The Moon is spiraling away from Earth at an average rate of 3.8 cm per year, so when it was formed it would have been much closer to Earth. Does it follow that tides would have been greater earlier in Earth's history? If so how large?

Astromike23 · 1 year

The short answer here is yes, definitely.

The current best theory is that our Moon formed from a giant impact very early in the solar system's history. It's a little difficult to say exactly how far away it formed, but it was certainly closer to the Earth than it is now.

The moon steals angular momentum energy from the resulting torque generated by the tides in combination with Earth's rotation. Without rotation, tides are generated at the point on Earth directly facing the Moon, and the exact point on the opposite side of the Earth. Earth's rotation shifts these tides from these points, generating a net torque that gives a velocity kick to the Moon, like this.

Note that this angular momentum of an ever-widening orbit had to come from somewhere, namely Earth's rotation rate. The Moon's increasing distance also means that the Earth's rotation has slowed down considerably - it's likely that a few billions years ago, one "day" was only a few hours long. This is also one of the reasons we need to insert a "leap second" every so often, as the Earth continues to slow down ever so slightly.

Now, to get back to your original question - just how much larger? Tidal forces are formed from the net difference of the gravitational force. Since gravity goes as 1/r² , the difference means differentiating with respect to r, so tidal forces go as 1/r³ . In other words, if the Moon were 2 times closer, tidal forces would be 8 times stronger.

There's some evidence that the Moon formed around 10 Earth Radii away (though this is heavily debated, with some saying it formed only 70% of its current distance). That's 10 radii distance is about 6 times closer to Earth, resulting in tides that were 216 times larger. Big. Note that if it did form at this distance, it would not have stayed that close for long - the tidal forces would be so large as to generate really whopping torques, moving the Moon outwards quite quickly.

tl;dr: A little over 200 times larger.

Interesting side note: This also means that the moon would've been 6 times larger in our sky, subtending 3 degrees instead of its current 0.5 degrees. Eclipses, both solar and lunar, would've been much more common.

EDIT: added a tl;dr.

hoeding · 1 year

As a followup question, How the heck do they measure that distance?

stefeyboy · 1 year

So when will the moon reach its furthest distance? Won't the torque of the rotation (pushing the moon away) and the gravitational pull of the earth (pulling it in) eventually equal to maintain it's location?

TheJoseppi · 1 year

"Tides were ~1000x higher than today. They would've gone inland as a wall of water as high as 10,000 feet, probably would've covered hundreds of miles. Then they would come back, scouring the land, taking debris from the surface of the earth back into the oceans."

source: Neil Comins via National Geographic documentary "Moon Mysteries Investigated"

CatastropheOperator · 1 year

I found this, which mentions tides from long ago. This one does mention the moon somewhat but doesn't give exact figures. And this last one comes from a site that doesn't look particularly scientific but it does have more information about the moon's effect on tides.

RichterSkala · 1 year

As a follow up question: Was it significantly larger during dawn of civilization, say neolithic age?

oncemoreforscience · 1 year

How close was the moon when it formed?

hot_coffee · 1 year

Relevant: BBC - Do We Need The Moon? (1080p)

anticommon · 1 year

What would happen if the moon completely left our gravitational field and flew off into the distance? Would the earth barrel down towards the sun, or perhaps fly off into the distance? Given how long this process takes, could it counteract the (to my understanding) ever increasing size of the sun as it dies?

justkauz · 1 year

Always been curious about this - If the moon is able to have an effect on the tides, would the pull of the moon when it was closer to earth also have had a noticeable effect on volcanic activity?

MrCLOVES · 1 year

I feel like this question has some flawed logic behind it.... This seems to presume that nothing has imparted additional momentum or force upon the moon since its formation, and since that can't be true (I think craters make a decent example of this), the current rate of increase probably cannot be assumed to be directly relevant to its historic rate.

As always, I remember Mark Twain having a pretty witty statements about extrapolation of current trends.

[deleted] · 1 year

Will we lose the moon at some point, or will it eventually crash back to Earth?

1 year

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SketchTeno · 1 year

1.)approx 4.53 Billion years old moon.

