Why do planets orbit the way they do?

Asked by pallen123 (1514) September 23rd, 2012

Tonight my five year old son asked me if the earth could fly into the sun and if not why? He said if there’s “no gravity in outer space and space men can fly around how come the earth doesn’t just move around in space.” And my answer was I’m not sure let me find out. So why does the earth hold it’s position in orbit with the sun and planets? Yes I took physics in college and no I don’t remember much.

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The Sun’s gravity pulls the Earth. Without it, the Earth would fly straight off, instead of around the Sun. So, it is a combination of the gravity of the Sun, and the weight and speed of the Earth.
Try taking a yo-yo and spinning around your head by the string. The string, or the Sun’s gravity, holds it close, and if you release the string, bye-bye yo-yo.

filmfann (43638)

Everything has its own gravitational pull, even in space, no matter the size. That’s why satellites stay where they are, and the moon stays where it is.

zenvelo (29827)

Good answer for a five year old,,,, the sun does pull in the astronauts, and so does the earth, but their orbit pulls them away so they stay balanced. you can demonstrate it by tying a weight to the end of a piece of string and swing it in a circle.

DrBill (16036)

I think this is the best way to explain it: Newton’s cannonball.
http://en.wikipedia.org/wiki/Newton's_cannonball

Let’s just take Earth’s gravity. This is a force, that pulls everything on Earth down at a certain rate.

So if you fire a cannonball, gravity will pull it down. Fire it with more energy, and it will take longer to fall.

What happens if you fire the cannonball so fast that it flies faster than gravity can pull it back down? Then the cannonball goes into orbit. Gravity is still pulling it towards Earth. But it’s flying as fast as gravity can pull it.

So when the moon orbits the Earth, or when Earth orbits the sun, the orbiting body’s motion—its speed and mass*—has to be in balance with the other body’s gravity. (When you get to things the size of moons and planets, their mass starts to play a big role too, unlike with the tiny little cannonball—because the force of gravity is proportional to both bodies’ combines mass.)

Qingu (21160)

If the Sun didn’t pull on the Earth they would move apart. The Sun pulls just hard enough to keep the Earth circling around it. The Earth is always falling toward the Sun, but also has considerable sideways motion to keep a circular orbit. If the Earth should slow down it would fall into the Sun, so your five-year-old’s intuition is correct!

“There’s no gravity in outer space.” Correction: There’s no gravity while orbiting another body, such as what Space Shuttle astronauts experienced. Gravity spreads through space by Newton’s equation (with a little fiddling by Einstein) to accurately account for celestial mechanics. There’s plenty of gravity in outer space, if you’re close enough to a massive body.

gasman (11261)

Wow, I wish I could give you a zillion points for this question. I thought I must have questioned or seen questions of every kind on here that has made my mind go nuts. I’ve always accepted that gravity exists but never pondered how until now. LOL Excellent question.

Pandora (26542)

Gravitational force does not disappear in space, but it does diminish. The equation for gravitational force is G x m1 x m2/r^2, where G is a constant, m1 and m2 are the masses of the two bodies, and r is the distance between them. Note that the gravitational force depends on both masses. For astronauts between Earth and moon, the gravitational force between Earth and moon is considerably larger than the force of either of them on an astronaut.

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@gasman, isn’t gravity (almost) as strong in near-earth orbit as it is on the ground? I thought astronauts were just as affected by gravity as people on land—it’s just that they’re falling as fast as Earth’s horizon curves away, so they’re in freefall.

Qingu (21160)

@Qingu , You are correct and what I said is not quite right. Low Earth Orbits, which is what astronauts travel along, are not sufficiently high to have much effect on gravitational force.

We experience gravity when something is pushing against us to counter gravitational force, like what we feel when standing on the ground or sitting in a chair. If we are in free fall in space there is nothing that pushes against us, so the feeling is of weightlessness. It is a little hard to picture, but traveling in a circular, or nearly circular, orbit is the same as free fall. Consider @filmfann ‘s yo-yo example. There is a constant force on the string in the direction between yo-yo and person swinging it. In the same way, a circular orbit requires a constant force toward the Earth, which is supplied by gravity. A person falling toward Earth and a person in orbit around it both accelerate toward the Earth. The difference is that the person in orbit has a tangential speed which keeps the person at a constant distance from the Earth.

@Qingu Quite right. There’s almost as much gravity in low Earth orbit as there is at the surface. Astronauts are “weightless” because they are in free-fall. If you jump off the top of a skyscraper you are equally weightless until hitting the ground.

gasman (11261)

Here is what I don’t understand, @gasman and @LostInParadise. If you’re in freefall, why don’t you feel the force of gravity? You’re being accelerated, right?

