Geeky astronomical coordinate question:

Yesterday was the Vernal Equinox. The RASC handbook says it happened
at 23:21 UT, but I wanted to check that in XEphem. I couldn't figure
out how to get it to tell me directly, so I went to the Sky View
and right-clicked on the Sun at different times to find out when
it crossed 0 declination. That happened four hours after the date
the RASC lists, at about 03:15. Most tables list the 23:21 time
as the time of the equinox.

Subsequent research, and running it on the JPL Horizons simulator

seems to indicate this is a difference between astrographic
coordinates (used by xephem) and apparent coordinates (RASC and
other tables). But none of my references are very clear what the
difference is and why they differ so much. The best I've been able
to come up with is that apparent coordinates are corrected for the
motion of the Earth during the light travel (approx 8 minutes)
from the body being observed, while astrographic coordinates are
only corrected for the other body's motion during that time.
But Horizons says both sets of coordinates are corrected for
light travel time.

Anybody know what the difference is, or know a good reference that
explains it?

Also, is there a better way to ask XEphem for the time of the equinox?

...Akkana


------------------------------------

Yahoo! Groups Links

If you're saying to ask Steve, I just sent him an email. However I was hoping not to bother him if someone else might know the answer.

Oddly I can't open the credits window. I get a "Error: No running window found" message.

--- In xephem@..., "ecdowney2002" <ecdowney@> wrote:

The short answer is Stephen L. Moshier via Michael Sternberg.

The short answer is Stephen L. Moshier via Michael Sternberg.

I've been looking at the techniques for the delta t calculation and trying to understand what is going on with the reference data.

The data in the tables don't match the years from the UNSO () where the deltat.c code says the data was taken from.

Just a quick example set from the deltat.c vs. the UNSO data:
deltat.c dt[1740]=12.0
Unso[1740]=13.5

deltat.c dt[1800]=13.7
Unso[1800]=12.6

deltat.c dt[1980]=50.54
Unso[1980]=50.54

Also the UNSO data stops at 1657, yet deltat.c has data from 1620-1656. Any one know where this comes from and why the early years like 1620 is so large (124.0)?

Thanks,
Eric

Hi everyone, i have the original 3 CD set of XEphem 3.7.1 that my
astronomy department bought some while ago and under Ubuntu 10.04, the
sky view mode crashed when right clicking in the sky background, does
anybody experienced this?

I fixed this because i found a precompiled XEphem 3.7.4 .deb file and i
replaced the xephem executable in /usr/bin and that fixed the problem

now in ubuntu 10.10 maverick i have some random rendering problem in the
application menus, File, Data, etc,... the menu list is not rendered
completely

So, i was thinking in compiling the latest version in ubuntu 10.10
anyway, because may be there is more online support for doing this than
for trying to fixe previous version's bugs.

is there any online manual for compiling XEphem in Ubuntu 10.10 for 64
bits? or there is any info about this bugs i mentioned?

Regards,

--
Santiago Roland.-
-------------------------
Los Molinos Observatory
IAU Code 844
Montevideo, Uruguay
-------------------------

As always, thanks for jumping in to help Serge. However, since the original post mentions the CD I think David is referring to the Commercial version. I think your advise is more appropriate to the Free version since most Commercial users never even look at the INSTALL file or the GUI/xephem directory from the source tarball.

So David, assuming the Commercial version, my first suggestion is to read the response to FAQ 3 in the UNIX section of the FAQ. (Go to and click on FAQ in the left column). I think what's going on is the ShareDir ahd/or PrivateDir are messed up. I think with the general information in the FAQ answer you can figure out what's going on.

Note also that the original Mac installation package arranges for your PrivateDir to be ~/Library/XEphem by listing that in ~/.xephemrc and it also creates that directory for you, so it might just be that you need to create that directory once by hand.

As a last resort I agree with Serge you could reinstall.

Let us know how it goes.

