A curious effect of this is that total solar eclipses past and future look different from total solar eclipses today. We live in a time when both the Sun and the Moon are roughly the same size (~32 arcminutes) when viewed from Earth. This coincidence results in the image we associate with total solar eclipses where the Sun’s corona is visible as a ring around the Moon.
This can't be true: you can choose a point behind any sufficiently round satellite where it appears the same size as the star.
Perhaps it's the only place we know of on a planet's rocky surface. But most of our solar system's planets with moons don't have rocky surfaces, and we can't detect moons in other systems, so you really mean it's not true from the surface of Mars, Pluto or Charon.
Additionally, the moon varies in apparent diameter by about 13% from apogee to perigee, and the sun by about 3%. So while it might appear "exactly" the same size at some point in its orbit, mostly it's just within 5%.
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).
We are actually pretty close to having found "exomoons"(natural satellite orbiting an exoplanet), with several promising candidates having been identified already.
David Kipping's Cool Worlds lab at Columbia has done some promising research on this, some of which is detailed in excellent videos on their youtube channel.
38000km is just 10% of the current Moon-Earth distance and on the time scale of billions years, there are other things to worry: "the Sun will likely engulf Earth in about 7.59 billion years." https://en.wikipedia.org/wiki/Future_of_Earth#:~:text=Earth%....
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 meanwhile).
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)
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.
> 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 this? Seems off to me, but I'm very physics-naive.
Also, good to know it's drifting away, and not towards the earth!
Sure. The earth is spinning faster than the the moon is going around the earth (earth rotates in one day, the moon goes around in about a month). When the tides transfer energy and momentum to the moon from the earth's spin, it slows down the earth's spin and gives the moon more energy. This moves the moon to a higher orbit, so it moves a bit farther away from us. The source of energy for the whole process is the earth's spin. It takes a lot of energy to lift a moon, relative to the amount of energy in the earth's spin, which is why it has such a dramatic effect on the day length.
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)
Earths rotational velocity is decreased (longer days) and added to the moon which increases it's potential energy (distance relative to earth), increases it's orbital height, and increases the period of it's orbit (the lunar month).
But once the lunar month = earth day the transfer will stop and the moon will slowly approach earth again, until it hits the roche limit and becomes a ring.
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 accelerate relative to the Earth. It's not actually accelerating, though; it is moving in a straight line at a constant velocity, the straight line just happens to be curved completely around the Earth.
The tidal forces are literal forces, and forces cause acceleration. So, the moon isn't quite moving at constant velocity. The change in velocity means the moon isn't quite travelling in a straight line through spacetime. The orbit changes, and in this case gets higher and slower relative to the Earth.
Another way to think about it. If you're in a space ship at a point X1 in an orbit, you can steer the nose of the ship in the direction you're moving relative to the Earth, and fire your rocket engine. You're now going faster. The opposite end of your orbit, point Y1, will now be higher in altitude than it would have otherwise been. Your relative speed at Y1 will indeed be slower than where you would have been had you not fired your engine, but when you circle back to X1 again your speed will still be higher. When you get to Y1 again, you could fire your engine a second time and increase your speed even more. You'll no longer end up back at X1, but a new point X2 at a higher altitude than X1 was. Your relative velocity at X2 will be lower than it was at X1.
In space, "speed" isn't really velocity, but acceleration. Big rocket engines make you go fast! In The Martian, the main character makes a comment to that effect when he talks about NASA convincing him to strap himself into a hodge podge death rocket, by claiming he'll be the "fastest" astronaut in history.
In a future where humans practically travel to a distant star, a "fast enough" space ship would be one that can maintain constant non-trivial acceleration for many decades. You would accelerate to the halfway point, then turn around and decelerate the rest of the way. Assuming you got fast enough relative to the destination, weird relativistic effects would become obviously apparent and the travellers would perceive space and time compressing.
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 objects).
If the moon wasn't drifting away and was in a static orbit, would the Earth's rotation slow and the moon's orbit speed up to the point where they're in sync?
Fair warning: everything I know about orbital mechanics is from Kerbal Space Program.
The moon cannot orbit at a higher speed while keeping the same semi-major axis (average distance to the orbital centre of mass).
If you suddenly doubled the orbital speed of the moon right now, the apoapsis (the highest point in its orbit relative to the orbital centre of mass) would increase significantly.
