Why Does RA Count Down During Retrograde Motion?

[Dustinsfl]

How can I determine Mars' orbital position in February 2009?

Is there a website that tracks past positions?

By position, I want to know where in its rotation around the Sun it was radians or degrees.

 

[gneill]

Take a look at the JPL Solar System Dynamics web site. You can use the online ephemerides system to find the orbital elements and position for Mars at any time. It may take a bit of reading and understanding of the documentation to determine exactly what you want, but it's definitely worth the effort to learn how to use this tool.

 

[Dustinsfl]

Take a look at the JPL Solar System Dynamics web site. You can use the online ephemerides system to find the orbital elements and position for Mars at any time. It may take a bit of reading and understanding of the documentation to determine exactly what you want, but it's definitely worth the effort to learn how to use this tool.

So I found Mars and it was fine but Vesta and Ceres are weird.

The ephemerides are acting strange for Vesta between July 1-30 2011 and Ceres July 1-30 2015.

Instead of the right ascension (in degrees counting up) it is counting down. Since the asteroids are rotating counter clockwise like us, why is ra counting down?

 

[gneill]

The ephemerides are acting strange for Vesta between july 1-30 2011 and Ceres July 1-30 2015.

Instead of the right ascension (in degrees counting up) it is counting down. Since the asteroids are rotating counter clockwise like us, why is ra counting down?

Presumably that would depend upon the chosen observer location (coordinate system origin). Did you make the observer location Sun centered or solar system barycentered rather than Earth-centered?

 

[Dustinsfl]

How do you make it sun center? I couldn't pick that.

When I pick observer location, I can only pick Earth locations.

 

[gneill]

How do you make it sun center? I couldn't pick that.

When I pick observer location, I can only pick Earth locations.

It's been a long while since I've used the system, so my memory is hazy. But there should be a way to select any solar system object as the observer position. Barring that, use the coordinates of the Sun at the same instant to determine the Sun-Earth vector and do a bit of vector algebra to shift the coordinates.

 

[Dustinsfl]

It's been a long while since I've used the system, so my memory is hazy. But there should be a way to select any solar system object as the observer position. Barring that, use the coordinates of the Sun at the same instant to determine the Sun-Earth vector and do a bit of vector algebra to shift the coordinates.

I was able to figure it out. It wasn't too user friendly at first.

Thanks.

 

[phyzguy]

So I found Mars and it was fine but Vesta and Ceres are weird.

The ephemerides are acting strange for Vesta between July 1-30 2011 and Ceres July 1-30 2015.

Instead of the right ascension (in degrees counting up) it is counting down. Since the asteroids are rotating counter clockwise like us, why is ra counting down?

It's called retrograde motion. All of the outer planets do it when the Earth catches and passes them in their orbits. Here is an explanation.