Elwood




--- In xephem@..., Serge Montagnac <obs.psr@> wrote:

On 11/28/2010 05:40 AM, daffyddsant wrote:

I used Apple's Migration Assistant to copy files from an old Mac to a new one prior to wiping the old Mac. It brought XEphem over, but the links are messed up. When I type /usr/local/bin/xephem I get an error message saying /Users/dewi/Library/XEphem: No such file or directory . (Both the xephem directory and the xephem executable are where they ought to be.)

Of course it can't find "dewi"--dewi was the home directory on the old computer. Can I edit a file or files to tell XEphem that it's on a new Mac, or do I need to reinstall (bought the disk version)? If I can edit (or if I need to delete something) kindly advise how/what.

Many thanks,

David

Hi David,

What you have to do is to search and check if you have a real
"xephem-3.x.x" directory
somewhere, and if it is made of usual source tree : xephem-3.x.x/GUI/xephem/
if you find it, you have, first to type that as root (administrator) in
a terminal.

# cd /usr/local/bin
# ln -s <path of xephem-3.x.x>/xephem-3.x.x/GUI/xephem/xephem xephem
# exit
$

which create a link to xephem exec file and permits you to type as
normal user

$ xephem

if it runs ... OK you may have to fix your own parameters in your own
.xephem/XEphem file
... read Elwood's xephem-3.x.x/INSTALL file (3a) section carefully.
afterward, if you get other problems, you will have to check if there is
a /usr/local/xephem/
directory in your Mac.
if NOT ... , you just have to read back the INSTALL file to learn how to
create or fix /usr/local/xephem
and copy necessary data in it.

This is faster to do than it seems and will be a good training about
xephem and Unix installation
routines, but if don't succeed ... I think wou will have to reinstall
the package.
Tell me back about ...

Good luck ... ,
Serge.


xephem-3.x.x/GUI/xephem/--
Criticism is something we can avoid easily by saying nothing, doing nothing, and being nothing
-- Aristotle

--------------------------
Serge Montagnac

obs-psr@
--------------------------

On 11/28/2010 05:40 AM, daffyddsant wrote:

I used Apple's Migration Assistant to copy files from an old Mac to a new one prior to wiping the old Mac. It brought XEphem over, but the links are messed up. When I type /usr/local/bin/xephem I get an error message saying /Users/dewi/Library/XEphem: No such file or directory . (Both the xephem directory and the xephem executable are where they ought to be.)

Of course it can't find "dewi"--dewi was the home directory on the old computer. Can I edit a file or files to tell XEphem that it's on a new Mac, or do I need to reinstall (bought the disk version)? If I can edit (or if I need to delete something) kindly advise how/what.

Many thanks,

David

Hi David,

What you have to do is to search and check if you have a real
"xephem-3.x.x" directory
somewhere, and if it is made of usual source tree : xephem-3.x.x/GUI/xephem/
if you find it, you have, first to type that as root (administrator) in
a terminal.

# cd /usr/local/bin
# ln -s <path of xephem-3.x.x>/xephem-3.x.x/GUI/xephem/xephem xephem
# exit
$

which create a link to xephem exec file and permits you to type as
normal user

$ xephem

if it runs ... OK you may have to fix your own parameters in your own
.xephem/XEphem file
... read Elwood's xephem-3.x.x/INSTALL file (3a) section carefully.
afterward, if you get other problems, you will have to check if there is
a /usr/local/xephem/
directory in your Mac.
if NOT ... , you just have to read back the INSTALL file to learn how to
create or fix /usr/local/xephem
and copy necessary data in it.

This is faster to do than it seems and will be a good training about
xephem and Unix installation
routines, but if don't succeed ... I think wou will have to reinstall
the package.
Tell me back about ...

Good luck ... ,
Serge.


xephem-3.x.x/GUI/xephem/--
Criticism is something we can avoid easily by saying nothing, doing nothing, and being nothing
-- Aristotle

--------------------------
Serge Montagnac

obs-psr@...
--------------------------

Ok folks, please try RC11. It should fix all Download issues. Please let
me know if there are any problems.

Elwood