Daylight moon.
August 20, 2011 11:50 AM   Subscribe

Is it possible for a moon to have an orbit such that it is only ever visible from its planet during daylight hours? If so, what are the necessary requirements for this to happen?
posted by curious nu to Science & Nature (14 answers total) 2 users marked this as a favorite
Well, I can see two ways - one, that it doesn't actually orbit, but is in a fixed position between the planet and the sun. (Whether this is physically possible I dunno, though.) Or that it follows a normal orbit but happens to be always in eclipse when it's on the dark side, which seems like it'd be pretty possible with a very large planet and/or a very small moon.
posted by restless_nomad at 11:57 AM on August 20, 2011

Lagrangian points may hold some of the answer.
posted by kithrater at 12:03 PM on August 20, 2011

Put a small moon in geosynchronous orbit around the planet. The angle of the rotation of the moon around the planet should be so that it falls into the shadow of the the planet at night.. Remember, the moon is only visible because it reflects the sun's light. A lunar eclipse happens when the moon comes across earth's shadow. Your hypothetical moon would always be in the planet's shadow.
posted by Brandon Blatcher at 12:05 PM on August 20, 2011 [1 favorite]

In principle, the moon could be placed in Lagrange point 1. The problem is that Lagrange 1 is not stable. An object there is balanced on the head of a pin, and any perturbation of it will cause it to fall out of Lagrange 1 and go into a more normal orbit.

NASA is maintaining a research satellite in Lagrange 1 right now, and the guidance team for it are constantly manipulating its position with onboard rocket engines to prevent it from leaving that position.

Lagrange 4 and 5 are the only ones which are eternally stable. But they're visible from the night-time side of the planet.
posted by Chocolate Pickle at 12:06 PM on August 20, 2011 [2 favorites]

BB, a moon in geosynchronous orbit would be visible at dusk and dawn, and in fact for quite a while after sunset and before sunrise. The planet's shadow is really quite small by comparison to the whole orbit.

Also, geosynchronous orbits are also not eternally stable. Our communications satellites in geosynchronous orbit also have rocket engines to keep them in place. (And when the fuel is nearly gone, they use the remaining fuel to dump the satellite into a parking orbit which is internationally recognized as the satellite garbage bin.
posted by Chocolate Pickle at 12:08 PM on August 20, 2011

OP, the answer is that it could only stay in Lagrange 1 if someone was actively maintaining its position there. (As is the case with SOHO.)
posted by Chocolate Pickle at 12:11 PM on August 20, 2011

A moon at the L1 point would always face the daylight side of the planet, yes. But on the other hand, it would always be a new moon, so even during the day you wouldn't be able to see it.

The size of a geosynchronous orbit around a planet is determined solely by the planet's mass and the length of a day. For a geosynchronous orbit to do what you want, the planet would need a very low density (so it takes up more space and has a bigger shadow) and a very high rotation speed (so the altitude of a geosynchronous orbit is lower).

For instance, Saturn is the least dense gas giant in the solar system, and has a synchronous orbit altitude of about twice its radius. A moon at that altitude would be eclipsed for 1/3 of each night.
posted by teraflop at 12:23 PM on August 20, 2011

Anything other than a new moon would be visible at night. The moon would have to be on a line in between Earth and the Sun. If it deviated from the ecliptic it would be visible before or after sunset. If it deviated perpendicularly from the ecliptic it was be visible from one of the poles just beyond the terminator.
posted by Stonestock Relentless at 3:29 PM on August 20, 2011 [1 favorite]

I mentioned perturbations above which can knock satellites out of Lagarange 1 and/or geosynchronous orbit.

The biggest source of such things is the moon's gravity, but it turns out that Venus and Jupiter also have to be taken into account. One of the guys on the SOHO navigation team told me that their orbital predictions kept going wrong, early on, and they eventually figured out that the reason was that they hadn't been including Venus in their simulations.
posted by Chocolate Pickle at 6:15 PM on August 20, 2011

As Stonestock Relentless says, the only point in the sky that's never visible at night from any point on the planet is the Sun itself (or any point on the line between the Earth and Sun, such as L1 and L3).

A smallish satellite in a very low orbit would be in the sky at night but only visible near dusk or dawn, because it would be in shadow (eclipse) in the middle of the night, and if it's small enough you won't be able to see it by earthglow. If you watch the sky at dusk you can often see satrellites appear or disappear as they pass out of / into the Earth's shadow.

Geosynchronous orbit is far too high for that, though.
posted by hattifattener at 8:09 PM on August 20, 2011

I'm inclined to say that, as such, this is not possible. It's possible if we fudge your requirements a bit. But really, a satellite orbits around its primary, and that means going all the way around. Your requirements for an orbit that stays in daylight at all times are pretty much non-Keplerian.

Now, if we fudge things we can come up with stuff. Like a binary star system, so there's more daylight than half the rotation period of the planet. Or the satellite has a super-low albedo, so reflects practically none of the sun's light. Or a perpendicular orbit essentially directly over the terminator such that it never actually enters full darkness -- that would in some ways come the closest. It would really ultimately be non-Keplerian, though, because the orbit in our universe would in all probability maintain its notional perpendicularity only twice a year.
posted by dhartung at 11:20 PM on August 20, 2011

Dhartung: Behold the cleverness that is the sun-synchronous orbit! You set up the orbit so that the torque from the equatorial bulge causes it to precess at exactly one revolution per year, so that the satellite is always in daylight over the terminator. (Not to be confused with statites).

It is irrelevant to the OP's question, though.

To be more analytical: Consider a satellite that is anywhere other than on the line connecting the Earth and the Sun. There will be two areas on Earth where the horizon plane goes between the satellite and the Sun— you can construct this geometrically if you like. In one of those areas, the satellite will be above the horizon and the Sun below, which is what curious nu wishes to avoid. This leaves only the L1 and L3 points, which aren't stable against perturbations; or powered orbits. (Taking non-point-size of objects, and atmospheric refraction, into account only makes it worse.)
posted by hattifattener at 12:12 AM on August 21, 2011

A properly-configured Molniya orbit might do this.
posted by Rash at 9:13 AM on August 21, 2011

Thanks for the answers! This was prompted by noticing a daytime moon yesterday and thinking about how much I enjoy the sight; got to wondering if you could have a situation where you only (or mostly) saw it during the day.
posted by curious nu at 10:50 AM on August 21, 2011

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