View Full Version : Future Moon Orbital Questions

2017-Jan-11, 09:20 PM
So imagine my surprise the other night to be "channel surfing while intoxicated" through what seem to be hundreds of channels I didn't know I had, only to see the Bad Astronomer's face go blipping by! It was the only reason I stopped! I figured somebody was going to take him out of context to prove mermaids can't be found because they're all hiding from the megladons, as predicted by the second coming of Niburu.

(That fact that that sentence can be parsed out on a science board, even in jest, should be a forever stain on the Discovery Channel...)

Back tracking a few clicks I chance upon a science channel I didn't know existed and it doesn't make me cringe ever ten minutes! Who knew!

But to my point.

As the moon coasts inward billions of years from now, the inner Hill sphere limit seems to be 11,000 miles before lunar break-up. The questions I have are thus.

Is that inner orbit accurate?

And more importantly...

Who can tell me how fast the Moon's orbit would be prior to destruction? An average of the last couple of thousand years would be close enough.

Who much sky would the Moon take up that close?

Also any input whatsoever about the tides would be welcome. Ever question I try to form just sort of sticks when I try to imagine the Moon that close and presumably "that fast".

I'll even entertain the possibility of lunar induced tide tracks carved through whatever continents exist at that time.

2017-Jan-11, 10:00 PM
I think your questions are mostly moot, Don. My understanding is that the Moon will continue to recede from Earth for many billions of years, which is well after the Sun is expected to enter its red giant phase, during which it will likely expand to encompass Earth's orbit.

Jeff Root
2017-Jan-11, 11:36 PM
The speed of a body orbiting the Earth is a function of the total
mass of the Earth plus the body, and of the distance between them.
Most bodies orbiting Earth have such tiny mass that only the mass
of the Earth needs to be considered. Earth's mass is 81 times that
of the Moon, so if only a rough approximation of the orbital speed
is needed, even the Moon's mass can be ignored. So you can find
the speed in km/s at any altitude above Earth's surface in km from
the graph at the top of this web page:


To use distance from Earth's center (r) rather than altitude from the
surface, just subtract Earth's radius of 6371 km to get the altitude
to look for on the graph.

11,000 miles is 17,700 km. If that is altitude above Earth's surface,
then the orbital speed is about 4 km/s. If it is distance from Earth's
center, then 17,700 - 6371 = 11,330 km, giving an orbital speed of
about 4.5 km/s.

-- Jeff, in Minneapolis