Celestia Forums

Afternoon Gents,
I've been using Celestia and I notice that it's possible to travel between planets and stars intantaneously - no regard to the speed of light.
Fair enough - but gravity also travels only at the speed of light. This would mean that, supposing we consider two orbital bodies interacting eg: Jupiter and the Sun, which are about 40 light-minutes apart, then Jupiter would be reacting to the position of where the Sun was *40 minutes ago*.

I'm curious as to whether Celestia simulates these effects (I doubt it) and what effect it would have on orbits?

Celestia doesn't do the physics for propagation of the planets, it interpolates an ephemeris. Most ephemerides do include lightspeed gravitational effects.

Whether Celestia calculates the planet positions mathmematically, or using a look-up table, there's still a possible problem.

What if I'm close to Earth at time t, and looing at Jupiter. The light, and Jupiter's gravity, is from a previous time, t-40, ago.

So far so good. Now in Celestia, I move (instantly) to Jupiter's orbit and look back at the Earth. What I'm seeing is the Earth calculated at time t, not t-40. I'm actually seeing the future position of Earth, not how I would actually see Earth from Jupiter at that time.

Does Celestia work like this, thus any non-Earth views are physically incorrect for their given time?

Although it's not ideal, there is a "light time delay" effect available: to toggle it, type a - (minus sign)

Thanks for that - I didn't notice it. However, it only seems to give the rotation of the planets a time lag, not their positions (the latter being very complicated - though it's not going to stop me trying).

I believe it also adjusts planet positions.
View the transit of Venus of 8 Jun 2004 from some convenient place on the Earth - anywhere in Europe should do. Stop time when Venus is crossing the Sun's limb. Toggle light delay on and off and watch Venus shift back and forth between its "simultaneous" position and its apparent position.

Grant

I stand corrected - it does. I tried just what you suggested before making my prior post, and it didn't work before, and now it does! (I have noticed this light lag effect is a bit buggy and sometimes refuses to come on).

Incidently, I was glancing over the code and noticed that the planet positions are not determined by look-up tables - they are actually calculated from Keplar's laws.

In fact, not many (if any) gravitational effects seem to be calculated in Celestia - even spacecraft trajectories are given to it in the form of a table.

Makhno wrote:Incidently, I was glancing over the code and noticed that the planet positions are not determined by look-up tables - they are actually calculated from Keplar's laws.

No, they're calculated from the vsop87 ephemerides - the Keplerian data in solarsys.ssc is just for information, and the real work is done by the CustomOrbit call.
So they're accurate enough to reproduce planet-on-planet occultations.
Many of the major satellites have their own ephemerides calcs, so Celestia does pretty well on mutual events there, too.

Grant

Incidentally, the finite speed of gravity doesn't do to the solar system precisely what you might expect. You might think that the right thing to do is to take a simple Newtonian model, then add in a speed-of-light delay that makes the gravitational force vector point toward the gravitating body's previous position. But if you try that in a dynamical simulation, the whole solar system rapidly becomes unstable. The unmodified Newtonian model without light delays is actually more accurate.

There was a guy named Tom van Flandern who perennially got into arguments on the sci.physics newsgroups by claiming that this meant the speed of gravity was infinite. He was wrong. What it really means is that this modified-Newtonian model is an inadequate way of modeling light-speed delays; what you really want is to use general relativity.

General relativity is a field theory, and in other field theories there are seeming paradoxes similar to these. For instance, in electromagnetism, causal effects only propagate at the speed of light (after all, light is an electromagnetic wave). But if you work out in detail the electric field of a point charge that moves in a straight line at constant velocity, you will find that everywhere it points toward the charge's current position, not its light-speed-delayed position. The field isn't exactly the same as the field of a stationary particle (it is flattened in the direction of motion, and there's a magnetic field too), but adding in a simple light-speed delay to the force vector is the wrong thing to do. The relativistic invariance of the theory means that, in a sense, the fields themselves can travel along with the charge and keep up with it as long as its motion is constant. If the charge changes its velocity, then you'll see the effects of the acceleration propagate outward at the speed of light.

Gravity is a lot like this, only the light-speed delay acts in an even more subtle way because of the self-interaction of the field. Gravitational fields can not only keep up with the straight-line constant motion of a mass, but can also to some extent compensate for accelerations as long as they are free-fall accelerations. The cancellation of the delay effect is not 100% perfect, which is why a binary neutron star will emit gravitational waves at the speed of light. But it does make the waves much weaker than they would otherwise be.