Re: [guide-user] Epoch question & UCAC2

Bill J Gray Feb 5, 2012

Hi David,

> (1) Could someone explain to me the difference between the "mean of date"
> epoch and "apparent" epoch (Settings...Formats...Epoch:)?

"Mean of date" epoch means precession is included, but not nutation and
aberration. "Apparent" means all the effects are included, which makes a
difference of up to about 40 arcseconds.

You may also notice that for certain objects (such as planets), Guide
will give you the J2000 position (a position measured relative to a "fixed",
inertial coordinate system of the sort a physicist would want), a "mean
of date" position (one where the coordinate system moves along with the
overall precession of the earth), and an "apparent" position.

Truthfully, I don't see a lot of utility to "mean" coordinates, except
that they get you pretty close to apparent coordinates without having to do
all the work. For example, I'd be pretty confident that some telescope
systems that aren't good to one arcminute anyway figure that after correcting
for precession, their results are close enough; there's no real need to
continue to computing apparent coordinates.
> (2) How do I get the UCAC2 numbers to show up? IOTA is not yet using UCAC3
> for asteroid occultation predictions, but is still using UCAC2.

Getting UCAC-2 to display isn't trivial, but it's not too bad:

Go into Settings... Toolbar, scroll down about 2/3 of the way through the
list of options, and you'll see a "UCAC-2.0 data via Internet" option.
(Which should be changed to UCAC-2, no .0; I'll fix this.) You can turn
on a toolbar button for this, and/or set a hotkey to perform this function.

After that, use Extras... Toggle User-Added Datasets, and scroll down
to "UCAC-2 data (downloaded from VizieR)". Double-click on it, turn it
On, and check the Labels box.

Go to the area of interest, and click on the toolbar button or hit the
hotkey. Guide will pause a bit while it gets data from VizieR, then show
you the UCAC-2 stars, labelled by UCAC-2 number.

> Also, regarding UCAC2, I have often seen star designations written as 2UCAC.
> I'm assuming these are identically equal to the UCAC2 values.

Yes, as are "2U" identifiers.

-- Bill