I watched a program about the moon recently. It said that when the moon was formed, it was only 15,000 miles away from the earth, causing massive tides travelling at huge speeds.
On one of the Apollo missions, they left a reflector on the moon. Pretty much daily, a man in Texas fires a laser at the reflector, and has measured that the moon is moving away from us at a rate of 3.4cms a year. Its movement away from us explains our much diminshed tidal surges. Will there come a time when it has moved so far away from us that we will have no tides? Will there be any other consequences?
The tidal bulge is caused by the gravitational pull and the slight difference between the pull on the near and far sides of the planet. As long as there is a Moon we’ll have tides, and even if the Moon was gone, we would still have tides from the Sun’s gravity.
The other issue is that the tides are slowing down the Earth’s rotation. Eventually the Earth will become locked to the Moon, in much the same way as the moon is locked to the Earth. Two things happen then. The Moon stops moving away, and the tidal bulges caused by the moon stay in the same place - so tidal flows stop. As pointed out above, the Sun’s tidal effects will remain.
Exactly what the state of the oceans will be is another matter.
In principle the Earth-Moon system will eventually become locked to the Sun - as Mercury is now. No tidal flows at all then. No alternating day/night either.
In fact, this is not quite true. The annual and lunar rotational periods will theoretically settle into a harmonic rotational/revolutional pattern, but because of the magnitude difference in those periods, it will not be a simple everything remains facing everything else pattern, because that system would not be stable.
The arithmetic is beyond me, but I am pretty sure the time scale we are imagining here is much greater than the probable lifetime of the sun. Other events will also play a part in the eventual transfer of momentum among the many bodies in the solar system, and that arithmetic is pretty much beyond everyone working together with every single computer in the world, except by the classical method of inspection.
Do you have a cite for this? Wouldn’t his location have to be in sync with the reflector? Or what is the minimum area he’d have to be in, for this to work?
According to this it’s about 50B years before mutual tidal lock. Given that the Earth-Moon system will be a goner when the Sun goes red giant in 5B years, I don’t think a lunar-tideless Earth is a concern.
The wiki says that the beam has to be aimed precisely in order to hit the reflector. But other than that, you could hit the reflector anytime the moon is above the horizion.
As it was a TV program, I’m afraid my cite is the program, which I know isn’t very helpful. But you did get to see the man driving his motorbike to the observatory, and then seeing him trying to find the reflector. He said it could take up to four hours a night to pinpoint it. Not a very rigourously scientific explanation, I’ll admit - but it’s the best I can do.
The trick is that the reflector is not a simple mirror. It is a retroreflector. It is designed to reflect light back towards the source regardless of the angle.
Also worth noting is that spring and neal tides result from the interplay between solar and lunar tides. At full and new moons, moon and sun are “pulling in the same direction” – either ~0 or ~180 degrees opposed to each other, so we have spring tides: higher high tides, lower low tides. At the quarters, sun and moon are pulling 90 degrees off from each other, so we have neap tides: relatively low high tides, relatively high low tides. I don’t have a cite off hand, but I recall the statement that solar influence on the tides is about 10% of lunar.
The Sun’s tidal force is 46% of the Moon’s. The total gravitational force from the Sun is much, much greater than that from the Moon, which is why we (primarily) orbit the Sun, not the Moon.
