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dodecahedron
2009-Jan-20, 03:03 AM
Now that I have your attention I shall present my humble query.

Would there ever be a time where the Earth appears to be a double star from Mars? I know Ganymede is naked-eye visible from Earth but would Earth's moon be visible from Mars?

01101001
2009-Jan-20, 05:12 AM
Now that I have your attention I shall present my humble query.

Would there ever be a time where the Earth appears to be a double star from Mars?

Trouble is the Moon is dark, like asphalt.

But it would be visible with a good telescope and maybe with very good eyesight: APOD: 2003 May 26, The Earth and Moon from Mars (http://apod.nasa.gov/apod/ap030526.html)

NASA Goddard: Can an Astronaut on Mars distinguish Earth from its Moon? (https://pumas.gsfc.nasa.gov/examples/index.php?id=7c) (Follow link to PDF document):


The Earth-Moon distance then covers about 17' (17 minutes of arc), and thus the Moon should be easily visible from Mars; this distance is about the same as the visible radius of the Moon, seen from Earth.

agingjb
2009-Jan-20, 10:54 AM
Back of the envelope suggests that the Moon is 1/100th as bright as Earth, that's 5 magnitudes. If Earth is -2 (ish) from Mars at a good elongation, that puts the Moon at 3 (ish) - easily visible and mostly easily separable.

antoniseb
2009-Jan-20, 01:19 PM
If the Earth is 60 million miles from Mars, the Moon could be as far as a quarter degree from the Earth in the Sky. This is half the diameter of the Moon as seen from Earth. The Moon would be 12 or 13 magnitudes dimmer than we see it. So it might well be as bright as first magnitude from Mars when seen at optimal brightness.

Gillianren
2009-Jan-20, 05:57 PM
Is Ganymede naked-eye from Earth? I hadn't heard that.

Tucson_Tim
2009-Jan-20, 05:59 PM
Is Ganymede naked-eye from Earth? I hadn't heard that.

No. Galileo discovered the 4 major moons of Jupiter with his newly-built telescope.

Kyle Edwards
2009-Jan-20, 06:28 PM
Is Ganymede naked-eye from Earth? I hadn't heard that.

You would have a better chance of seeing Callisto. It can get up to 9 or 10 arc minutes from Jupiter, and is roughly the same brightness as Uranus. You would have to be at a very dark site and know exactly where to look.

Tucson_Tim
2009-Jan-20, 06:39 PM
You would have to be at a very dark site and know exactly where to look.

Just curious, have you been able to do that? Seems like Jupiter would be too bright.

Kyle Edwards
2009-Jan-20, 07:00 PM
Just curious, have you been able to do that? Seems like Jupiter would be too bright.

No, I just think it would be possible, and I may of course be wrong.

For comparison, the two stars in the "double-double" in Lyra are 3.5 arc minutes apart, so Callisto would get between 2 and 3 times that distance from Jupiter. It would be interesting to try at a dark site on a night with good transparency.

antoniseb
2009-Jan-20, 07:29 PM
I feel pretty certain that on many clear nights, I have looked at Jupiter and seen the plane of the moons, and known that one side was brighter than the other. Twice I verified which side using the Sky and Telescope maps.

Gillianren
2009-Jan-20, 07:30 PM
No. Galileo discovered the 4 major moons of Jupiter with his newly-built telescope.

Well, that's what I thought. The whole naming them after the Medicis and all that.

Tucson_Tim
2009-Jan-20, 08:55 PM
I feel pretty certain that on many clear nights, I have looked at Jupiter and seen the plane of the moons, and known that one side was brighter than the other. Twice I verified which side using the Sky and Telescope maps.

Antoniseb, you must have Chuck Yeager eyes! :)

Arneb
2009-Jan-20, 09:00 PM
Here (http://www.denisdutton.com/jupiter_moons.htm) is a nice treatment of this interesting question.

Tucson_Tim
2009-Jan-20, 09:06 PM
It is sort of like Uranus - it is a naked eye object at 6th mag - but you have to have very good eyesight, know exactly where to look on a moonless, clear night, with no light pollution, at a time of night when Uranus is high in the sky. I've never been able to see it without binocs.

Uranus wasn't discovered until 1781 by Sir William Herschel using his telescope.

antoniseb
2009-Jan-20, 09:14 PM
Antoniseb, you must have Chuck Yeager eyes! :)

Had... things went down hill in my late 30's. These days, I wear glasses for distance vision.