365daysDate: May 25, 2009

Title: Tides: More Than What Floats Your Boat


Podcaster: Robert Morehead from Columbia Astronomy

Organization: Columbia University Astronomy

Description: Anyone who has spent a day on the beach knows about tides, but this cosmic phenomena is at play throughout the Universe. We’ll explore this gravitational tug-o-war and see its effect on the evolution of astronomical systems, from the sandy shores of Earth and ice-covered seas of Europe, to fiery star-skimming exoplants and the delicate dance between galaxies.

Bio: Robert Morehead is a former burger-flipper and science outreach > educator who is celebrating the IYA by finishing a B.S. degree in astrophysics at Columbia University. Actually, he will have graduated a week before this podcast airs! While at Columbia he has worked on a robotic telescope for lunar observation and modeled tidal evolution in exoplanet systems. He plans to continue on to grad school and would like to continue working on exoplanets. His hobbies are numerous and all of them are nerdy.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by [brief dramatic pause] Anonymous on behalf of The Onion Router Project. The Onion Router Project provides free, open source software securing privacy on the Internet for all of Earth’s inhabitants. Find out more at


Robert: Hello everyone, and welcome to Columbia Mondays! My name is Robert Morehead, and I’m a graduating senior at Columbia University in the City of New York. Joining me is…

Cameron: Cameron Hummels, I’m a PhD student, fourth year, in the same department.

Robert: Great! Thanks for coming in and helping me with the podcast today. Today we are going to talk about Tides.

Now, almost everybody knows about tides here on Earth, and that they are caused by the gravitational forces of the Moon (and to a lesser extent, the Sun). But there is way more to tides than that! Tides and tidal forces play an important role in shaping astrophysical systems throughout the Solar System, and beyond.

First, a little background on tides. If you’ve spent anytime at the beach, you probably noticed two low tides and two high tides each day. Why is this? Well, as you probably know, the moon’s gravity pulls a tiny bit more on the near side of Earth, causing the ocean to bulge up. And of course, the land bulges up too; it’s just that the ocean, since it’s liquid, can respond to these gravitational tugs more easily. OK, but why are there TWO tides? It’s because there are two tidal bulges, one on the side closest to the Moon and one exactly opposite on the far side. It makes more sense if you compute the difference in tidal force on both sides. If you go through the calculus and trigonometry, you can convince yourself that the difference in tidal forces across an object, in this case the Earth, goes like the inverse of the distance cubed instead of the inverse squared like simple gravitational force. So the difference in tidal force is a lot more than you’d expect from just gravity alone, and stretches the object kind of like a football, making two tidal bulges.

What I think is more interesting is the effect tides have on astronomical objects and their evolution. In the Earth-Moon system, tides are actually causing the day to get longer and the Moon to get further away. It works something like this: the Moon’s gravity raises the tidal bulges on Earth, and likewise Earth’s gravity raises tidal bulges on the Moon. Now, these bulges don’t form instantaneously; it takes time to move all that water around and to heave the land up. So the bulge itself actually gets carried forward by the Earth’s rotation instead of being directly underneath the Moon. In physics terms, this causes unbalanced torques, allowing the transfer of angular momentum between the Earth and the Moon, causing the Moon’s orbital velocity to increase, and thus its orbital distance to also increase, while the Earth’s rotation slows. Everybody got that? Yeah, it still confuses me a bit, too, so let me give you an analogy that I like.

Imagine there is a really long rope or chain connecting the bulge on the Earth to the Moon. Since the Earth rotates in the same direction the Moon orbits, as the the Earth turns it pulls the Moon along, speeding it up in its orbit and making its orbit higher; but since for every action there is an equal and opposite reaction, the Moon tugs back and causes the rotation of the Earth to slow down. This is exactly what happens, except the “rope” is gravity. Now this is a really slow process — the Moon’s average distance from Earth increases by about 4 centimeters a year, and our day gets longer by only about 2 milliseconds a century. What is really cool is that if you wait long enough, there will one day be a time when Earth’s rotation period, the Moon’s rotation period, and the Moon’s orbital period will be the same. When this happens, the same side of the earth and the same side of the Moon will face each other constantly and the Moon will stay above the same point on Earth’s surface. Only one side of the Earth will get to see the Moon. This process is already partially complete. The Moon is what we call “tidally locked”: its rotation is already synched up with its orbit and that’s why we only see one side of it now. The entire process will be completed in several billion years, so it is not something you or I will see. In fact the Sun will be in its red giant phase by then, and the Earth and the Moon will be barbecued, so no one will see it.

