Planetary Orbits: Decay or Spiral?

[gloo]

So, the moon goes around the earth, the Earth orbits the sun etc...
Are the orbits eventually decaying orbits? That is, given enough time, will the moon hit the earth, Earth go into the sun etc...?? Is that why galaxies are spiralling?

 

[gloo]

actually, don't bother, i found other old posts on the question. I was asking b/c if the orbits don't decay, then the law of thermodynamics don't apply in our ability to extract tidal wave energy from the moon orbits. A debate i had with a friend of mine who said orbits will be eternal.

 

[D H]

The orbits of satellites in low Earth orbit decay, but that is because the Earth's atmosphere extends (very thinly) 1000 km above the surface of the Earth. Satellites well above the atmosphere do not suffer any noticeable decay.

Theoretically orbits will decay because orbiting bodies emit gravitational waves. In the case of the Earth orbiting the Sun, this energy loss is the equivalent of a couple of light bulbs. The Sun is losing mass in the form of electromagnetic radiation and solar wind. This means the Earth is imperceptibly moving away from the Sun, not toward it. The incredibly tiny energy loss due to gravitational waves is too small to matter.

The Moon is perceptibly moving away from the Earth. The tidal interactions that make the Moon move away from the Earth is many, many orders of magnitude larger than the incredibly tiny energy lost through gravitational waves.

Gravitational waves are only significant when the orbiting masses are very large and when the orbital radius is very small.

 

[gloo]

wow! thanks D.H! that was pretty enlightening.