What is or was the Earth satellite with a relatively circular orbit that takes longest to make a single orbit?
Say the ratio between the diameters is not greater than 1.5 (big/small)
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The four in my answer have eccentricities about 0.4, there are some answers about month or longer roughly circular orbiting space junk as well: What artificial satellite has the farthest orbit around the Earth? – uhoh Dec 08 '20 at 12:06
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Define "relatively" : ratio of major and minor axes would be a good place to start. – Carl Witthoft Dec 08 '20 at 12:39
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3The Moon, maybe? – Oscar Lanzi Dec 08 '20 at 14:31
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Gravity wells have a fuzzy upper limit. The furthest away you can place something and have it remain close to the Earth are the SEL1 and SEL2 Lagrange points. These have very circular orbits around the Earth, with an eccentricity comparable to the Earth's orbit around the Sun.
Both have an orbital period around the Earth of exactly a year. By distance, L2 is slightly farther away than L1, although if one measure "slowness" in relative velocity, L1 moves slightly slower.
As such, it's presently a tie between:
This is subject to change since these locations require active station keeping.
SE - stop firing the good guys
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I still don't understand how they have a circular orbit around the Earth. Is there any diagram or video? – Joe Jobs Dec 10 '20 at 20:35
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1@JoeJobs the orbit is circular because the satellite remains at the same distance from the Earth as it stays between it and the sun, taking 365 days to go around the Earth (near as damn it - Earth's orbit isn't quite circular) – Dec 10 '20 at 21:15
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@JoeJobs while not gravitationally bound to Earth, over the course of one year it will circle Earth once, an so it does meet at least the broad definition of "orbit". – uhoh Dec 10 '20 at 21:43
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That circle has the Sun in its center, not the Earth. Sorry I dont understand. I think i need to see a video or pic – Joe Jobs Dec 10 '20 at 21:54
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@JoeJobs Imagine three people holding on to a long straight horizontal pole and walking round so that the pole turns through 360 degrees. Ignoring the ground underneath them (since the real thing is in space) each of them can imagine themselves standing still turning on the spot and the other two circling around them. None of them is right or wrong, it just depends on your viewpoint. That's how Sun, Earth and an object at SEL 1 or 2 move. – Steve Linton Dec 11 '20 at 18:31
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Sorry the only thing I can visualize is two circling around the last one – Joe Jobs Dec 12 '20 at 15:12