't mean the same side always faces the Sun. We can clearly observe the phases of the moon as the face of the moon goes from fully illuminated to fully dark and back again with a smooth change in illumination between. If the moon were totally flat and had no orbital inclination, any spot on the moon would be illuminated half the time - you can look at a phase diagram, pick any location on the moon, and see that it's in darkness for half of the phases. Topography and inclination complicate things somewhat, however.

Shackleton crater is interesting since it's located at the south pole of the moon, the rim is elevated, and the moon has some orbital inclination - this allows you to see "over" the horizon to the Sun that's technically on the other side of the moon. The crater's rim is in sunlight when that side is facing the sun, but it remains in sunlight even when it rotates around to the other side, due to its elevation and the moon's inclination. The result is a location that stays in sunlight most of the time. Conversely, the depression of the crater stays in darkness almost all the time due to local topography.

As an analogy, consider a very tall building built at the south pole - the tip of the building could be in sunlight even if the ground at that location is in shadow. No matter how the planet rotates, the tip will always be in sunlight, although different parts of the tip will be illuminated at different times. The crater itself is like a bowl that's being illuminated edge-on - the inside is in perpetual darkness no matter how you rotate the bowl.

  • $\begingroup$ "axial wobble": You may want to look for the term "physical libration". $\endgroup$ Aug 25 at 10:54

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