[vsnet-chat 5160] Re: About courios observation, big confusion

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

Re: About courios observation, big confusion

> I Have verified that the correct position seem to be the first. On my images
> at second position seem visible a faint star with a magnitude fainter then
> 12.5.

> fainter then  magMin 10.5 p of  HYAFH and brigther then 12.2 of GSC4900:329.
> If need I will send the images.
> 
> On the other hand, on images, the star appear of orange coulor and
> comparison with the near star
> GSC 4900:477 9.8/0.4 and GSC 4900:491 10.3/0.4, the different magnitude is
> well visible.
> 
> What is the truth?

   Toni, you may wish to learn the astronomical concept of "equinox", i.e.
the earth has a changing rotation pole projected on the sky.  The equinox
used in the diskette files was 1950.0 (well I hear some people murmur
about the difference of "B" and "J" ...).  Most of current programs and
star charts use the 2000.0 equinox.  This makes the difference.
[ Please also note "equinox" and "epoch" is different.  I have frequently
seen people trying to convert Hipparcos catalogue (epoch 1991.25) to 2000.0
positions assuming that the equinox is 1991.25! ]

   Even considering this, the Megaster position is still different from
the actual position of the star by several tens of arcseconds.  This is
presumably from an inacurrate original position.  Please note the GCVS
has a colon (:) after the position, which indicates that there remains
an uncertainty of the position.  In such cases, you should be more
conservative in actually identifying the star, unless you have an independent
source of accurate identifications.

Regards,
Taichi Kato

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