Qingu (21160)

Astronauts typically feel sick for the first few days of weightlessness. I don’t know how much of this is due to having a constant “sinking felling” and how much is due to the disorientation of having the body expecting to feel gravity but not being able to experience it.

It’s actually an excellent question, and one that puzzled even Newton and Einstein.

If every body in space has a gravitational pull on every other body, then why hasn’t the universe simply collapsed?

One of Newton’s responses (since the question had been posed to him by a Bishop), was that “God has the universe in perfect balance, so that each body perfectly offsets the other bodies that would start such a sequence, and if it gets out of balance, then God can reset it again.” But he didn’t like that answer.

Next he postulated that the universe, if infinite, has no geometric “center”, so there’s no common place for it to collapse into. But that still didn’t explain why the phenomenon doesn’t occur locally (on a galactic level, if you want to think of a galaxy as “local”).

Einstein attempted to prove that the universe is still expanding, so the effects of gravity which would lead to a series of such collapses have not had a chance to occur yet.

I believe that the current thinking in astronomy is that the unknown properties of dark matter and dark energy have a sort of repulsive effect (a form of anti-gravity, perhaps?) that both causes the (continuing) universal expansion and also prevents the “local” collapses.

My ideas on this are cloudy, I know. It’s something I never did study or think about very much. I recommend Physics of the Impossible: A Scientific Exploration into the World of Phasers, Force Fields, Teleportation, and Time Travel by Michio Kaku, which I also didn’t understand, but which got me thinking about such things.

CWOTUS (24354)

@CWOTUS, only dark energy is repulsive.

Dark matter is just matter that doesn’t respond to the electromagnetic force (ie light). So we can’t see it. Dark matter is still pulled in by gravity, though; that’s how we know it exists.

Qingu (21160)

Thanks, @Qingu.

So, if I try to come on to a 25-year-old sweetie and she calls me “repulsive”, I can smoothly segue into, “Yeah, honey, I’m dark energy!” I’ll keep it in mind, anyway. But it sounds like a line better suited to Blackberry. In 35–40 years. Maybe.

Like I said, I didn’t understand about half of the book. I have enough trouble dealing with gravity I can feel and stuff that I can see… and the stuff I miss seeing that occasionally hits me.

CWOTUS (24354)

@CWOTUS, I don’t think anyone understands dark energy. :)

One way to think about the “missing” stuff is that it’s sort of like the world of bacteria. Before the 1600’s and 1700’s, people literally had no idea that microscopic life and structures existed. But once we invented microscopes it turns out there is a whole world of microscopic life—that there is, in fact, more of it, and more kinds of it, than macroscopic life.

When you think about the tools that physicists observe the universe, from our little orbit around our planet, it makes sense that there’s a lot of stuff that we simply cannot see (let alone understand) yet.

Qingu (21160)

The planets revolve around their orbit because the sun’s gravity keeps the planets in their orbit. It is almost similar with the earth and the moon. The moon orbits around the earth because the gravity of the earth pulls the moon. – http://curious.astro.cornell.edu/question.php?number=481

Why do planets orbit the way they do? I don’t know, because my interest in astronomy doesn’t extend that far, but the way they do it is in elliptical orbits as discovered by Johannes Kepler in 1605. He, evidently, had an excellent grasp on this question and looking into Kepler might give you some insight as to the answer you seek.

Esoterica: My interest in Kepler was serendipitous. In the mid eighties, I was bicycling on a dirt road through some apple orchards on a small island (Ven) in the sound between Denmark and Sweden and came across some interesting stone ruins. In the village it was explained to me that these ruins were what was left of Tycho Brahe’s personal observatory. I had no idea who he was, but there was a little library that explained this famous Dane’s historical importance in the grand scheme of things. He evidently had a lot of correspondence with this Kepler fella down in Prague. Kepler was a Jesuit priest and an astronomer who discovered that the planets ran in elliptical orbits, versus round ones. This was considered a ground breaking discovery which later made him famous and put him in the encyclopedia. But the thing that jumped off the pages at me was a small mention that Kepler’s mother was her village’s herbal physician/midwife/healer who had run afoul of church authorities and was burned at the stake as a witch. The guy must’ve really loved his science to tolerate that and become a priest anyway—I suppose the church was the only institution in which he could pursue his love for mathematics and later fund his research, as they were in control of the universities. They also had the only libraries to speak of and becoming a Jesuit would allow him access to all that. But man, that must have been tough to swallow. So, I really don’t understand much about what the guy actually did, but I can tell you a little about his mom.

It’s funny—the stuff you can run into on a bicycle that you would blow right past in a car.

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