Why isn the moon headed to the earth?
This is an example of people providing more information than necessary. For the purposes of your question, the Moon is in a stable orbit around the Earth because it circles the Earth exactly fast enough to balance the mutual gravitational attraction between the Earth and Moon. Manmade satellites can be in a variety of orbits, some geosynchronous (above the same point on the Earth) and some not. Low orbit satellites that are not in geosynchronous orbit will see the greatest amount of atmospheric drag which will degrade the orbit over relatively short periods of time. Even high orbit satellites that are in geosynchronous orbit may have orbits that are eventually disturbed by complex gravitational interactions between the Earth and the Moon within the span of a human life. (Note that “geosynchronous” doesn’t mean “a stable orbit around the Earth” because, for example, when you’re standing on the Earth you’re necessarily in a geosynchronous orbit, though you’d fall if you had nothing to st
As others pointed out, the transfer of energy from the earth to the moon means that the moon gets farther away, the lunar month gets longer, and the earth’s day gets longer. These tiny effects have been directly measured with atomic clocks and lasers. But the effects have also been verified on a larger scale. By looking at and counting sedimentary layers from millions of years ago, geologists can determine the number of days between monthly high and low tides. They have also counted growth rings on buried corals to determine the number of days in a year. One study shows that 900 million years ago, the day was only 18 hours long and that there were 481 eighteen-hour days in a year. (The length of a year has not changed, just the number of days in the year.
There are a lot of interactions going on between celestial bodies, and the relationships have puzzled many of the greatest minds in Western civilization for millenia. Don’t feel too stupid. The distances between bodies like the sun, earth and its moon DO change. The orbits of the planets aren’t even circular, and while periodic, they are constantly perturbed by changing proximity to other planets which pull and tug on them. Even distant stars exert SOME pull on us… Those pulls and tugs are clues as to where new objects can be found, too. Many of the outer planets were identified first by their periodic impact on the expected positions of others. It’s really pretty cool to know this, and should give you a real appreciation of the minds of folks like Galileo. Your presumptions regarding celestial stability sound rooted in a really short time frame… even 1000 human lifetimes is barely enough to tell that ‘average’ distances have changed. As clord said, atmospheric drag accounts for th
Man-made satellites decay because they are all close enough to be affected by the Earth’s atmosphere. While the atmosphere is extremely thin at satellite heights, there’s still enough air to gradually slow down satellites, which causes them to eventually fall to Earth. There’s a wide range in orbital heights. Mir, the international space station, and a variety of satellites are in low earth orbit, which decays rapidly (more atmosphere). Many communications satellites are in geosynchronous orbit, which is much much farther away from the Earth, and these orbits decay rather slowly (which is causing a space litter problem). The moon is a good long way away, and isn’t affected by Earth’s atmosphere. The Moon is actually getting further and further away from Earth, at about 1.5 inches/year. The Moon is stealing rotational velocity from the Earth, slowing the Earth’s rotation and speeding the Moon up (which makes it orbit further away). Eventually the Earth and Moon will match rotations, and
The moon is in orbit because when it was captured by Earth … or was formed in a collision with a Mars-sized object (the giant impact hypothesis).
