Planetarium programs adjusted for apparent position?

[Blue Fire]

Here's a possibly dumb question that occurred to me recently:

Regarding the apparent versus actual positions of objects in planetarium programs (or goto controls for scopes for that matter), are the views they display on your screen variously adjusted to the "apparent" position of all the various objects like comets and planets? For example, a telescope view of the positions of Jupiter's moons (relative to Jupiter itself) may be noticeably different than the actual positions of the moons since it takes close to an hour for light to get to us from Jupiter.

If it is the case that apparent positions are calculated according to the travel time of light from the source, it seems like an awful lot of extra programming and calcs for the software.

Or, am I just nitpicking here? Or don't you think the differences between apparent and actual positions are significant? Obviously, objects like nebulae and galaxies and stars would be a different problem. It seems to me that the display of those objects would have to be based on apparent position to begin with with no calc from actual position since it might be difficult to determine the "actual" position due to uncertainty of the exact motion of these objects which may be, for example, a thousand or more light years away.

Anybody know for sure how these programs handle these issues? Call me Curious under the skies.

[Darryl]

If it is the case that apparent positions are calculated according to the travel time of light from the source, it seems like an awful lot of extra programming and calcs for the software.

Very interesting thought. I've never given that consideration before. I would say it must be calculated based on the time it takes for the light to travel here. I say this because the moons are normally in the right place when comparing the display on the planetarium and the views through a scope.

Or, am I just nitpicking here?

Maybe , but it's a very interesting thought and its got me thinking.

Or don't you think the differences between apparent and actual positions are significant?

It is significant if you are viewing the moons of the planets and objects in our solar system.

Anybody know for sure how these programs handle these issues? Call me Curious under the skies.

Not a clue. I am sure curious now.

[Blue Fire]

Thanks for the reply, Darryl! It's not like it's an earthshaking issue, but I must admit that the curiosity factor is starting to get the better of me. I guess I could always try an email or two to some planetarium publishers. I have to agree that the accuracy of the apparent positions of objects in our solar system like Jupiter's moons is right on. I just wonder if the software is based on apparent positions to begin with rather than having to make calculations from the actual positions first. It's kind of an itchy thing for me and I think I'm gonna have to start scratching soon.

[dmill1220]

HI Blue fire,

You have got my attention on this one also!

I have always gone by the assumption that its apparent positions that are usually given and not actual positions.

Let us know what you find.

I'm getting itchy too!

Dennis

[Blue Fire]

Well, so far, I have gotten a response from the folks that make Planitarium Gold version 7. And the tech support guy said:

Honestly, I do not have that details. We need to ask programmer who has been adjusting this software over 10 years

I don't know if they actually will ask the programmer and get back to me or not.... But I'm not giving up yet. More emails to go out yet. And some posts to other forums.

Stay tuned!

[Darryl]

I am staying tuned. It's one of those things that's just plain interesting to know.

Regards,

[Blue Fire]

Here's a reply I got to my inquiry on the BAUT forum from boom stick:

In the solar system simulating programs I've seen, planets, the moon, comets, asteroids and earth are stored as their orbital data. This is then used to calculate their actual positions at a given time and their apparent position in the given location. This way commonly available astronomical data can be entered into the program directly.

To the best of my knowledge stars and deep sky objects are stored as their apparent positions usually on January 1st 2000 (so called J2000 epoch) in the celestial coordinate system. Again, the programmer doesn't have to know anything about stars. He can use a star catalog.

For simpler software the stars' celestial coordinates don't change which is no biggie unless you want to know the exact positions of say the stars in the big dipper in 6000 BC. Fancier software like Starry Night also has the proper motion of stars in store, which it can use to estimate the positions at a given time accurately.

This seems to make sense to me so far.