View Full Version : Question on calculating orbits...

Zavatar

2005-Feb-11, 11:33 AM

Actually, two questions :P

Question One:

I was wondering, how exactly do I calculate an orbit for a planet ( or any other body for that matter ) ? I know I have to have some information on the position and speed of the body at the instant I want to calculate from, but how do I use that information? I've tried Google, but I can't find anything useful... or maybe I'm just to dumb to figure out the things I do find...

Question Two:

I know that astronomers calculate orbits by looking at an object, finding it's speed and position and working from there. My question is how do they find the object's speed and position in the first place? I know they take CCD images and use them to monitor the movement of the object, but how can they calculate that from just a few specks on a computer screen?

If anybody could help me out I'd be very thankful :D

badprof

2005-Feb-11, 03:29 PM

Hi Zavatar,

Calculating an orbit is actually quite complex. What is needed is a mimimum of 3 images taken on different nights (or some other suitable interval). From these three images, the position of the object is obtained. The motion of the object is a combination of the movement of the Earth and the actual movement of the object. The motion of the Earth is well known so it can be removed from the apparent motion of the object, leaving its actual motion.

From these three positions, a definitive orbit can be derived via some fairly complex maths, using a proceedure first developed by Gauss.

Modern computers can do this in a fraction of a second. (thank goodness!!!! :D )

Hope this helps a little.

Regards,

Maurice

Kelfazin

2005-Feb-11, 04:57 PM

This (http://www.gettysburg.edu/academics/physics/clea/A101ol15.html) website has good info some basic orbital mechanics.

v) You can use Newton's laws to calculate the shape of a body's orbit and its speed as it goes around. Gravitational law lets you calculate the force on the body, which in turn lets you calculate how its velocity and position change with time. So given an initial position and speed, you can trace the future motion of the body. This is, of course, how NASA plans the orbits of satellites and space probes, but you don't need to be a rocket scientist to calculate orbits.

peter eldergill

2005-Feb-12, 09:26 PM

How did Kepler determine that the orbits were ellipses? He didn't have Newton's method or anything. Orbital mechanics have always been a mystery to me

I've survived a week so far of teaching grade 9 science. Can't wait till the astronomy unit!

Later

Pete

Normandy6644

2005-Feb-12, 10:04 PM

How did Kepler determine that the orbits were ellipses? He didn't have Newton's method or anything. Orbital mechanics have always been a mystery to me

I've survived a week so far of teaching grade 9 science. Can't wait till the astronomy unit!

Later

Pete

Kepler I believe just analyzed the data he got from Tycho Brahe. It took Newton to explain why they were ellipses.

Maddad

2005-Feb-12, 10:05 PM

Kepler had years of Tyco Brahe's observational data to work with which he inherited after Brahe's death. He'd try to match orbits with circles and then ovals before he tried elipses. They worked; the others didn't. Because he was Brahe's assistant, he knew the zeal the man had for precision observation, so he trusted the numbers that he inherited and kept trying to find a simple model to make them work. His persistence paid off.

peter eldergill

2005-Feb-13, 02:27 AM

Sorry, I wasn't clear...I don't understand how Kepler analysed the data to even compare the data to a circle. How do you translate postition in the sky into a 2-d equation of an ellipse (or circle, for that matter

I do know that he had tons of info from Brahe (who wasn't a very nice person as I understand). I saw this on an old show called "Connections" or the similar one "The Day the Universe" changed. These shows were on the learning channel years ago when that station actually had some quality programming

Pete

Zavatar

2005-Feb-14, 12:43 PM

Thanks for all the answers folks :D

I know I'm probably going over my head here, but does anyone know a link to someplace where they lay out the math in full for calculating an orbit?

TravisM

2005-Feb-14, 01:06 PM

PM Me and I will send you the source code of a program I wrote about a year ago, it's a point simulation of newtonian gravity, i used basic trig without anything too fancy. It's probably wrong, but you can see the loop in the program where it adjusts the acceleration of every point vs. every other point... Truely neat, and fun to make too.

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