I'm told that our Moon is moving further away from Earth with each passing century. Why is this?

Question

When two objects like the Moon and the Earth are in mutual orbit, surely it would require added energy to increase their distance apart against the force of gravity. Where is this added energy coming from?

Answer 1

The tidal bulges on Earth are carried ahead of the Earth-Moon axis by a small amount as a result of the Earth's rotation. This is a direct consequence of friction and the dissipation of energy as water moves over the ocean bottom and into or out of bays and estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, and this causes the Moon to slowly recede from Earth at the rate of approximately 38 millimetres per year. Due to conservation of angular momentum, the Earth's rotation is gradually slowing, and the Earth's day thus lengthens by about 17 microseconds every year (this would make each Earth day one second longer every 60,000 years or so, by one minute longer every four million years, and by four hours longer in 1 billion years' time. Looking back, the day was a mere 23h in length when the Dinosaurs roamed the Earth 65 million years ago). See tidal acceleration for a more detailed description and references.

So the Moon is gradually receding from the Earth into a higher orbit, and calculations (by e.g. NASA at the Jet Propulsion Laboratory) suggest that this will continue for about two billion years. By that time, the Earth and Moon will become caught up in what are called a "spin-orbit resonance" in which the Moon will circle the Earth in about 47 days (currently 29 days) and both Moon and Earth will rotate around their axes in the same time, always facing each other with the same side. Beyond this, it is hard to tell what will happen to the Earth-Moon system.

Answer 2

The Moon is NOT being pulled further away by the Sun's gravity. Think about it: the Moon and the Earth are at about the same distance from the Sun, with tiny monthly variations. The Sun IS pulling the Moon, but it's also pulling the Earth with very nearly the same force per kilogram. That means that the centripetal acceleration of the Moon is the same as the centripetal acceleration of the Earth, and one isn't moving toward the Sun faster than the other.

Besides, if that were the explanation, you would expect to see the Moon's orbit becoming more elongated toward the Sun. But the Earth-Moon distance is growing all around the Moon's orbit, so there must be another factor at work.

The Moon causes a tidal bulge in Earth's oceans, meaning that water facing toward and away from the Moon is stretched out a bit. The effect isn't much, compared to the diameter of the Earth, but it's real. If the Earth were not rotating, the tidal bulge would line up perfectly with the Earth-Moon line. However, the Earth IS rotating, and it sweeps the tidal bulge forward a tiny bit. Because the mass of the tidal bulge is "ahead" of the Moon, it causes the Moon to accelerate in its orbit. A higher orbital speed means a larger orbit, and so every century the Moon moves a little bit further out due to tide-induced acceleration.

Of course, kinetic energy can't just be created like that. As the Moon gains kinetic energy, it robs it from the Earth. Due to the Earth's tidal interaction with the Moon, the Earth's day is also slowly getting longer. We won't see any noticible change in our lifetimes, but in a few million years we'll have to invent new clocks if we're still around as a species.

Answer 3

You are correct that it is a matter of energy. If the earth and the moon were equally solid (no oceans and no plastic crust on earth) the distance would remain constant. However, the earth loses some mechanical energy because of the drag of the tides (moving bulge) within the oceans and to a lesser extent within the crust) slowing the earth's rotation. The earth is trying to pull the bulge away from the moon that causes it. Energy can not be lost from the earth/moon system and the moon must speed up a little increasing its orbit.

Answer 4

You've got some crummy answers here. It is not due to the Sun's gravity on the Moon (Which causes it to orbit the Sun along with the Earth). It is DEFINITELY not due to the expansion of the universe, which is completely undetectable on these scales.

It is due to tidal forces. The Moon's gravity raises tides on the Earth, causing the Earth to be stretched elongated rather than spherical. Ideally, the elongation would point right at the Moon, but because of friction, points slightly in front of the moon in its orbit. The gravity of the elongated bit pulls the Moon in the direction it is orbiting, and thus adds energy to its orbit. Result: it gradually moves further away. The Moon's gravity on the elongation works against the rotation of the Earth, and as a result the Earth's rotation slows down at the same time.

Obviously, these effects are very small. We only detect them with atomic clocks and laser beams bounced off the Moon.

Answer 5

Friction by the tides is slowing the earth’s rotation, so the length of a day is increasing by 0.002 seconds per century. This means that the earth is losing angular momentum. The Law of Conservation of Angular Momentum says that the angular momentum the earth loses must be gained by the moon.

