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I was looking at a full moon the other night with my telescope and noticed a black dot move across the moon quite rapidly. This happened a few times and others who were with me were able to confirm.

There is no way the ISS would make its orbit around the Earth that quickly for us to be able to see it a few times. Therefore there must have been others. How many are there, and how long does the average satellite take to make a full orbit around the earth?

The dots we saw looked much like this: enter image description here

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    $\begingroup$ This seems like it belongs over on Space Exploration, but it's honestly too broad. There are lots of satellites, and they have all kinds of different orbits. $\endgroup$ – Nathan Tuggy Sep 17 '15 at 17:06
  • $\begingroup$ I suspect this is a joke question. Although artificial satellites do transit our one natural satellite, there would not be several of them in a few minutes, and they certainly wouldn't show anywhere near the level of detail in the Photoshop'd image above. $\endgroup$ – user21 Sep 17 '15 at 21:55
  • $\begingroup$ We didn't see them in the detail as above but we definitely saw a similar (obviously more blurry) shape about 3 or 4 times within a few minutes. Being as there are thousands of satellites in orbit around the Earth, this shouldn't be so unbelievable. @barrycarter $\endgroup$ – B537B7725DC58715F6E6BFA7AFC20C Sep 17 '15 at 22:00
  • $\begingroup$ I'm pretty sure the above image is NOT faked by the way - there are several enthusiasts out there who photograph International Space Station passes of the moon, or even of the sun with appropriate protection. There are quite a few such images on the web. $\endgroup$ – Andy Sep 18 '15 at 8:55
  • $\begingroup$ @barrycarter The photo shows an Earth-orbiting satellite transiting the moon. It does not show a moon-orbiting satellite. $\endgroup$ – gerrit Sep 18 '15 at 13:11
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There are over 1000 operational satellites, and probably as many again defunct or non-operational satellites in orbit, plus a great deal more small pieces of space junk.

About half of these are in a low earth orbit (LEO). Satellites in LEO have a period of between 90 minutes and 130 minutes. But you would not expect a satellite to follow the same path in the sky in two orbits, as the earth will have moved between orbits of the satellite. So it is very unlikely that you will see the same satellite pass in front of the moon twice.

Satellites in LEO are moving fast! They will cross the moon's disc in less than a second, and transits are rare, as each is a path only 7km wide. at any one time, a transit is visible from less than 100th of 1% of the Earth's surface. To obtain the awesome photo that you have linked, the astrophotographer would have had to use some orbital prediction software to calculate the exact time of a solar transit by the ISS and travel to a carefully calculated position (and prayed for clear skies). You could be observing the moon all year and never see a transit.

Most of the remaining satellites are in Geostationary orbits, They orbit at a great distance over the equator. As their orbital period is exactly one day, they appear to hang in space above a point on the Earth. Such satellites are too distant to be seen as "dots".

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Im not sure of the number, upwards of 2k probably, Godards lists it at 2271, but who knows how accurate that is as some are inactive, or not listed due to government privacy. The period would depend on the satellite, its orbutal distance from the earth and its velocity.

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