Our moon has been slowly drifting away from Earth over the past 2.5B yrs
phys.org
phys.org
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%.
Edit: only by a few percents; the distances are too large to appreciably change the visible size of the sun in the sky.
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.
But at the time also the Earth's average distance from the Sun will grow, wouldn't it?
(I'm not an astrophysicist, I just read about this idea a few years back and it stuck in my mind).
(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)
Sun will be loosing a lot of mass later, but that will be way after Earth has been baked to a crisp.
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!
I don't know offhand whether that would happen before the Moon drifts too far away to remain in Earth's orbit, however.
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.
Actual scientists of HN: have at me.
Knowing what I know about that game I read this as: Warning; I actually know what I'm talking about here.
“Will the Moon ever leave the Earth’s orbit?” => https://youtu.be/IM_euz9PUiw
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).
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)
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.
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.
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.
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.
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.
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.
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.
Just in case any of you were thinking of patenting this idea, I'm afraid that someone beat you to it: https://patents.google.com/patent/US3216423A/en
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.
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.wikipedia.org/wiki/Lunar_distance_(astronomy)
what is wrong here?
The path of the moon away from the Earth is not a straight line, it is a tightly wound spiral.
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.
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.
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.
What would have been the effect on tides?
There are many religions that use the lunar cycles as basis for their calculations: https://en.wikipedia.org/wiki/Lunar_calendar
Simply because there is a lot more rock, and as molten magma deforms it absorbs a lot of energy - it has a pretty high viscosity.
Which I guess is also true.