Reading http://what-if.xkcd.com/26/ it is mentioned that the earth day has slowed from 22 to 24 hours over the last 500 million years due to tidal drag. This made me curious:
How long until earth is tidally locked to the moon?
At that point, how long will a day be? 2 weeks? Or will the moon-earth combination circle faster in relation to the sun due to the earth slowing down?
In what orbit will the moon end up? Will we have daily lunar eclipses? Or same as now?
Any ideas on the type of flora and fauna that could inhabit an earth like that? In what ways will they have to adapt?
What will climate be like? Will we have -100C nights and +100C days? Or is the atmosphere enough to moderate the swings?
You could probably google 1 and 2.
regarding 3, by my reasoning: the moon has to move further away from the earth for it to be tidally locked. Considering that it is pretty much just big enough to cover the sun from our perspective now, it won’t be when it is further away. Therefore no total eclipses.
I know that for the second question a day will be much longer than 2 weeks because the Moon currently takes twice as long to orbit the Earth and over time that period will increase as the Moon gets further away.
Just how long, I have no idea (a Google search just brings up stuff about the Moon being tidally locked in the sense that the same side always faces Earth).
Also, if it takes 500 million years for the day to become 2 hours longer, it would take about 160 billion years just to increase to the current orbital period of the Moon, assuming a linear rate of slowdown in Earth’s rotation (probably not, with the rate slowing down by exponential decay, so it’d take even longer). Of course, the Moon would also slow down, so I’d say hundreds of billions of years, if not trillions - or more likely, never. Similarly, any effects on life and climate are pretty much a moot point.
Regarding #3, we’re lucky to be living in a short (geologically speaking) era when total eclipses are a possibility. A mere 1.4 billion years from now, the moon will have drifted too far away for there ever to be another total solar eclipse - though presumably this means we will have longer lunar eclipses, if that’s any consolation.
From an article I recall reading about 20 years ago: Tidal lock will occur in about 50 billion years. Then the earth day and lunar month will be about the equivalent of 47 of our current days. Then, the sun will be the only body creating tides. This will cause the earth’s rotation to further decay causing the moon to be pulled back in. When the moon gets too close to the earth, it will break apart and the earth will have rings, like Saturn.
-calculate kinetic energy in current system (earth spins once per 24 hours, moon orbits once per ~30 days at altitude of 240,000 miles).
-Now slow the earth down to some low RPM. All of that kinetic energy you just took out of it has to be transferred to the moon, propelling it to a higher velocity (higher kinetic energy) AND a higher orbit (higher gravitational potential energy). The moon’s new speed and altitude are coupled by an equality between gravitational attraction and centripetal acceleration for a circular orbit. Moon’s new orbital period must match earth’s new RPM.
This ignores the viscous losses associated with the tides and with crustal distortion of the earth; it’s difficult to say how much energy that would eat up over time.
One way is to look at the energies involved. It would take about 36% of Earth’s rotational energy to eject the Moon out of Earth’s orbit. However, as Machine Elf pointed out, energy is lost in various places.
A second way is to look at the angular momentum of Earth-Moon system, which is always conserved. Both Earth and Moon are rotating around their common barycentre, and both are rotating about their axis. If I ignore Moon’s rotation about its axis, I get that Earth would be tide locked to the Moon when they are 921,000,000 km apart (about 2.4x current distance). Moon’s orbit (and Earth’s rotation) is about 2500 (105x24) hours at this point.
Looking from Earth, the Moon would remain in the same place - visible from half of Earth’s surface. Solar eclipse would not be possible. Lunar eclipse would be both shorter and much less common.
Probably never. In about about 5 billion years the Sun will turn into a red giant and expand past Earth’s orbit.
No idea. However, Earth’s atmosphere and oceans are quite good at redistributing Solar energy.
You’d get a synchronous lock first, and then the planes would gradually shift until they were aligned and the shape of the orbit would shift to a more perfect circle. Or rather, the plane-shifting and circularization are going on all the time, too, but they’re a slower process.
The Earth will be engulfed, consumed and burned into a molten rock by the Sun as it expands into a Red Giant during its death rattle long before a Moon/Earth tidal lock has any serious effect… IMO
Good point. Current geostationary satellites are not significantly affected by the gravity of other satellites in that orbit; with the Moon there we’d have to crowd them all into the Lagrange points, L4 and L5, and with a bit of station keeping, L3.
They wouldn’t be stationary, though; trojan satellites loop around the Lagrange points in curious orbits, so it wouldn’t be a simple matter to lock onto them any more.
They might not be perfectly stationary, but you could make those “curious orbits” quite small, smaller than the width of the beam of your antennas, to make the lock just as simple as it is now.
