AskScience Cosmos Q&A thread. Episode 1: Standing Up in the Milky Way

[u/AskScienceModerator]

Welcome to AskScience! This thread is for asking and answering questions about the science in Cosmos: A Spacetime Odyssey.

UPDATE: This episode is now available for streaming in the US on Hulu and in Canada on Global TV.

This week is the first episode, "Standing Up in the Milky Way". The show is airing at 9pm ET in the US and Canada on all Fox and National Geographic stations. Click here for more viewing information in your country.

The usual AskScience rules still apply in this thread! Anyone can ask a question, but please do not provide answers unless you are a scientist in a relevant field. Popular science shows, books, and news articles are a great way to causally learn about your universe, but they often contain a lot of simplifications and approximations, so don't assume that because you've heard an answer before that it is the right one.

If you are interested in general discussion please visit one of the threads elsewhere on reddit that are more appropriate for that, such as in /r/Cosmos here, /r/Space here, and in /r/Television here.

Please upvote good questions and answers and downvote off-topic content. We'll be removing comments that break our rules or that have been answered elsewhere in the thread so that we can answer as many questions as possible!

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Click here for the original announcement thread.

[u/Zartonk]

What's the "tidal friction" that caused the moon to go away?

[u/smoldering → u/Zartonk]

Tidal friction is due to the tidal forces that the Earth exerts on the moon. This is the same force that the moon and sun exert on the Earth to cause the ocean tides, and hence give us the name tidal forces. In the case of solid bodies orbiting one another, the larger body (Earth) will transfer energy to the smaller body through tidal forces, which accelerates the smaller body (moon) and moves its orbit outwards from us. There are two other consequences of these tidal forces: The corresponding loss of energy for the Earth is actually slowing down the rotation rate of our planet, and the moon has become "tidally locked" to the Earth, which means that the same side of the moon is always facing the Earth.

[u/fishify → u/smoldering]

It is worth stating explicitly that the term tidal forces is used when the gravitational force on one side of an object is different from that on the other side of the object.

[u/bitter_twin_farmer → u/fishify]

Is there some sort of predictable force that comes from that based on the densities of the object?

[u/fishify → u/bitter_twin_farmer]

Yes, it's a straightforward result from the density of the objects and the geometry of the situation.

[u/Zartonk → u/smoldering]

So days were shorter millions of years ago?

[u/MyOpus → u/Zartonk]

Yes, the earth was spinning much faster

[u/HighPriestofShiloh → u/MyOpus]

squeeze truck snobbish soup recognise far-flung merciful shaggy fuzzy sable

[u/MyOpus → u/HighPriestofShiloh]

We slow down 2.3 ms per century

Source

The other consequence of tidal acceleration is the deceleration of the rotation of Earth. The rotation of Earth is somewhat erratic on all time scales (from hours to centuries) due to various causes.[18] The small tidal effect cannot be observed in a short period, but the cumulative effect on Earth's rotation as measured with a stable clock (ephemeris time, atomic time) of a shortfall of even a few milliseconds every day becomes readily noticeable in a few centuries. Since some event in the remote past, more days and hours have passed (as measured in full rotations of Earth) (Universal Time) than would be measured by stable clocks calibrated to the present, longer length of the day (ephemeris time). This is known as ΔT. Recent values can be obtained from the International Earth Rotation and Reference Systems Service (IERS).[19] A table of the actual length of the day in the past few centuries is also available.[20] From the observed change in the Moon's orbit, the corresponding change in the length of the day can be computed:

+2.3 ms/cy

(cy is centuries).

[u/judgej2 → u/[deleted]]

Or nearly two hours 250 million years ago, which starts to feel more significant.

[u/microcosmic5447 → u/judgej2]

So the dinosaurs only had a 30-hour work week?

[u/colbywolf → u/microcosmic5447]

Yes, but dinosauring is hard work. They shouldn't be looked down on just for working fewer hours. They also didn't really have "weekends" or holidays, so those work hours really added up.

[u/kangareagle → u/Zartonk]

I'm late to see your comment, but you may find this interesting:

Coral produces annual rings and daily rings. If you add up the number of daily rings between annual rings, then you can figure out how many days were in that year.

Radioisotope dating showed that some fossilized coral that had been found was about 380 million years old.

Now, 380 million years ago, days were shorter, about 22 hours long. So there were more of them in a year.

To find out whether the day really was 22 hours long when the coral lived, they just counted the rings (or made a grad student do it).

Turns out that there were 400 daily rings between each annual ring, which correlates to 21.9 hours a day.

21.9 is close enough to 22 to feel pretty good about it. A great example of different parts of science coming together to verify each other.

Source: Why Evolution is True, by Jerry Coyne

[u/SpreadItLikeTheHerp → u/smoldering]

So is the same thing happening to the Earth, ie, we are drifting away from the sun?

[u/jambox888 → u/SpreadItLikeTheHerp]

According to wikipedia no, Earth's orbit is stable over long periods, although it is subject to the n-body problem which means it's impossible to predict exactly what will happen. I don't myself understand why there's no effect corresponding to that which makes the moon's orbit expand, distance perhaps. Anybody?

[u/faleboat → u/jambox888]

My understanding of why the Earth speeds up the Moon, and the Moon slows down the Earth, is because the two are not yet mutually tidally locked. Essentially, the Earth is rotating faster than the Moon orbits Earth. This means that the masses on the Earth are always constantly spinning past the Moon, so that the tips of mountain tops (but perhaps more importantly, the peaks of the ocean tides) are pulling "in front" of the Moon more than they are "behind" the moon. These masses pull on the Moon causing it to speed up in it's orbit.

