Which phases of Earth’s moon (if any) would we see from Mercury?
All of them. But they won’t coincide with what’s observed on earth at the same time.
I’m thinking to the naked eye the phases of even earth would not be noticeable and the moon less so – they’re pretty tiny.
Venus’ phases are not all that visible to the Mk I eyeball on earth.
Earth won’t have any phases to speak of as seen from Mercury by eyeball alone; the parallax from the orbit of Mercury is such that at aphelion it is about 25 degrees off-angle but illumination of the atmosphere extends to about 12 degrees beyond 12 degrees, so at most you would see about a 12-13 degree sliver of not-quite-complete-darkness at the extreme. Given that the distance at closest appraoch would be roughly comparable to Venus when it is visible from Earth, the “naked eye” view would essentially be a bright blobby spot, although given that the Bond albedo of the Earth is half of that of Venus you might not even see that much. A telescope would be needed to make out any details.
You won’t see any phases of the Earth’s moon because aside from the occasional eclipse (because of the coincidence that the umbra cast by the Earth is almost exactly the diameter of the Moon at its orbital distance) you will always see essentially the full phase of the Moon minus some slight sliver at aphelion (see below).
Stranger
Since the Moon is outside of Mercury’s orbit the only phases seen would be full or gibbous. If Mercury and the Moon were 90 degrees apart with Mercury at aphelion and Earth at perihelion then you would see the least gibbous phase of the Moon at 115.4 degrees illuminated or just under 2/3 of the way across. This is because that arrangement is the one where Mercury can most see around the side of Mecury so to speak.
Simple geometry (sun-earth-mercury angle) shows that the illuminated fraction (even ignoring atmosphere in the case of the Earth) can never drop below 94 or 95 percent or so, corresponding to Stranger_On_A_Train’s 25 degree phase angle. So you will always see the nearly-fully-illuminated fuzzy blobs as in the actual photograph
Wow. I guess I’m misunderstanding something basic about the phases of the moon. Or about orbital geometry.
The phases of the moon as viewed from a different planet will not be the same as those seen from Earth, the geometry is different. E.g., Venus will not form a crescent on Mercury the way it sometimes does from Earth.
Apparently. I need to sit down and visualize it, perhaps with twenty seven eight-by-ten colour glossy photographs with circles and arrows and a paragraph on the back of each one explaining what each one was. But…not right now. 8 beers in.
Think of it this way. When we have a new moon, an observer on the surface of the earth looks up and sees the moon as all dark. But an observer on the other side of the moon, looking from the same direction as the sun is shining, would obviosly see the wholly illuminated other side of the moon - a full moon.
From which it can be seen that the apparent phase of the moon depends on the position of the observer.
Suppose you’re holding a light bulb in a dark room. There’s a model of the moon a ways off. The moon will always appear full, since the light completely illuminates the side facing you (and nothing else).
Move the bulb a little ways away, and the situation doesn’t change much. It’s not quite a full moon anymore, but it’s close.
I get that. But is there no time that mercury and the moon are in the same geometrical alignment (though vastly larger distances) as one would observe on earth for a waxing/waning moon?
This might all be obvious to me in the morning. 9 beers in now.
The Moon rotates fully around the Earth, so what we see as “phases” of waxing and waning are due to the changes in illuminated aspect, e.g. we see it from behind (as a “new” moon), in front (“full”), and to the side as a crescent. From the sunward direction, it is always in some phase of illumination unless totally eclipsed by the Earth, and at the orbit of Mercury (aphelion of 0.467 AU, perihelion of 0.308 AU) only a small sliver beyond the illuminated horizon can ever be seen. You can figure out how much via basic trigonometry of the right triangle formed by Mercury at maximum aphelion and advanced or proceeding Earth by 90 degrees, e.g. α=arctan(0.467/1.00) = 25.03 degrees.
Stranger
You’re never going to get a new moon from a mercurian perspective, because the orbit of Mercury lies within the orbit of Earth. So there can never be a situation in which a Mercurian observer looks at the moon from one side, while the sun shines solely on the other.
But, yeah, you can have a situation where Earth’s moon, seen through a sufficiently powerful telescope, would look somewhat gibbous from Mercury. In round figures, Mercury is 50 million km from the sun while Earth is 150 million km from the Sun. So ( except when they are in a straight line) the three bodies will always form a triangle, one of whose sides is 50m km, the second side of which is 150m km and the third side of which must be more than 100m km but less than 200m km. For any triangle with such a configuration you can calculate the angles and hence calculate how the light from the sun would strike the moon (or, indeed, Earth) from the perspective of an observer on Mercury.
Mars, Jupiter, Saturn, Uranus and Neptune (and all their satellites) lie outside the Earth’s orbit. We only ever see these worlds at a full, or near-full phase. No crescents.
This is how the Earth and Moon would appear from Mercury.
Good analogy. So I wonder, how close to half-full does Mars ever get at its most (least?) gibbous?
And what would it take for another planet’s orbit to appear half-full from Earth? I’m guessing that would be crossing our orbit 3 months ahead or behind.
That works best on Thanksgiving day. (circles, arrows, etc.)
This image gives some indication of the variation in Mars’ aspect. Indeed, Mars is the most variable of all the planets in brightness, because it is sometimes near us, and sometimes very far away. But it always presents a nearly-full phase.
In a right triangle, the hypotenuse is the longest side. Therefore, if the other planet is farther away from the Sun than the Earth (superior planets), you will not be able to get a right angle.
Venus also varies considerably in distance from the Earth, but because Venus is closer to the Sun than us, we do see the full gamut of phases of Venus (this was one of the observations that Galileo made with his telescope, that convinced him that the Sun was the center of the Solar System). And Venus is furthest away from us when it’s full, and closest when it’s new, so the two effects tend to largely cancel out, and Venus stays close to constant brightness.
(though strictly speaking, the phases also make Venus’ or Mercury’s brightness much more variable than Mars’, since at perfectly new phase they both have 0 brightness. But we don’t usually worry about that, because at those times they’re too close to the Sun in the sky to see anyway)