Please explain planetary motion (planets don't fall into the sun) to a layman

Factual Questions
1

Please explain planetary motion to a layman. From What I understand, the fact that the planets are circling around the sun is what prevents them from falling into the sun. But why does it not work like a whirlpool motion, the more they circle, the closer they get, the faster they get, the closer they get… etc. Please help me understand.

Speeds of various bodies:

Mercury - 105,947 mph (170,505 kilometers)
Earth - 67,000 mph (30,000 Kilometers)
Jupiter - 29,236 mph (47.051 Kilometers)
Neptune - 12,253 mph (19,720 Kilometers)

Time each body takes to circle the sun:

Mercury - 88 days
Earth - 365 days
Jupiter - 4,330 days (11.86 Earth Years)
Neptune - 60,190 (164.79 Earth Years)

Distance from the Sun:

Mercury - 57,910,000 km
Earth - 149,600,000 km
Jupiter - 778,500,000 km
Neptune - 4,503,000,000 km
Mass of Objects:*

Sun - 1.989E30 kg
Mercury - 328.5E21 kg
Earth - 5.972E24 kg
Jupiter - 1.898E27
Neptune - 102.4E24 kg

was hard to find Mass expressed in units I could Understand
Speed of sun through Galaxy: 7 KM/s (25,200 KM/h)
Speed of Solar System: 250 KM/s (900,000) KM/h)
Speed of Galaxy: 600 KM/S (2,160,00 KM/h)
Rotation around own axis:

Mercury - 63 days
Earth - 24 hours
Jupiter - 9.9 hours
Neptune - 16 hours

*had trouble finding this data expressed as KM/s or KM/h.

2

IANA astronomer, but I think that it’s the relative lack of friction. In a whirlpool, you have a medium that’s in contact with itself all the way in, and the friction slows the outer parts to the point where they move inward. Also, ‘Nature abhors a vacuum’. You have a low pressure area in the middle of a whirlpool, with relatively high pressure outside of it. The high pressure naturally moves to the low pressure area, moving things in. The Sun isn’t a low-pressure area though, and there’s no high-pressure area surrounding it.

But then, I’m a layman. I’m sure some Stranger will happen by to explain it. :wink:

3

The simplest explanation is to start with orbital dynamics. Visualize a cannon. You fire the cannon, the cannon ball falls to earth at a certain distance. Put more gunpowder in the cannon and the velocity of the ball increases and it goes farther. Now visualize putting in enough gunpowder so that falls 1000 miles away, 5000 miles away, and eventually continues to fall missing the earth. The cannon ball is now in orbit.

Similarly, the planets are all falling toward the sun, but because of their orbital velocity, they continue to miss it.

4

Newton’s Second Law of Motion - Objects in motion tend to stay in motion.

This means that the Earth (for example) will track a straight line at constant speed if no forces are applied to her. But we do have a force being applied, namely, the force of gravity from the sun. Right now the Earth is in a “perfect” balance between shooting off into intersteller space and being dragged into the sun.

A better example of this effect is a satellite in low Earth orbit. It is constantly “falling” back to Earth, but because of the curvature of the Earth, the Earth’s surface is constantly curving away from the satellite. As long as the satellite speed is “perfect”, then it will forever stay 200 miles over the surface. Any slower and it will spiral in and crash, any faster and it will spiral out.

A whirlpool is a bad analogy, as this requires a fluid medium to exist. Interplanetary space is a effectively a vacuum so any fluid structures wouldn’t be comparable. These orbits are strictly a function of gravity.

5

I too an not an Astronomer.

How about visualising a heavy bucket on the end of a strong rope.
You stand in your garden and rotate yourself steadily. The bucket is kept whirling around your head, but never gets closer.

Now this is not exact :eek: , but it sort of represents an orbit:

  • you are the Sun, pulling things towards you because your massive mass has a huge force of gravity
  • the bucket is a planet, which swings around you but has too much speed to crash into you
  • the rope is the Sun’s force of gravity
6

are these the same principle?

7

Ok, thanks, that I can kind of visualize

8

The problem with that analogy is that the only speed that make the bucket “fall into you” is a speed of ZERO. Any other speed will put the bucket into some sort of orbit.

And just to be clear, the orbits are ellipses, so those speeds are “average speeds” and those distances are “average distances”. Each planet is at one of the ellipse’s foci.

9

It depends how much detail you want, but it comes down to the conservation of energy and the conservation of angular momentum in a gravitational potential. At the closest point of approach a body orbiting a much, much larger body can’t have less energy or angular momentum than at anty other point on its trajectory which limits how close the orbiting body can get to the larger body.

