Celestia Forums

For those people who are always complaining that the Sun isn't at the origin in Celestia, this is for you . . .

Well, the Sun still won't be at the origin--the origin is the solar system barycenter, which usually lies outside of the Sun. There's a new custom orbit for the Sun relative to the Solar System barycenter.

I didn't yet make the modifications to nearstars.stc that will set the
solar system barycenter as the origin. Here's are the definitions you should use for the Sun in nearstars.stc:

Barycenter "Solar System Barycenter:SSB"
{
RA 0
Dec 0
Distance 0
}

0 "Sol:Sun"
{
OrbitBarycenter "Solar System Barycenter"
CustomOrbit "vsop87-sun"

SpectralType "G2V"
AbsMag 4.83

RotationPeriod 609.12 # 25.38 days
Obliquity 7.25 # correct orientation relative to ecliptic
EquatorAscendingNode 75.77 #
RotationOffset 23.00 # standard meridian
}

HHUUURRRRRAAAA !

Was about time ! Finally, accuracy won !

Now old scripts creators, do your homework ! Hehehe !


Question : the planets are orbiting what ? The Sun, or the barycenter ! Hehehe !

Sounds good to me.

How was the previous origin set for Celestia, out of curiosity. Was is something completely arbitrary?

chris wrote:For those people who are always complaining that the Sun isn't at the origin in Celestia, this is for you . . .

Well, the Sun still won't be at the origin--the origin is the solar system barycenter, which usually lies outside of the Sun. There's a new custom orbit for the Sun relative to the Solar System barycenter.

I didn't yet make the modifications to nearstars.stc that will set the
solar system barycenter as the origin. Here's are the definitions you should use for the Sun in nearstars.stc:

Barycenter "Solar System Barycenter:SSB"
{
RA 0
Dec 0
Distance 0
}

0 "Sol:Sun"
{
OrbitBarycenter "Solar System Barycenter"
CustomOrbit "vsop87-sun"

SpectralType "G2V"
AbsMag 4.83

RotationPeriod 609.12 # 25.38 days
Obliquity 7.25 # correct orientation relative to ecliptic
EquatorAscendingNode 75.77 #
RotationOffset 23.00 # standard meridian
}

Solar System Barycenter <= 206.27AU Distance => sol

Malenfant wrote:Sounds good to me.

How was the previous origin set for Celestia, out of curiosity. Was is something completely arbitrary?

Originally positions of stars had to be specified with by right ascension, declination, and parallax. Since distance is proportional to 1/parallax, positioning something at the origin requires an infinite parallax. I just worked around this by giving the Sun a very large parallax--one that put it near the origin, but not exactly on it.

--Chris

Eh ! What a "hack" that was !

Cham wrote:HHUUURRRRRAAAA !

Was about time ! Finally, accuracy won !

Now old scripts creators, do your homework ! Hehehe !

Most scripts should still work fine . . . However, cel: URLs will need updating unless I implement some backward compatibility fix.

Question : the planets are orbiting what ? The Sun, or the barycenter !

The VSOP87 custom orbits for the planets are in heliocentric coordinates, so everything still orbits the Sun. But, it's possible to redefine them in barycentric coordinates if you want to use barycentric xyz or SPICE orbits.

I think the impact of the Sun not being at the origin was very much exaggerated. As far as I know, there were no real bugs attributed to the Sun being somewhere other than the origin. It would very slightly affect the positions of stars, but I don't believe any bugs were reported against this.

The main thing I like about this change is that I can observer the Sun move relative to the solar system barycenter. The dominant influence of Jupiter is clear: the center of the Sun is always displaced from the barycenter in roughly the direction of Jupiter. The displacement from the barycenter is noticeably larger when Jupiter and Saturn are aligned in the same direction. It will be a neat demonstration in educationally oriented Celestia activities.

--Chris

chris wrote:the center of the Sun is always displaced from the barycenter in roughly the direction of Jupiter.--Chris

You mean in the **opposite** direction of Jupiter ?

chris wrote:

Cham wrote:HHUUURRRRRAAAA !

Was about time ! Finally, accuracy won !

Now old scripts creators, do your homework ! Hehehe !

Most scripts should still work fine . . . However, cel: URLs will need updating unless I implement some backward compatibility fix.

Is there going to be a simple way to fix the URLs? eg write a small conversion program that adds the relevant numbers to the right place in the url and then outputs the correct one?

I definitely prefer having a non-arbitrary origin, but if it can be done so that all the scripts and celURLs aren't broken then that's even better

For those people who are always complaining that the Sun isn't at the origin in Celestia, this is for you . . .

Well, the Sun still won't be at the origin--the origin is the solar system barycenter, which usually lies outside of the Sun. There's a new custom orbit for the Sun relative to the Solar System barycenter.

Whell that can be applied to other stars with planets