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Message: <blockquote data-quote="InvaderSquoosh" data-source="post: 653782" data-attributes="member: 4529">Ok, I haven't the time to double check my math, but here it is:If Mars were to orbit Earth and appear the size of our Moon (1/2 degree), Mars would be around 1.5 million kilometers from Earth compared to 380,000 kilometers for the Moon. Because Mars is pretty massive with respect to Earth, the center of mass of the system is somewhat beyond the earth's surface. I didn't have time to do the math for that part, but it was estimated from the center of mass of the Earth-Moon system being about 1600 kilometers below the surface of the earth. Ignoring the center of mass, the period of Mars' orbit around Earth is 1.8 years. Because this is longer than the Earth's year, Mars would appear to circle the sky in the opposite direction of the Moon (E to W instead of W to E). The implications of this setup on tides is simple: Tides would occur with the same daily regularity due to the sun, but the high and low tides would occur nearly yearly instead of monthly. So the port at Procampur would experience a high tide once every two years and those conditions would persist for almost half an year!A moon like phobos would probably not be in a stable orbit at only 10,000 km out. It would need to be at a lagrange point in the system, but then that would be boring as it would appear to be locked in step with Mars.If you want Mars to orbit the Earth monthly, then there will be incredible consequences. Mars would be the distance of the Moon, roughly speaking, and around twice the angular size. The impact on tides would be devastating. Having Mars at 1.5 million km out is already stretching it. I haven't thought out the gravitational conseuqences of that setup in full, especially the part of having the center of mass of the system beyond the Earth's surface.</blockquote>

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