Mars’ moons are easily visible at night from the surface of the Red Planet. Phobos — the nearer, larger, and brighter of the two — would be obviously non-round. A sharp-eyed observer would be able to make out the moon’s largest craters and see the jaggedness of its terminator — the dividing line between light and dark. Phobos rises in the west and sets in the east in the span of around four hours a couple times each martian day and is up at some time each night. It also noticeably changes phase between rising and setting. From the Curiosity rover’s point of view, on some evenings Phobos rises out of the slowly fading blue-gray twilight in crescent phase, transits as a quarter moon, and sets as a gibbous moon. Near fall or spring equinox, it goes into
eclipse before becoming full. Near the start or end of totality, Phobos would remain faintly visible. And on a few days each year, it also transits the Sun during the daytime in an
annular or even a partial
solar eclipse.
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