Thus the moon is slowly receding from Earth at about 4 cm (1½ inches) per year, and the rate would have been greater in the past.

Something the other above neglect to add is that the moon could never have been closer than 18,400 km (11,500 miles), known as the Roche Limit, because Earth’s tidal forces (i.e., the result of different gravitational forces on different parts of the moon) would have shattered it. But even if the moon had started receding from being in contact with the earth, it would have taken only 1.37 billion years to reach its present distance.

NB: this is the maximum possible age — far too young for evolution (and much younger than the radiometric ‘dates’ assigned to moon rocks) — not the actual age.

http://www.creationontheweb.com/content/view/764

Answer 6

The above answers are incorrect. I searched the internet for about 30 seconds to tind this nice answer:

Yes, at a rate of about 3--4 cm/yr.

The tidal bulges on the Earth (largely in the oceans), raised by the
Moon, are rotated forward (ahead of) the Earth-Moon line by the
Earth's rotation since it is faster than the Moon's orbital motion.

Using a similar picture as from the previous question, we'd see
(looking down from the north pole):

Earth Moon
____
/ ) ___ ^
/ / / \ |
(____/ \___/ Moon's orbit &
Earth's rotation
(Ocean) Tidal bulge this way
*greatly* exaggerated.

The gravity from these leading and trailing bulges impels the Moon
mostly forward along the direction of its motion in orbit (the Moon's
orbit is not exactly in the plane of the Earth's equator). This force
transfers momentum from the rotating Earth to the revolving Moon,
simultaneously dragging the Earth and accelerating the Moon.

In addition to causing the Moon to recede from the Earth, this process
also causes the Earth's rotation to slow and days to become longer (at
a rate of about 0.002 seconds every century). Eventually the result
will be that the Earth will show only one face to the Moon (much like
the Moon only shows one face to the Earth). A lower limit to how long
it will take for the Earth and Moon to become tidally locked is 50
billion years, at which point the month and the Earth's "day" will
both be approximately 50 (of our current) days long. However, this
estimate is based on the assumption that liquid water seas would be
present on Earth's surface to provide the tidal interactions
necessary. But as the Sun evolves, the seas will evaporate and tidal
interactions will be much slower (solid planet distortions only). The
oceans will evaporate about 1--2 billion years from now, so the actual
time will probably be much longer.

Considerably more detail on the evolution of the Earth-Moon system can
be found in an article by J. Burns in the book _Planetary Satellites_
(ed. J. Burns [Tucson: University of Arizona]) and in Sir Harold
Jeffries' _The Earth_, 3rd ed (Cambridge Univ Press, 1952).

Answer 7

Just as the Earth's gravity influences the moon's orbit, the moon's gravity affects Earth's rotation. The moon's gravity is actually slowing down the Earth's spin rate (albeit at an extremely slow rate). As the Earth's rotational period slows, so does the Moon's. With this loss of angular momentum, the Moon's orbital distance increases at a rate of approximately 4cm per year. This has been discovered by laser measurements to the moon, reflecting a laser beam off of reflectors left on the lunar surface by the Apollo astronauts.

Answer 8

Well, the others above have it about right; The moon is robbing us of our rotational energy, and that energy is being added to the moon's orbital energy, pushing it a little furher away each day.

Eventually (if Earth were to last that long), it will be tidally locked with the moon, always facing one face toward it, and the lunar orbit would remain fixed & constant.

Answer 9

The Moon is gaining angular momentum from the Earth's rotation, due to the action of tides. As the Moon gets farther away, the Earth's spins more slowly.

The Moon pulls tidal bulges into Earth's oceans. As the earth spins, these tides wash up on the shores of continents. This causes friction against the Earth's spin, slowing our daily rotation.

The immediate effect of this friction is that the tidal bulges are not directly aligned with the Moon, but are pushed eastward by the spinning earth. This means that the Earth's mass is not fully symmetrical, and has a bit of extra mass on the eastern side when viewed from the Moon -- which is the "forward" side of the Moon's orbit. This little bit of extra mass pulls the moon forward in its orbit, causing it to slowly get farther away from the Earth.

Answer 10

It's due to conservation of momentum. Tidal friction caused by gravity ihas / is slowing down the Earth and moon's rotation. But the momentum has to go somewhere and as the Earth is much larger than the moon, the moon is slowly being pushed away from Earth.