Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Our moon has been slowly drifting away from Earth over the past 2.5B yrs

phys.org

Post reply on HN
Print view View original

61–70 of 89 posts

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#61

Post by goodcanadian »

unnouinceput wrote: post #54
kubafu wrote: post #33

Earlier quoted context omitted.

And another factor will be the Sun growing in size due to hydrogen exhaustion: https://en.wikipedia.org/wiki/Sun#After_core_hydrogen_exhaus... But at the time also the Earth's average distance from the Sun will grow, wouldn't it?

No, it won't. Earth will either be a scorched planet or will be engulfed by the Sun. And it won't take those 5 billion years either for life to vanish from Earth. We're currently near the inner edge of Goldilocks zone (Mars is just a bit outside of the outer edge) and we have only like 500 million years. Either start colonizing other planets or we die, there nothing in between (assuming we don't destroy ourselves mea…

While I am not generally a proponent of waiting until the last minute to worry about a problem, I think with a time frame of hundreds of millions of years, we can afford to procrastinate a bit.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#62

Post by onos »

ncallaway wrote: post #25
chowells wrote: post #19

Earlier quoted context omitted.

If the moon's orbit is increasing in radius, doesn't that mean its orbital velocity is decreasing? Which way is energy being transferred? This stuff confuses me.

I haven't done the math, but I strongly suspect that the potential energy (from moving away from the source of gravity) is increased more than the kinetic energy (from the reduced velocity) decreases. So, I would think when the moon moves away from the earth, its total energy increases. Thus, the earth's energy decreases (in the form of slightly reduced rotational kinetic energy)

That’s right. The radius is set by the force

mv^2/r = GMm/r^2.

—> mv^2/2 = 0.5 GMm/r

—> Kinetic plus potential = - 0.5 GMm/r

This goes up with r.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#63

Post by plasticchris »

ben_w wrote: post #56
unnouinceput wrote: post #54

Earlier quoted context omitted.

No, it won't. Earth will either be a scorched planet or will be engulfed by the Sun. And it won't take those 5 billion years either for life to vanish from Earth. We're currently near the inner edge of Goldilocks zone (Mars is just a bit outside of the outer edge) and we have only like 500 million years. Either start colonizing other planets or we die, there nothing in between (assuming we don't destroy ourselves mea…

Third option, we can take advantage of repeated gravitational slingshots to transfer orbital kinetic energy from Jupiter to Earth via an asteroid going repeatedly between both planets, gradually increasing Earth's orbit. (I'm not an astrophysicist, I just read about this idea a few years back and it stuck in my mind).

I guess Jupiter would be more efficient but it would be really neat to do this to mars bringing it nearer to the habitable zone/earth

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#64

Post by tomrod »

goodcanadian wrote: post #61
unnouinceput wrote: post #54

Earlier quoted context omitted.

No, it won't. Earth will either be a scorched planet or will be engulfed by the Sun. And it won't take those 5 billion years either for life to vanish from Earth. We're currently near the inner edge of Goldilocks zone (Mars is just a bit outside of the outer edge) and we have only like 500 million years. Either start colonizing other planets or we die, there nothing in between (assuming we don't destroy ourselves mea…

While I am not generally a proponent of waiting until the last minute to worry about a problem, I think with a time frame of hundreds of millions of years, we can afford to procrastinate a bit.

Given how far we've come in the past 150 years, no sense in waiting for an existential threat. Let's get crackin'!

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#65

Post by andsoitis »

soliton4 wrote: post #58

-- quote -- These have shown that the moon is currently moving 3.8 cm away from the Earth every year. If we take the moon's current rate of recession and project it back in time, we end up with a collision between the Earth and moon around 1.5 billion years ago. -- end quote -- 3.8 cm * 1.500.000.000 = 5.700.000.000 cm = 57.000.000 meter = 57.000 km distance between earth and moon = 384,399 km according to https://en…

> we end up with a collision between the Earth and moon

My understanding is that it isn’t that the moon and earth collided, but rather that the moon was formed from earth stuff when another body collided into earth.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#67

Post by castratikron »

soliton4 wrote: post #58

-- quote -- These have shown that the moon is currently moving 3.8 cm away from the Earth every year. If we take the moon's current rate of recession and project it back in time, we end up with a collision between the Earth and moon around 1.5 billion years ago. -- end quote -- 3.8 cm * 1.500.000.000 = 5.700.000.000 cm = 57.000.000 meter = 57.000 km distance between earth and moon = 384,399 km according to https://en…

I guess that assumes that the moon's starting point was the surface of the Earth and then started floating upward very slowly to where it is now.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#68

Post by zenexer »

soliton4 wrote: post #60
JaimeThompson wrote: post #59

Earlier quoted context omitted.

The rate at which the moon moves away from the Earth isn't constant. I can't find the details right now so I could be totally wrong.

thats true, however the author explicitly stated that the collision point of earth and moon at 1.5 billion years was calculated by assuming the moons CURRENT rate of recession

At no point does the author make any assertion that the rate was constant. Presumably, the current rate of recession is just one variable that gets plugged into an equation.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#69

Post by sidewndr46 »

zaarn wrote: post #43
TheSpiceIsLife wrote: post #38

Earlier quoted context omitted.

Yes, sorry, I should have also wrote "where one can stand".

The Moon Kerberos of Pluto is actually very very close to the size of the sun in the sky (of by being 0.0099° compared to the suns 0.0089°, around 11%), so the earth isn't the only planet (if you count Pluto as planet, otherwise, but it's still a solar body you can stand on where a moon is roughly the size of the sun).

Doesn't Pluto have some weird elliptical orbit? Does it not matter enough to change the apparent size of the sun as seen from Pluto's surface?

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#70

Post by soliton4 »

andsoitis wrote: post #65
soliton4 wrote: post #58

-- quote -- These have shown that the moon is currently moving 3.8 cm away from the Earth every year. If we take the moon's current rate of recession and project it back in time, we end up with a collision between the Earth and moon around 1.5 billion years ago. -- end quote -- 3.8 cm * 1.500.000.000 = 5.700.000.000 cm = 57.000.000 meter = 57.000 km distance between earth and moon = 384,399 km according to https://en…

> we end up with a collision between the Earth and moon My understanding is that it isn’t that the moon and earth collided, but rather that the moon was formed from earth stuff when another body collided into earth.

the "collision" between earth and moon is entirely hypothetical in this context. yes there was probably a colision involved in the creation of the moon, but that is an entirely different storry that has no relevance to this thread besides the fact that it also involves the earth, the moon and the word colision. if you must continue a conversation about the origin of the moon please do it in a new thread, as any further exploration in this context will lead to more confusion and will ultimately do more harm than good
Post reply on HN