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RickArcher
Post  Post subject: Huge Moons / Planets in Sky  |  Posted: Tue Jan 10, 2017 8:56 am

Joined: Tue Jan 03, 2017 1:18 pm
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If you're fed up with my other topic hear's one you might find more interesting. I've often wondered, do you know how in science fiction they have a tendency to show a moon or planet as huge in the sky, as per this image from Star Trek's pilot episode:-

http://www.kethinov.com/images/startrek/TOS1x00e.png

Is this actually possible? Do Newton's laws of motion and gravity make it impossible for, say a moon to look this big in the sky of its planet, and vise versa the planet in the sky of the moon? The planet will always look bigger in the sky of the moon than the moon will look in the sky of the planet, but it could never look this big could it? It seems to me there would be a limit to the size.

Rick


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marnixR
Post  Post subject: Re: Huge Moons / Planets in Sky  |  Posted: Tue Jan 10, 2017 10:08 am
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well, there's always the Roche limit which would imply that tidal forces would tear a moon apart if it gets too close to its primary
on the other hand if you have a situation like with Pluto and Charon,which is closer to a double planet, then maybe your scenario could apply ?

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janus
Post  Post subject: Re: Huge Moons / Planets in Sky  |  Posted: Tue Jan 10, 2017 6:03 pm

Joined: Mon Dec 28, 2015 7:11 pm
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Well, there isn't really any law that says a planet and moon have to have any particular size ratio, though the aforementioned Roche limit does set a minimum distance between the the two.

So let's assume we have a Earth-like planet with a Mars-sized moon. The Roche limit would be somewhere between ~9000 and ~17000 km. With the lower limit a rigid body and the upper one for a fluid body. Thus we could easily put our Moon ~26,000 Km from the Earth and stay outside of the Roche limit. This is ~1/15 the present distance of the Moon. Mars has twice the diameter of the Moon, thus it would appear to be ~30 times larger in the sky then the Moon does now or have an angular size of 15 degrees. As a comparison, the Moon's apparent size is approximately equal to that of a US quarter seen from a distance of nine feet. Our hypothetical moon would have an apparent size equal to the same coin viewed at a distance of 3.6 inches. Another way to look at it is that the field of view for a 35mm camera is ~38 degrees, so this moon would take up ~39.5% of the width of a picture taken with this camera. This seems to be pretty close to the apparent size of the moon shown in the image you linked to. ( Assuming the picture was supposed to have been taken with that focal length of lens. If the shot were taken with a longer focal length, such as 50mm, then the FOV would have have been only 27 degrees, and our moon could have been smaller/further away and still appear as large in the image. This is a common photography trick; by shooting with a longer focal length, you can make bac ground objects appear larger in comparison to foreground objects.)

That large a body orbiting that close would likely tidally lock the rotation of the planet to it. At that distance we get a period of ~11 hrs, so that would also be the length of the day. In this case, being tidally locked is good thing in terms of the planet's habitability is concerned, as the tides caused by our moon would be huge, and having them fixed with respect to the planet would be good. (you would still have a daily tidal variation due to the planet's star.)

And if 11 hrs still seems to short, there is no reason you couldn't have a moon that is even larger than Mars (which has only about 1/10 the mass of the Earth), then it could orbit further away, appear just as large and give you a longer day.

Such a scenario would likely be rare, but I don't think impossible.


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RickArcher
Post  Post subject: Re: Huge Moons / Planets in Sky  |  Posted: Wed Jan 11, 2017 10:13 am

Joined: Tue Jan 03, 2017 1:18 pm
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Thanks very much for that extensive reply Janus. Much appreciated. So it could look that big? Wouldn't it be fantastic to see a view like that in the sky? Almost like being in outer space ourselves (at night) in a high orbit around the other body.

I'm having problems with your second paragraph though - tidal locking. Do you mean that the rotation time of the Earth would be the same as the oribtal time of the moon, thereby meaning the moon would always be in roughly the same place in the sky, only one side of the Earth would be able to see the moon?


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janus
Post  Post subject: Re: Huge Moons / Planets in Sky  |  Posted: Wed Jan 11, 2017 4:28 pm

Joined: Mon Dec 28, 2015 7:11 pm
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RickArcher wrote:
Thanks very much for that extensive reply Janus. Much appreciated. So it could look that big? Wouldn't it be fantastic to see a view like that in the sky? Almost like being in outer space ourselves (at night) in a high orbit around the other body.

I'm having problems with your second paragraph though - tidal locking. Do you mean that the rotation time of the Earth would be the same as the oribtal time of the moon, thereby meaning the moon would always be in roughly the same place in the sky, only one side of the Earth would be able to see the moon?


Yep. that's basically what it would mean.


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