If you slowly accelerate the moon in the direction of its orbital velocity consistently over a long time period, the moon will slow down relative to the Earth, but it's semi major axis will increase.
The moon is drifting away because the Earth is rotating. Eventually the Earth will be tidally locked with the moon, the same side of the Earth facing the moon at all times. When this happens, the moon will (for the most part) stop moving away from the Earth. Well, that’s what would happen if we didn’t get eaten by the sun when it morphs into a red giant. For further research, check out…
This is called tidal locking, and if the universe consisted of only the Earth and moon, this would in fact happen. However, the big heavy Sun also affects both the Earth and moon's rotations.
So why do I say that it has happened? Because the moon, having significantly less mass than the Earth, is almost tidally locked to the Earth. That's why we always see the same side of the moon. So the Earth's rotation hasn't synchronized with the moon's revolution, but the moon's rotation has nearly synchronized with the Earth's revolution (actually both the Earth and moon revolve around their common barycenter).
Moon is below escape velocity, so it's not leaving. In fact when 1 day = 1 lunar month the movement away will stop, and reverse. It will continue to get closer to earth until it hits the Roche limit and becomes a ring.
> wish they explained why the moon being that much closer would have such a dramatic effect on the day length. Can someone explain this? Seems off to me, but I'm very physics-naive
Earth-Moon momentum is conserved.
Think of a pregnant woman (the Earth) spinning on a seivel chair.
The woman gives birth to her child (the Moon) and she takes the child in her arms and extends it at arm's length.
Their rotation slows down, just like an iceskater slows down when spinning and extending their arms.
The Earth does not have phisical arms holding the Moon, but it has gravity and the Moon also has gravity that affects the ocean tides -- the tidal effects are like tiny tiny arms that both the Earth and the Moon use to push eachother away (and lose a lot of energy in the process also).
The Earth is losing rotational momentum at the expense of the Moon, which is gaining momentum and increasing speed in traveling around the Earth which increases the centrifugal force which means the Moon goes to a higher and higher orbit and further and further away from Earth.
> Think of a pregnant woman (the Earth) spinning on a seivel chair. The woman gives birth to her child (the Moon) and she takes the child in her arms and extends it at arm's length.
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.
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 difference?
Because of those prior mentioned forces causing the location of the sun to be slightly out of alignment from where it started the day prior. Or something like that.
Tropic/Solar days are determined by the position of the sun, as per the name.
Sidereal are based upon the position of the stars. or so I understand.
To be clear. I am not an expert. I am just regurgitating what I read from NASA.
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 / 365.
> 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.
To first order I think the explanation is just conversation of angular momentum. The moon earth system has to have it's angular momentum constant over time. If the moon is further away in a stable orbit, the earth must rotate slower.
-- 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
> 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
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.
Yes, but the rate of energy transfer is related to the ratio of an earth day (which used to be shorter) and the lunar month (which also used to be shorter).
Taken in the opposite direction that moon will stop moving away once the a earth day = lunar month. At which point it will start getting closer, until it hits the roche limit and becomes a ring.
I was disappointed in this article that doesn't really explain what's going on.
Yes, the moon is moving away from earth, because the energy transferred by tides. Said energy transfer is slowing the spin of earth, which lengthens the day. This transfer will stop once a day = a lunar month. At which point the moon will slowly decrease it's orbit until it hits the Roche limit and becomes a ring.
Fun to think about, granted it's billions of years and the sun may become a red giant before then and either alter earths orbit or consume it.
All nations of the world must band together to build a harpoon, of immensity and grandness the likes of which has never seen before. The moon cannot be allowed to escape.
I feel sorry for our future colleagues software developers, that will have to deal with calendars, public holidays and all other stuff related to the lunar calendar.
Lunar calendar? How about the rotation of the earth? I imagine the further out it gets, the further out our moment of inertia, the slower the earth spins. Days get longer, but the length of a year stays the same. That'll be a fun one. We would probably have day length deniers/365 purists who believe nothing is changing for centuries.
Despite the rapid devolvement of the Moon’s rotational at its own polar axis (to a point of having one face of the Moon being tidal locked toward Earth) since its birthing from Earth, I remain curious about the breakage of such tidal locking as Moon’s orbit dramatically continues to expand.
