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Does the Prime Meridian of Mars Ever Parallel to the Celestial Prime Meridian?

-- | April 26, 2016

Question: Is there ever day in Mars history where the Mars prime meridian is parallel to the prime meridian on the celestial sphere? If so what is it? For example on earth on 1950-3-21T04:35-21 spring equinox the earth prime meridian is parallel to the celestial sphere prime meridian. I am wondering if that ever happens with Mars and what that date would be if it ever happen. I don’t think its as easy as just saying spring equinox on Mars means the same thing. — Steve

Answer: This is kind of a tricky question as the celestial meridian is defined as the great circle passing through the celestial poles, the zenith, and the nadir of a particular location. So, if you are standing on Mars the prime meridian of Mars will be exactly parallel to the celestial meridian twice per year, once during the Mars spring equinox then again during its autumnal equinox. Note that the two meridians cross (though are not strictly parallel) once each day.

Jeff Mangum

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