If there is a mechanism for the orbiting body to transfer energy and angular momentum then it could spiral into the larger body of course.

10

But this is a giant mis-statement. If the satellite is slower than the “ideal” speed it will fall into a lower but faster orbit - it is losing potential energy and gaining kinetic. Similarly if it is moving faster it will ascend into a higher but slower orbit - losing kinetic energy and gaining potential. Indeed, nearly all stable orbits are elliptical because of exactly this - drop a little lower, pick up speed, move a little higher, lose speed, rinse and repeat perpetually. It certainly will not spiral in or out.

A satellite will only drop out of orbit if it gets low enough to experience atmospheric braking - which, admittedly, needn’t be very large, or at least not initially, because any energy lost to the atmosphere is gone for good.

11

Does that mean if the orbits were perfect circles they would collapse into the sun?

12

Nope, unless you have a way of transferring energy/momentum then an object orbiting in a circle in a gravitational potential remains orbiting on that circle forever.

13

No … I agree that the satellite is losing potential energy, but that energy is being transferred to the Earth via the force of gravity. Angular momentum must be conserved, so as the satellite is losing this, the Earth will be gaining it. This effect is a function of mass, so a 100 kg satellite returning to Earth won’t change the rotational speed of the 5.97 x 10[sup]24[/sup] kg Earth very much. Think of a figure skater in a spin who draws her arms in, she’ll spin faster. For another example, hold a rock chest-high and drop it. It only accelerates straight down, it won’t move left, right, forward or backwards without an additional force being applied.

A circle is just an ellipse where the two foci are coincident. There’s nothing really special about a circular orbit.

14

Again, no. Put a stationary object of mass m at height h above the Earth’s surface, where h is small enough that the force of gravity g can be considered constant throughout our discussion, and the object has potential energy equal to mgh. As it begins to fall its velocity v increases and its kinetic energy equals 1/2 mv[sup]2[/sup]. The sum of its remaining potential energy and its kinetic energy exactly equals the potential energy it started with, neglecting losses due to friction. I have no idea why you think it would be transferring any energy to the Earth. The angular momentum of the system may change but there is nothing to say that angular momentum can’t be transferred back the other way again.

The only special thing about a circular orbit is that for the most part they don’t exist - you would have to set your initial conditions extremely carefully to achieve one. An elliptical orbit, such as those of all the planets, is stable over a very long timescale.

15

A handy little Newton’s cannon game that demonstrates orbits quite nicely:

16

IME the critical thing that confuses laymen is that they have zero practical experience with frictionless non-1G environments.

So they “build in” the idea that stuff slows down on its own and that stuff falls down on its own. And then they try to reason about orbits with those biases built in. Which produces wrong results.

In most of the universe, frictionless is normal. Friction like we live with every day is what’s weird and oddball.

In most of the universe, essentially zero gravity is normal. Gravity like we live with every day is what’s weird and oddball.

The OP is 100% right that there’s *some *similarity between an orbit and a vortex (AKA whirpool). Stuff closer to the center has to rotate about the center faster to remain in orbit. Stuff which is farther out than appropriate for its speed will slide inwards.

But that’s where the similarity ends, because true vorticity requires friction. And friction is the other big thing that’s different between space and down here. Absent friction, once a chunk of stuff is going the right speed in the right direction for its altitude above the central object, it’ll keep orbiting essentially forever unless some new influence pops up to change things.

The specific chunks we see orbiting in our solar system today are the survivors that ended up at the right speed in the right direction at the right altitude. Over the eons there’s been plenty of colliding, splitting, and coalescing as the chunks jostle for position. The more settled things get, the less jostling there is. The current set-up pretty much completely settled and is going to be stable as-is for millions of years unless something from outside the solar system happens along to disturb it.

17

Perfectly circular orbits are rare just because perfect anything is rare. But orbits that are really, really close to circular are quite common, for a variety of reasons: Most of the messy real-world things that perturb orbits tend to do so by making them get more circular with time.

18

Has someone said “Play Kerbal Space Program” yet?
Play Kerbal Space Program.

19

Sure, but this still doesn’t add to the tangential component of the satellite’s velocity vector. The confusion is that as we increase the perpendicular component, then the magnitude of the satellite’s velocity vector increases, but none of this increase is in the tangential component. That’s the issue here, this tangential component wasn’t fast enough to keep the satellite in orbit, no amount of perpendicular force will stop the satellite from crashing to the ground. There’s no stable orbit below that doesn’t require a tangential force being applied.

Thank you for the clarification of the exact physics involved.

LSLGuy - well said …