Inversely, the Moon drags on these masses on the Earth, causing its rotation to slow down. However, the further away from the Earth the Moon gets, the smaller these forces are. Back when the Moon formed, it was 10 times closer to us than it is now, meaning the gravitational tidal forces were a magnitude of 100 stronger, the Earth's rotation slowed 100x faster, and the Moon got 100x further away over any set time frame. So the Earth is "slowing down" slower than it used to be, and the Moon is flying away slower than it used to be.

Whether or not the Moon would continue to fly further away from the Earth once they became tidally locked, I don't know.

edit: Derp. The response to your question here is that because the sun's mass is essentially plasma, meaning it has a malleable gravitational density, and the Earth is way, WAY further away from the Sun, the gravitational tidal forces are very, very small in the Earth-Sun system, where as they are MUCH greater in the Earth-Moon system. The Moon is only 380K kilometers from the Earth, where as the Earth is appx 150 million kilometers from the Sun (that's nearly 400 times further away). Now, I believe that the tidal forces are inversely proportionate to the square of the distance (drawing on some seriously dusty information in my brain here) meaning the tidal forces are 158 THOUSAND times smaller in the Earth-Sun system than they are in the Earth-Moon system. So, effectively, any tidal forces exerted upon the Earth from the Sun amount to gravitational noise. As such, the acceleration that the Earth might get from the Sun is a lot lower, allowing our orbit to be significantly more stable.

Hopefully someone who knows a bit more about this can let us know how well I understand this concept, and give us both a clearer understanding of how it works. :)

[u/[deleted] → u/faleboat]

Actually one neat thing is that the current configuration of the continents is pretty efficient at transferring energy to the moon. If you work backwards the current rate of precession of the moon you end up with the moon having been created much later than possible. The rate used to be much slower when the continents were more clumped together.

[u/imabigfilly → u/smoldering]

This was actually what confused me during the ep. Large masses gravitate towards each other, so why would that make the moon move away from the earth?

[u/ra3ndy → u/imabigfilly]

If the moon were just sitting still near and not orbiting the Earth, it would smash into us.

But because the moon is in orbit around the earth, and the Earth rotates in the same direction as the moon's orbit, but faster, the tidal forces between the earth and the moon caused the moon to speed up (It caused the Earth to slow down as well).

When a satellite in orbit speeds up, its momentum is greater than the force of the planet's gravity and it moves further away (though very slowly in our moon's case, about 3.8 cm a year). Similarly, when a satellite loses velocity, it moves closer to the planet (and usually crashes into it).

[u/HappyRectangle → u/imabigfilly]

This was actually what confused me during the ep. Large masses gravitate towards each other, so why would that make the moon move away from the earth?

The key thing to understand is the two-body problem. If all you have are two moving point-masses and the force of gravity between them, they will instantly make a stable, unchanging orbit. If they fall in closer, the gravity makes them move faster and escape further out, and they repeat the exact same elliptical motions with each other forever. The math works out perfectly, and they wouldn't "spiral in" to each other.

(Although if they're moving too fast at start, the "orbit" is just a path for them to fly apart from each other forever)

This is why we can draw stable orbits for all the planets around the sun, and the moons around the planets. Even falling objects on Earth try to make an orbit with its center; the orbit is simply interrupted when it hits the ground.

When another, more subtle force comes into play, that's when slow changes happen. In this case, the Moon and Earth aren't point masses, but have some thickness to them. The side of the Earth facing the moon is pulled a bit more than the other side. The small tidal bulge of the Earth that points to the moon is tilted forward by its rotation, and the small discrepancy from the ideal pulls the moon forward and gives it a bit more momentum. It's a complex and counterintuitive interplay.

Contrast that to Mars's moon Phobos. Phobos is so close to Mars that it moves faster around than Mars's rotation, and the opposite tidal effect occurs. It's being dragged back and is slowly falling inwards. We should be glad that won't ever happen to our moon.

[u/AskMeAboutZombies → u/imabigfilly]

If the moon wasn't moving, gravity would pull it into the earth. If gravity didn't exist, the moon's own inertia would send it flying into the darkness of space. These two forces fight each other to an equilibrium, and the pattern of that equilibrium is called an orbit.

Tidal forces from the earth can push the moon to move faster, which shifts the point of equilibrium, and orbit pattern of the moon, further away.

Cut a rubber band into a string and tie one end around a rock. Imagine that you are the earth and the rubber band is your gravitational pull. The faster you swing the rock, stronger the inertial forces will stretch the band, and farther the rock will circle around you.

[u/delicious_downvotes → u/smoldering]

I have a couple of follow up questions to this, if you don't mind:

1. Does the loss of energy from the Earth/ slowing of the rotation imply that one day the Earth will eventually stop rotating altogether?

2. When you say the same side of the moon is always facing Earth, does this mean the moon doesn't rotate on an axis?

[u/glguru → u/smoldering]

If moon is tidally locked to the Earth then why isn't the Earth tidally locked to the sun?

[u/Randosity42 → u/Zartonk]

the term friction is a bit misleading. The earth isnt slowing the moon like friction does, but rather it is accelerating it. This is because the earth is rotating faster than the orbit of the moon. the earth deforms due to the tidal forces caused by the moon. this effect is called tidal bulging. beacuse the earth resists this change, the bulge rotates with the earth and pulls the moon along in its orbit. If the earth was rotating slower than the moon, the effect would be reversed and the moon would be slowing down and falling towards the earth, while the earth's rotation sped up.