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Does Mars have phases?

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#1 RobSter

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Posted 22 February 2005 - 04:49 PM

I was playing with starry night backyard earlier on the comp and noticed it said that mars had an 84% full phase sometime in august or something, and it increased to 100% by early november. I thought all outer planets always had a full phase because we see their lit side?

#2 Greg K.

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Posted 22 February 2005 - 05:09 PM

Outer planets have phases but always at more than 50% because they are never between us and the sun.

#3 tjensen

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Posted 22 February 2005 - 05:15 PM

Hey Robin,

The outer planets never go thru a complete phase cycle like the moon or the inner planets. But the geometry does change enough between Earth and them to change the amount of the illuminated surface we see. At opposition, you will have 100% illumination. On either side of that it will be less and can appear quite gibbous.

Hope that helped.

#4 sixela

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Posted 22 February 2005 - 05:19 PM

No -- there's a quadrature, an opposition, a conjunction and everything in between, and each position has its phase.

Outer planets are always lit more than 50%, but we do see some of their unlit side except at opposition (and conjunction, but it's rather hard to see that Mars is completely lit with the Sun just next to it, at least not unless you want to make a trip out of the atmosphere and use an optical tool with very good baffling ;) ).

When they're "behind" the sun and in quadrature there's little difference between e.g. Mars and Venus. But when they're close, inner planets pass in front of the sun again (and take their characteristic crescent appearance) and outer planets go in opposition (the best time to observe them, given they're highest in the sky in the middle of the night and they're close).

With Mars, oppositions make the planet much closer to Earth - the last opposition was about the closest Mars gets, but this year Mars is going to be very close again (though not as close as two years ago - there are roughly two years between opposition as Earth has to play catch-up with Mars).

#5 RobSter

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Posted 24 February 2005 - 04:34 PM

Oh yeah course, it's obvious now. Our moon is an outer 'planet' before and after full moon.

#6 RobSter

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Posted 24 February 2005 - 04:35 PM

Thanks

#7 imjeffp

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Posted 24 February 2005 - 09:45 PM

Wasn't describing the phases of Venus vs. the other planets what got Galilleo kicked out of church?


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