2.)Moon abouts 363,104 KM at its current closest...

(centimeter is 1/100, killometer is 1000/1...=100,000 CM/KM. American converting metric w/ windows calculator cause he's dumb.)

moon at curren closest is 36,310,400,000cm/

3.) 3.8 CM/ year further away

4.) divided the 36,310,400,000cm by the 3.8 cm

We get 9,555,368,421 years to zero when the eart and moon rubbed cheeks and snuggled

(if a constant rate or departure [which i assume it has not been due to a number of things like planetary expansion, increaesed size of orbit, slowing the rate of departure and who knows what else])...

5.) BUT WAIT! the moon has only around for abouts 4,530,000,000 years.

we grab the 3.8cm per year and multiply it by 4,530,000,000 (the number of years we assume to have had a formed moon)

and we get 17,214,000,000cm further away from earth than it was when formed.

OR

About 172140 km further away. That is give or take 47% furthr away now than when formed.

Q.)How does this affect the tide you might ask? A.) I have no idea.

xlleimsx · 1 year

I have some doubts regarding the giant tidal waves: Did Earth really have waves as high as 2 kilometers or was it just that the pull of the moon rose the sea level 2 km above of what we have right now?

McSasquatch · 1 year

The object that impacted Proto-Earth to form the moon was about the size of the planet Mars.

Elrundir · 1 year

Reading this (and the answers) has brought another question to mind. Does this mean that the moon will eventually free itself from the Earth's gravitational pull? If so, what might happen to it at that point?

Or will it eventually reach some sort of equilibrium (since many of the answers have said it would have spiraled away from Earth at much higher speeds billions of years ago) where it will stop moving away from Earth entirely?

Javadocs · 1 year

Similar question: Is Earth, or any other planets in our solar system for that matter, increasing their distances from the Sun, like the moon is from Earth?

atmoura · 1 year

I'm no scientist, but one of my science profs said the moon used to cause earth tides where the land would literally move. Is this true?

Dalmahr · 1 year

My first time really posting here. Not sure if this is a separate question or could be answered here... But since the moon is spiraling out. Would it be possible to some how out it into a more permanent orbit?

sitdoggy · 1 year

Wait, if there are tides, does that mean that we're slightly lighter when the moon is directly overhead?

1 year

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SlattCatt · 1 year

Does this mean that when early animals on earth were kicking about, they were having to deal with almighty swell all the time? What sort of effect would this have ocean creatures, and shore dwelling creatures?

[deleted] · 1 year

Is this spiraling a constant value, liner in its increase, or exponential, or some other rate?

InvidFlower · 1 year

For curiosity.. with days getting longer. How long will it be until every day is 24 hours and one second long?

Theonlydreadcthulhu · 1 year

Idt that that 3.8 is a steady rate though. Of course it was closer, and there would be much larger tides, but also remember that barring the Ice Age, most of the time there was a whole lot more liquid water on the surface for the moon to push around.

Deergoose · 1 year

How can we even calculate such a tiny number, 3.8cm when dealing with such large distances? It's amazing.

Is there a point where the moon will get so far away that it will fly out of earth's orbit? Or will it just get larger and more elliptical or something ( sort of like Pluto )

nanotechnophobe · 1 year

here you go, simply amazing. the distance of the moon - italo calvino

clam_chips · 1 year

I wouldn't say 3.8 cm per year is exactly 'spiraling away'.

DevourerOfCookies · 1 year

Wait, a little simple math pointed me to the conclusion (based on the 3.8cm assertion) that it would take 42,000ish years to move a mile, So if the moon is 4.5 billionish years old does that mean its 106,000 miles farther then where it started?! that's nearly half its current distance, 239,000 miles, from earth! o.o

balleklorin · 1 year

I cant really answer your question, but huge tidal fluctations is mostly due to shape of the coastline and as a result of wind (together with the moon). This is why you have HUGE differences between low and high tides in the English Channel. Even if the moon was alot closer I dont think you would have extreme tides, unless the coastline had a shape that supported it.

HeresABookForYou · 1 year

The Moon

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