Terminator

Mercury
The terminator of Mercury. Credit: NASA/JHUAPL/Carnegie Institution

The MESSENGER spacecraft continues its steady stream of lovely shots from the first planet. Here, we see the crater Rembrandt (center) and the terminator, the dividing line between night and day. Night and day are kind of a big deal on Mercury, considering daytime temperatures can spike around 800°F, and then drop to about -300°F at night.

Sent by: MESSENGER | From: Mercury | Sent: July, 2011 | Credit: NASA/JHUAPL/Carnegie Institution | Image source

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With the period of rotation at about 59 days and the orbital period around 89 days (Earth days I presume) resulting in a 2/3 ratio (exactly?!) what is the situation at Mercury’s equator regarding the onset or movement of the terminator? I seem to recall reading an early SF book/article that required the human explorers to walk something like fourteen miles a day to remain in the so-called twilight zone. Further towards the poles the velocity would be in relation to the cosine of the latitude (I think). Can you comment on this, aside from the fact that my memory is lousy and my math is worse?

Paul Swift

I love the idea of the 14-mile walk to stay in the safe zone. But I’m afraid I can’t comment on that off the top of my head. You’ve made me curious. I’ll see what I can find out.

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