Kevin's Watch

Wikipedia wrote:Most significant moons in the Solar System are tidally locked with their primaries, because they orbit very closely and tidal force increases rapidly (as a cubic) with decreasing distance. Notable exceptions are the irregular outer satellites of the gas giants, which orbit much farther away than the large well-known moons.

Pluto and Charon are an extreme example of a tidal lock. Charon is a relatively large moon in comparison to its primary and also has a very close orbit. This has made Pluto also tidally locked to Charon. In effect, these two celestial bodies revolve around each other (their barycenter lies outside of Pluto) as if joined with a rod connecting two opposite points on their surfaces.

Damelon wrote:It sticks in my mind that we are able to observe 57% of the Moon's surface because of that wobble.

There are two ways to determine the length of a day on any celestial body. The sidereal day and the solar day. A sidereal day on Mars is the length of time that it takes the planet to rotate once on its axis. A sidereal day on Mars lasts 24 hours 37 minutes and 22 seconds. The solar day is how long it takes the Sun to return to the meridian. This position changes slightly each day, but a solar day on Mars lasts 24 hours 39 minutes and 35 seconds.

So, you were sitting there wondering how long is a year on Mars? It lasts 686.98 Earth days or 1.88 Earth years. Now, given that a day(sol) on Mars is longer than on Earth, you would get a different answer. If you were standing on Mars, a year would take 668.5921 sols. In other words, Mars rotates 668.5921 during one orbit about the Sun.

Wikipedia wrote:It was thought for some time that Mercury was tidally locked with the Sun. This was because whenever Mercury was best placed for observation, the same side faced inward. Radar observations in 1965 demonstrated instead that Mercury has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun, which results in the same positioning at those observation points. The eccentricity of Mercury's orbit makes this 3:2 resonance stable.

Venus's 583.92-day interval between successive close approaches to Earth is equal to 5.001444 Venusian solar days, making approximately the same face visible from Earth at each close approach. Whether this relationship arose by chance or is the result of some kind of tidal locking with Earth is unknown.

Damelon wrote:Murrin is right. I'm not aware of a moon that has any kind of rotational period in the solar system.

EDIT: There is a slight bit of wobble though. It sticks in my mind that we are able to observe 57% of the Moon's surface because of that wobble.

peter wrote:My questions are as follows; Has this always been, and will it remain so; why has this peculiarity developed with our moon and not with any of the moons we observe orbiting our sister planets [I'm guessing it has to do with having liquid waters on the surface of Earth that exert gravitational effects back upon the moon equal and opposit to the effects of the moon on them. On this point, as a student ['A' Level] I asked my physics teacher why this was so and he said it was 'coincidence' - this sounds like bullshit to me, and this guy had a physics PhD!].

Vizidor wrote:

peter wrote:My questions are as follows; Has this always been, and will it remain so; why has this peculiarity developed with our moon and not with any of the moons we observe orbiting our sister planets [I'm guessing it has to do with having liquid waters on the surface of Earth that exert gravitational effects back upon the moon equal and opposit to the effects of the moon on them. On this point, as a student ['A' Level] I asked my physics teacher why this was so and he said it was 'coincidence' - this sounds like bullshit to me, and this guy had a physics PhD!].

My guess would be that if we were able to set up a cosmic weighing apparatus it would show that the total mass of each, the Earth, the Moon, and the Sun, would be roughly equal in weight, when formulated in correlation to their distances. The fulcrum of all three, planetary, planetoid, and solar, being La Grange points.

This is only an idea based on how the Moon and the Sun have equal circumferences, which can be observed during a solar eclipse.
"Nothing unusual or unique about it!" - as Murrin pointed out.
The balance is maintained indefinitely, because solar space being a vacuum holds all three orbs in a constant free-fall.

The idea that the Moon will eventually stop rotating may not be entirely correct, I believe, just mathematically correct.
Doesn't the Moon flip over every 10,000 years or so? Meaning that the dark side would then face the Earth. Wouldn't that set up another 10,000 year orbit, just like a pull on the string of a spinning top?

Vizidor wrote: During a total eclipse of the Sun there is a corona of light, so maybe that's what you meant by apparent circumference. My point being, for all intents and purposes, the Moon and the Sun are the same size. To us. And that tells me they are also in a cosmic position to have equal weight.


The 10,000 year thing was told to me sometime ago by a very clever person. Maybe he was like peter's A level PhD tutor, and simply talking bull.

Fist and Faith wrote:We could say the tides are a scale measuring the relative mass of moon and sun. In which case, the moon is "bigger", since it causes stronger tides than the sun does.