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

71–80 of 89 posts

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

#71

Post by soliton4 »

zenexer wrote: post #68
soliton4 wrote: post #60

Earlier quoted context omitted.

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.

"If we take the moon's current rate of recession and project it back in time" is a very straight forward way of saying, lets assume the moon receeded from the earth at the current rate.

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

#72

Post by vikingerik »

elevaet wrote: post #6

> We found that the moon was around 60,000 kilometers closer to the Earth then [2.46 billion years ago] (that distance is about 1.5 times the circumference of Earth). This would make the length of a day much shorter than it is now, at roughly 17 hours rather than the current 24 hours. I wish they explained why the moon being that much closer would have such a dramatic effect on the day length. Can someone explain thi…

That explanation is worded somewhat backwards as for cause and effect.

It's not that the moon being closer caused the day to be shorter. It's that if we extrapolate backwards from the current values of the day length and the rate of slowing, we calculate that the day must have been 17 hours back then. The cause-and-effect is that the 17-hour rotational period became 24 hours by tidal deceleration.

Other posts have given the cause - conservation of angular momentum in the Earth-Moon system. Angular momentum transfers from the Earth's rotation to the Moon's orbit.

If what you're concerned about is the magnitude of the effect, that's pretty well explainable - the planet now rotates 25% slower when the moon is 25% farther.

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

#73

Post by John23832 »

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 sun would grow in size, it would also be losing mass. This reduces the centripetal force of revolution. The orbit would indeed expand.

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

#75

Post by snowwrestler »

soliton4 wrote: post #71
zenexer wrote: post #68

Earlier quoted context omitted.

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.

"If we take the moon's current rate of recession and project it back in time" is a very straight forward way of saying, lets assume the moon receeded from the earth at the current rate.

No it’s not because you are projecting backward in time in the presence of a gravitational field that varies with square of radius.

The path of the moon away from the Earth is not a straight line, it is a tightly wound spiral.

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

#76

Post by twawaaay »

John23832 wrote: post #73
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 sun would grow in size, it would also be losing mass. This reduces the centripetal force of revolution. The orbit would indeed expand.

The loss of mass is insignificant and we are talking 0,5B years.

Sun will be loosing a lot of mass later, but that will be way after Earth has been baked to a crisp.

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

#77

Post by sgtnoodle »

SJSque wrote: post #46
sgtnoodle wrote: post #39

Earlier quoted context omitted.

Its relative velocity with respect to a Cartesian frame of reference fixed on the surface of the Earth may be dropping, but that's not particularly meaningful. Think of the moon being in free fall, without any external forces acting on it. It would be moving at a constant velocity in a straight line, except the space and time it is in is curved due to gravity. Because of that curved spacetime, the moon appears to acc…

Just a comment on your second and third paragraphs: it's a bit odd that you invoke general relativity ("curved spacetime") for the Moon's basic orbit, and then discuss classical mechanics ("tidal forces are literal forces") for the second-order effects. Tidal forces are of course also gravitational effects, just differential (i.e., the result of the fact that we are not talking about point masses, but rather extended…

shrug I was just trying to help someone develop an intuitive understanding of orbital dynamics. As far as I know, all cows are indeed spherical.

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

#78

Post by Manu40 »

Telemark70 wrote: post #32
Manu40 wrote: post #15

Earlier quoted context omitted.

I was recently looking at some data about day lengths, and basically it all boils down to the fact that the earth doesn't orbit the sun in a perfect circle, which causes slight accelerations and decelerations; along with its wobble. That wobble as I understand is due to the effect of tidal forces due to the moons pull on our ocean. Tropical/Solar days = 24 hours But Sidereal = 23 h 56 min 4.0905 seconds Why the diffe…

The difference between the solar day and the sidereal day is caused solely by the progress of the earth in its orbit. After 23 hours 56 minutes the earth has made a full rotation relative to the stars. But it has to turn for a further 4 minutes to get the sun to be above the same place on the earth. The difference between the day lengths is one day divided the number of days in a year, i.e. approximately 24 hours / 3…

Yeah, that's basically exactly what I said but with different words.

Also, wobble does play a factor. You may want to look into it again some more.

https://singularityhub.com/2022/08/07/the-length-of-earths-d....

> Apart from these large-scale changes, over shorter periods weather and climate also have important impacts on Earth's rotation, causing variations in both directions. The fortnightly and monthly tidal cycles move mass around the planet, causing changes in the length of day by up to a millisecond in either direction.

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

#79

Post by ncmncm »

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…

No post body was provided.

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

#80

Post by AprilArcus »

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).

The limit on earth's habitability is determined by permanent sequestration of atmospheric CO₂ through the carbonate-silicate cycle, not insolation. Once CO₂ drops below a certain level, photosynthesis will no longer be possible and all remaining ecosystems will collapse.

(Don't mistake that as an endorsement of burning fossil fuels — climate change is operating at a rate measured in decades, CO₂ drawdown via the carbonate-silicate cycle operates at a rate measured in hundreds of millions of years)

Post reply on HN