That's
an interesting question. The tides do have effects on orbits, but not
quite what you'd guess. For starters, the tides on earth are mostly from
the moon, not the sun. Your idea about tidal friction draining energy
from other forms is completely correct, however.
So
let's start with the effects of the moon tides. The facts are that the
moon is moving away from the earth at about 3.8 cm per year and that the
earth's days are getting longer at about 2 milliseconds per century.
The earth's orbit around the sun changes by only a negligible amount.
These lunar tides mainly can drain energy from two sources:
1. the rotational energies of the earth and (to a much smaller extent ) the moon
2. the orbital energy of the moon.
One
effect is to slow the earth's rotation, gradually making days longer.
That's what's happening, and that's where energy is actually being
drained from.
The other effect is less obvious. Draining energy from the moon's orbit would actually cause the moon to speed up
while pulling it in closer to the earth. The reason is that in a
gravitational orbit like that, the change in potential energy is twice
as big and opposite in sign to the change in kinetic energy. So speeding
up and moving in closer is the way to losenet energy.
Adding
energy to the moon's orbit actually slows its orbital speed a bit while
increasing its distance to the earth and adding gravitational potential
energy. Since the moon is actually moving farther away and slowing
down, its gaining orbital
energy. How can that be? Although the tides cause a net energy drain to
heat, they're also transferring some of the energy drained from the
earth's rotation to the moon's orbit. It turns out that this must happen
in order for the angular momentum lost as the earth's spin slows to go
somewhere. Angular momentum goes up as the distance grows.
These
two effects will continue until the moon-tides stop, when the moon
orbits the earth in one day. The earth will have slowed its rotation
down to the point where the same side always faces the moon.
You
can see an example of something just like that. The moon rotates just
fast enough to always show the same face to the earth. Tidal friction
caused that.
The
end result will be that both the earth's and moon's rotational speed
(length of day) as well as the lunar month will be equal, about 47 of
our current days. This will happen far, far in the future, several
billions of years from now.
Sun tides would produce similar effects, but not as large. They also tend to make the days longer.
There are some nice articles on this:
