Many thanks to Brandon for his excellent stewardship and to everyone for their valuable contributions. Long live XEphem!

I have just released XEphem 4.2.0.

The change log:

• Elwood himself has contributed a new option that lets you switch XEphem from giving your telescope J2000 coordinates to giving it equinox-of-date coordinates.
• Sky View: the mouse wheel now zooms the view in and out.
• Sky View: DSS image download has been fixed by upgrading to HTTPS, and the window should no longer awkwardly resize once the image arrives.
• Data ? Download: added the URL of the Celestrak “visual.txt” file.
• Data ? Download: replaced “” with modern HTTPS.
• Data ? Field Stars: Fran?ois Meyer added support for the ucac5 catalog.
• The precession formula has been updated to the one from the 2000 Astronomical Almanac.
• A couple of compilation problems on modern Mac machines have been fixed.
• XEphem now uses more modern SSL setup routines if the code detects it’s being compiled against OpenSSL ≥1.1.
• Closing the INDI window with your window manager’s “Close” button no longer crashes XEphem.
• All references to “” have been changed to their new hostname “”.
• A few other small fixes.

I apologize for allowing so much time to pass since the previous release! Over 2022 there were several email threads that were gradually working on the problem of how to compile XEphem successfully on macOS. Unfortunately, I decided to wait to see if they would reach a perfect answer for all versions of macOS, and wound up waiting far too long — they all petered out without my understanding which versions of macOS needed what. From now on I will instead focus on making more frequent XEphem releases, and as macOS folks report on their various troubles, we can hopefully iterate towards a Makefile that works for everyone.

Now that we have a fresh release to work from, I’ll plan to go back through the open issues and pull requests and make comments and updates where it’s warranted. Enjoy!

Hello,

I have few questions about the settings to compare ephemeris generated by XEphem and almanac data.

Times are calculated for upper limb or sun center crossing the horizon?

Atmosphere pressure: 0 or 1013 hPa? Does it change refraction?

Menu Preferencies > Equatorial > Topocentric or Geocentric?
Other settings to consider?

Best regards

Stefano

Thanks to all have contributed to help me visualize why the moon elongation from the sun is only rarely approaching 0 or 180 degrees.

stefano

The ecliptic is the "mean invariant plane" of the solar system.
The solar system consists of the Sun, Jupiter and other debris!
So the Earth can be slightly above/below the mean plane (timescales
on the order of weeks/months to decent order).

Rather than spherical trig, use vectors. JPL Horizons offers up some
data but bear in mind the polynomials are not that accurate that far
back in time. You may have to play with a few of the observing
parameters. There are calendrical things to consider as well, like the
pesky 15 day leap between Julian date and Gregorian date not to
mention those leap years etc. Not to mention the 8 minutes or so of
light travel time. You will have to break that long a time period
up into segments. The problem with trig, on a computer, is the
Needle Angle problem. ?


So, if you take the Sun (not barycenter) vector to Earth and
cross that with Sun->Moon and look for the minimum point, then
you get a pretty decent estimate of the phase angle. The value
will not be zero, but close.

Here is full moon, on the order of 1.5 day resolution. Ephererides
every 1 day, so the resolution is pretty off. This is from the observer
table, vectors are avaliable as well if you want to try the maths.

?Date__(UT)__HR:MN, , , R.A.__(a-app), DEC_(a-app), ? ? ?Illu%, ? ? S-T-O, ?T-O-M, MN_Illu%,
?1322-Jun-30 07:00, ,m, ? 19 39 42.73, -20 13 31.5, ? 99.50468, ? ?8.0770, 0.0000, ?99.5047,
...
?1322-Jul-14 07:00, , , ? 07 23 24.24, +19 31 24.5, ? ?0.67674, ?170.5572, 0.0000, ? 0.6767,
?1322-Jul-15 07:00, , , ? 08 14 07.82, +16 15 10.1, ? ?0.14569, ?175.6283, 0.0000, ? 0.1457,

Column meaning: ?(Interesting tidbits here -- like for the Calendar.)
JPL is pretty cool for these things.

From the JPL Ephererides page...
?
TIME

? Times PRIOR to 1962 are UT1, a mean-solar time closely related to the
prior but now-deprecated GMT. Times AFTER 1962 are in UTC, the current
civil or "wall-clock" time-scale. UTC is kept within 0.9 seconds of UT1
using integer leap-seconds for 1972 and later years.

? Conversion from the internal Barycentric Dynamical Time (TDB) of solar
system dynamics to the non-uniform civil UT time-scale requested for output
has not been determined for UTC times after the next July or January 1st.
Therefore, the last known leap-second is used as a constant over future
intervals.

? Time tags refer to the UT time-scale conversion from TDB on Earth
regardless of observer location within the solar system, although clock
rates may differ due to the local gravity field and no analog to "UT"
may be defined for that location.

? Any 'b' symbol in the 1st-column denotes a B.C. date. First-column blank
(" ") denotes an A.D. date.
?
CALENDAR SYSTEM

? Mixed calendar mode was active such that calendar dates after AD 1582-Oct-15
(if any) are in the modern Gregorian system. Dates prior to 1582-Oct-5 (if any)
are in the Julian calendar system, which is automatically extended for dates
prior to its adoption on 45-Jan-1 BC.? The Julian calendar is useful for
matching historical dates. The Gregorian calendar more accurately corresponds
to the Earth's orbital motion and seasons. A "Gregorian-only" calendar mode is
available if such physical events are the primary interest.

? NOTE: "n.a." in output means quantity "not available" at the print-time.
?
SOLAR PRESENCE (OBSERVING SITE)
? Time tag is followed by a blank, then a solar-presence symbol:

? ? ? ?'*' ?Daylight (refracted solar upper-limb on or above apparent horizon)
? ? ? ?'C' ?Civil twilight/dawn
? ? ? ?'N' ?Nautical twilight/dawn
? ? ? ?'A' ?Astronomical twilight/dawn
? ? ? ?' ' ?Night OR geocentric ephemeris

LUNAR PRESENCE (OBSERVING SITE)
? The solar-presence symbol is immediately followed by a lunar-presence symbol:

? ? ? ?'m' ?Refracted upper-limb of Moon on or above apparent horizon
? ? ? ?' ' ?Refracted upper-limb of Moon below apparent horizon OR geocentric
? ? ? ? ? ? ephemeris
?
?'R.A.__(a-app), DEC_(a-app),' =
? Airless apparent right ascension and declination of the target center
with respect to an instantaneous reference frame defined by the Earth equator
of-date (z-axis) and meridian containing the Earth equinox of-date (x-axis,
EOP-corrected IAU76/80). Compensated for down-leg light-time delay,
gravitational deflection of light, stellar aberration, precession & nutation.
Note: equinox (RA origin) is offset -53 mas from the of-date frame defined
by the IAU06/00a P & N system.

? Units: RA ?in hours-minutes-seconds of time, ? HH MM SS.ff{ffff}
? ? ? ? ?DEC in degrees-minutes-seconds of arc, sDD MN SC.f{ffff}
?
?'Illu%,' =
? ?Fraction of the target objects' assumed circular disk illuminated by Sun
(phase), as seen by the observer.? Units: PERCENT
?
?'S-T-O,' =
? ?The Sun-Target-Observer angle; the interior vertex angle at target center
formed by a vector from the target to the apparent center of the Sun (at
reflection time on the target) and the apparent vector from target to the
observer at print-time. Slightly different from true PHASE ANGLE (requestable
separately) at the few arcsecond level in that it includes stellar aberration
on the down-leg from target to observer.? Units: DEGREES
?
?'T-O-M, MN_Illu%,' =
? ?Target-Observer-Moon LUNAR ELONGATION angle and illuminated percentage.
The apparent lunar elongation angle between target body center and Moon
center, seen from the observing site, along with fraction of the lunar disk
illuminated by the Sun. A negative lunar elongation angle indicates the target
center is behind the Moon.? Units: DEGREES & PERCENT

Computations by ...

? ? Solar System Dynamics Group, Horizons On-Line Ephemeris System
? ? 4800 Oak Grove Drive, Jet Propulsion Laboratory
? ? Pasadena, CA ?91109 ? USA

? ? General site:
? ? Mailing list:
? ? System news :
? ? User Guide ?:
? ? Connect ? ? : browser ? ? ? ?
? ? ? ? ? ? ? ? ? API ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? command-line ? telnet 6775
? ? ? ? ? ? ? ? ? e-mail/batch ?
? ? ? ? ? ? ? ? ? scripts ? ? ? ?
? ? Author ? ? ?: Jon.D.Giorgini@...

**

My astronomy professor did quite well with a rubber ball and a flashlight, but try Wikipedia.? I wrote a program showing how it works once.