I’m watching an episode of Stargate SG-1, and there’s a scene as they arrive at a new planet that I’ve captured here.
I assume that’s meant to be a planet in the sky, but I suppose it could be a moon, too. What I am curious about is whether one could be on a planet with something that big in the adjoining space, and still have an Earth-like climate, atmosphere, etc. Wouldn’t the gravity of such a large celestial body have a major effect on local conditions?
People can make the moon look humongous without altering a photographic image, just by using a zoom lens. Take an example of a person sitting in a chair looking at the moon. If you’re very close to that person, the moon is much smaller than the person’s head. The further away you stand from that person, the bigger the moon is in comparison to the person’s head. If you stand far enough back, the moon can be bigger than a skyscraper. If you then use a powerful zoom lens to take the picture, you’ll see a moon that looks monstrous.
As for the astrophysics aspect of your question, I can’t answer. Sorry!
Well, I understand that a picture like that can be created using “trick” photography or just plain photoshopped. (heh. “just plain photoshopped.” an unlikely phrase.)
I’m just wondering if a person could have a neighboring planet or moon that was actually as large as the one in the picture appears (turns out that, in the Stargate episode, it was in fact supposed to be a moon) and still have Earth-like conditions.
Tidal forces go as x^3 (x cubed) IIRC. So, for a given sized body, if its half as far away, the forces are 222, or 8 times greater. One tenth the distance, and its 101010, or a thousand times greater.
So, if you are on an earth sized/like body, but its moon (same size as ours) is 5 degrees across (still not very big in the sky really), things could still be relatively normal (i think). But you sure as hell wouldnt be living near the beach with those daily tides of thousands of feet.
There wouldn’t be any tides (or rather, the tide would be fixed and immobile) if the two planets or planet/moon were tidal locked to each other. In that case the “day” would be the orbital period of their revolution around their common center of mass. In an extreme case, you get a Rocheworld. My quibble with the picture is that if the other world were that close, it wouldn’t be spherical anymore, it would be signficantly oblong due to gravity.
But I am pretty sure I recall reading somewhere that back in the day, Earth actually had some mondo sized ocean tides. Maybe not thousands of feet, but even a few hundred would be damn impressive.
Now, the other thing I’m curious about: the perspective in the picture suggests immensity on the part of the planet/moon in the sky, relative to the planet on which you’re standing. Could that satellite be actually much smaller, but just really really close? I mean, would it still look that big in the sky?
Can a planet have a moon that is bigger than itself? It would have to have less mass, right?
Probably inspired by artist’s representations of views such as Jupiter as viewed from Amalthea, or Saturn from Mimas, many of which such images (such as those from Bonestell) were popularized back in the 50s and 60s.
If the planet is mostly rock/iron like the earth and the moon was mostly ice/gas, yes, then the “moon” could be bigger and yet “weigh” less (ie, have less mass).
It seems improbable to end up with something like that, but our own moon is somewhat improbable as well.
It would, but it wouldn’t necessarily be such as to render the planet uninhabitable. With a moon/parent planet/whatever looming that big in the sky, the world you’re standing on is almost certainly tidally locked to it, but that’s not a problem as long as you’re getting your sunlight from some other source, and your day is a reasonable length.
Most “moon” s can’t consist largely of gas, though. Without a lot of mass, atmosphere leaks away - molecular velocity exceeds escape velocity too easily. It’s why Earth’s atmosphere is low on helium (what we do have is largely the result of alpha decay), whereas Jupiter is all-around big enough to have plenty of everything.
I often think it would be way cool to live on a world with a humungous moon in the sky. (I also think it’s a shame our eyes aren’t keen enough to see the Andromeda Galaxy properly, since I learned here only a few years back that it’s plenty big enough, just too faint.) But what were the odds we’d have one just right for a total eclipse where you can see the Sun’s outer atmosphere? Cosmically speaking, that may well be a lot rarer than a ring system.
You’re right, Malacandra, our moon rocks! It is indeed an amazing thing, considering the odds.
Errr… are the odds that huge against it? If you were standing on, say, a stable structural anomaly at the surface of Jupiter, would none of its several groovy moons do the eclipse-corona thing?
When the moon first formed it was 1/10 as far away and the day was around five hours. So yes, you could have thousdand foot tides every 2 !/2 hours. Yet life developed then and some think that the tidal stirring may have been instrumental in the development. Over the next four billion years, the moon stole rotational energy from the earth and moved out to its present orbit while the earth’s day lengthened.
Extrapolating based on its current rate of 3.8 cm/yr gives about 172,000 km of total recession since it was formed. Currently, the moon is about 385,000 km away, so that’s around 45% of the total distance.
However, a linear extrapolation is very obviously wrong in this situtation. When the moon was closer, the tides would have been higher and the Earth’s rotation faster, both of which would have increased the tidal drag. I seem to remember that an analysis based on this decreasing tidal drag puts the moon too close to the Earth at its beginning.
But there’s also the fact that tidal drag is also influenced by the configuration of the continents. The more the continents block the westward flow water in the oceans, the greater the drag. Currently the configuration has high blockage, so there is greater than usual tidal drag.
