Xi Scorpii missing from stars.dat

[ajtribick]

Xi Scorpii (HIP 78727, HD 144070) is referenced in starnames.dat but is not present in the newest version of the stars.dat file, nor in versions going back at least to 1.12 (earlier than that I have not checked).

[granthutchison]

No parallax is provided for 78727 in the Hipparcos catalogue, hence the omission from Celestia.

Grant

[ajtribick]

Theta1 Orionis D (HIP 26224) is also missing, I checked, there is a parallax given.

[granthutchison]

Theta1 Orionis D (HIP 26224) is also missing, I checked, there is a parallax given.

But it's nonsensical (-1.85): a negative parallax doesn't determine a real-world distance.

Grant

[granthutchison]

By the way - I've added Xi Sco to stars.dat by hand, using the known distance to Xi Sco D (HD 144087, Hip 78738), a proper-motion companion that's far enough from the ABC conglomerate to yield a clean parallax.

Grant

[Bob Hegwood]

But it's nonsensical (-1.85): a negative parallax doesn't determine a real-world distance.

Mr. Hutchison or Mr. Syndrome,

At the risk of exposing my Brain-Deadedness again I wonder if one of you might
give an explanation of what a "parallax" is? I looked in the nine planets glossary
and didn't see the term. Could I trouble you for a quick (English) explanation?

Just curious as usual...

Thanks, Brain-Dead Bob.

[granthutchison]

OK.
Bob, hold your finger up a foot-and-a-half or so from your face. Close one eye, and look past your finger towards the far side of the room. Now, still watching the finger and the far side of the room, switch eyes (close your open eye and open your closed eye). See how the position of your finger seems to shift relative to the distant objects across the room? That's parallax, and it happens because of the distance between your eyes - you're getting two slightly different views of the same nearby object (your finger). If you knew how far apart your eyes were, and could measure the apparent shift in your finger's position accurately, then you could work out the distance from your eyes to your finger (bit of trigonometry involved).
Now scale up ... as the Earth moves from one side of the Sun to the other during its orbit, we can detect nearby stars shifting their apparent position relative to the distant background stars - it's as if your head is the width of Earth's orbit, your finger is the nearby star, the far wall of your room is composed of distant galactic star-clouds, and you waited six months between closing one eye and opening the other. Since we know the width of the Earth's orbit, and can measure the angular shift in the nearby star's apparent position (its parallax), we can work out its distance from us.
The angular shift is tiny - usually measured in fractions of an arc second (1/3600 degree), meaning that even the nearby stars are pretty far away. The distance of one parsec (3.26 light years) gets its name because stars at that distance show a parallax of one arc second.
The Hipparcos catalogue lists measured parallaxes for a large number of stars, and we convert that to a distance in lightyears for Celestia. But sometimes the automated systems in the Hipparcos satellite were baffled (often by multiple stars lying close to each other) and returned either no results or negative results. You can't do anything with such nonsense information, so the stars were left out of stars.dat.

Grant

[Bob Hegwood]

Hey, thanks *very* much there Dr. Hutchison.

I understood that explanation.

I know I'm a pain, but I really *am* just trying to understand some of this
terminology. Celestia is much more enjoyable when you can relate to what others
are trying to do with an effect, or when you can understand something that's
being discussed on the forum.

Again, thanks very much.

Take care, Bob

[ajtribick]

Theta1 Orionis D (HIP 26224) is also missing, I checked, there is a parallax given.

But it's nonsensical (-1.85): a negative parallax doesn't determine a real-world distance.

Grant

Using the Hipparcos Query Form at http://archive.ast.cam.ac.uk/cgi-bin/wdb/hipp/hipparcos/query, it seems that the components of Theta1 Orionis which are present in stars.dat are also reported to have negative parallax, only Theta2 Orionis has a positive parallax.

[granthutchison]

That's interesting. Theta1 Ori A and C seem to have been "saved" by their flagged association with Theta2 Ori - Celestia ensures that the components of multiples flagged in Hipparcos all end up with the same parallax, and Theta1 Ori A and C have acquired Theta2 Ori's parallax. But it's not clear why Theta1 Ori D wasn't picked up in the same way.
Looks like starnames.dat requires a little tweak in this region, since Theta1 Ori A has also been assigned the name Theta Ori B, which belongs to a non-Hipparcos star with a different HD cat number.

Grant

[ajtribick]

Ok, thanks for the info...

I take it you mean the program you are using to generate "stars.dat", not Celestia itself, that groups the multiple stars.

I'm currently using the new database (with Xi Scorpii) with 1.3.1 (I don't visit C stars all that often, although the results are slightly weird... )

[granthutchison]

I take it you mean the program you are using to generate "stars.dat", not Celestia itself, that groups the multiple stars.

Yes, sorry, that's right - stars.dat is an extract of Hipparcos generated by Chris, specifically for Celestia. (My role has been to make a few subsequent modifications to that extract, when dodgy data are encountered by Celestia users.)

Grant

[ajtribick]

A suggestion, in starnames.dat, I have added/changed these lines:

Code: Select all

78727:XI Sco A
78738:XI Sco D:Struve 1999 A
78739:XI Sco E:Struve 1999 B

...to highlight the association of these stars.