Tidal forces are not just a factor in the Earth-Moon system, they also play an important role elsewhere in the Solar System. For example, there is Io, a moon of Jupiter. Io is the closest large moon to Jupiter; it experiences very strong tidal forces. This causes Io to heat up internally due to tidal friction. It’s like when you bend a piece of wire back and forth, it gets hot right at the bend. The result is that Io is the most volcanically active body in the Solar System, with hundreds of active volcanoes, and large lava flows of molten sulphur. Another moon of Jupiter, Europa, has an immense sub-surface ocean under a layer of ice, tens of kilometers thick.

Tidal heating from Jupiter may also cause volcanism on Europa, kind of like the black smokers at the bottom of the mid-Atlantic ridge, and this may provide the chemical basis for life.

Tidal forces are important in exoplanet systems, too. In exoplanets that are close to their star, the planets often go around their star faster than the star rotates. Going back to the rope analogy, they get tugged back in the opposite direction of their orbit, and this causes their orbits to get closer in as they evolve — maybe even to the point that they will eventually fall into their star! Also, tides may be important in considering the possibility of habitable worlds around cool, dim, M-dwarf stars. Not only will these worlds be tidally locked like the Moon, but tidal heating might be an important contribution to the energy flow necessary to support life — unless that energy flow is so great that it turns it, that planet, into a volcano world like Io. But tides don’t just affect planetary objects, isn’t that right, Cameron?

Cameron: That’s right!

Robert: You know a thing or two about galaxies — how do tides affect galaxy interactions?

Cameron: Well, just so everybody’s clear, galaxies like our own Milky Way are large collections of gas, and stars, and dark matter. Our own galaxy, the Milky Way, is a hundred billion stars. So, galaxies tend to cluster. There are groups and clusters of fifty and hundred and thousands of galaxies close together; and when galaxies get close together, their gravity affects each other in the same way that we have tides in the Solar System. Except instead of just creating a bulge on one of the galaxies… as it approaches another one, it causes a bulge of stars, and these stars actually get ripped off of the satellite galaxies that are getting interacted with — in the same way that you can think of the bulge of water on the surface of Earth actually being lifted off the surface of the Earth toward the Moon, and away from the Moon. So, what this causes are tidal tails, tidal streams, that get ripped off where stars are removed from their host system; and they tend to trail out in the trajectory of the satellite galaxy as it’s going around the host galaxy, and you get these sorts of things, and astronomers actually see these when they observe the sky, when they observe the satellites of our own Milky Way that are ten or fifteen in number. You see, you potentially see these streams of stars coming off of satellite galaxies. So, it’s definitely an open field of research right now, and it’s very interesting.

Robert: Wow, thank you. Now, talking about tidal forces in an astronomy podcast, we’d probably be a little bit remiss if we didn’t mention black holes, and the fact that you get really strong tidal forces near the event horizon of a black hole — so much so, that like if an object were to fall into a black hole it would actually get stretched out and then ripped apart. There’s a great word for that; do you remember what the word for that is, Cameron?

Cameron: I believe it has something to do with Italian cooking…

Robert: It does — spaghettification, which is probably the coolest term in all of Astronomy!

Thank you very much for helping me out today, Cameron. I’m Robert Morehead and this has been a podcast of Columbia University here in the City of New York. For more information about public events of Columbia Astronomy visit Our next Columbia Monday podcast will be Maureen Teyssier on June 1st. Have a great day and keep listening!

End of podcast:

365 Days of Astronomy
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