Celestia Forums

I was curious how accurate Celestia displays the 4 Galilean moons of Jupiter and the jovian longitude (red spot as a mark).

As reference I took XEphem, whose code I know very well. It has very high scientific precision as to astronomical events.

Specific example:

Date/location: 01/18/02; 00:00:00 UT; Hamburg (53 N,9 E).

Results:

The position of the 4 moons is close to perfect!

As to the position of the red spot, Celestia is /3 hours/ late with the default jupiter.jpg map in the medres folder. With jupiter.jpg.shrox it is 2.5 hours /early/. After mapping the red spot to the corresponding jupiter map in XEphem, Celestia is still 2 hours late.

Nevertheless, an encouraging precision for this "young" program! (X)Ephem is far more than 10 years old and still being improved...

Bye Fridger

The orbits of the Galilean moons are implemented with enough accuracy that shadow transits and disk crossings occur at the right times. The same is true for the major satellites of Saturn and Uranus. Eventually, I'll add accurate orbital calculations for Phobos, Deimos, Triton, and Charon as well.

Remember that the Great Red Spot is an atmospheric feature of Jupiter. Jupiter's cloud bands exhibit differential rotation, so it's difficult to pin down a single rotation rate for the planet. I believe the value that I'm using now is actually the rotation rate of the magnetic field, which generally considered the 'true' rotation rate for a gas giant planet.

In any case, if you want the Great Red Spot to appear in the right planet (at least over the short term), you should edit solarsys.ssc add a RotationOffset for Jupiter. This sets the location of the prime meridian (which in Celestia is the vertical line down the center of the planet texture.) To adjust for an error of 3 hours, you should set the rotation offset to:

360 degrees * 3 hours / R

where R is the rotation period of Jupiter, 9.92425 hours.

Also remember that Celestia does not model light time delay. Jupiter is on average roughly 40 light minutes from Earth, so in Celestia events in the Jovian system will occur about 40 minutes ahead of ephemeris times.

--Chris

chris wrote:In any case, if you want the Great Red Spot to appear in the right planet (at least over the short term), you should edit solarsys.ssc add a RotationOffset for Jupiter. This sets the location of the prime meridian (which in Celestia is the vertical line down the center of the planet texture.) To adjust for an error of 3 hours, you should set the rotation offset to:

360 degrees * 3 hours / R

where R is the rotation period of Jupiter, 9.92425 hours.

Also remember that Celestia does not model light time delay. Jupiter is on average roughly 40 light minutes from Earth, so in Celestia events in the Jovian system will occur about 40 minutes ahead of ephemeris times.

--Chris

Thanks, I had shifted jupiter.jpg by an analogous amount. Solarsys.ssc is of course the right place, and it works, too. Being used to earth-based precision software, I had forgotten that the light time delay might not be included;-). Now the accuracy of my comparison is even better.

Bye Fridger

Unfortunately, Celestia (at least the latest MacOS X version, 1.2.2) is not accurate enough to simulate the mutual events of the Galilean satellites (the shadow of one moon passing over the surface of another moon). That's too bad, because those events are very cool to watch (it is possible to find pseudo-events of this kind in Celestia's virtual universe; they just don't correspond to real events).

- Hank