Lunar Crater Ephemerides with Horizons (using web interface http://ssd.jpl.nasa.gov/horizons.cgi)

[Dr. Diane Wooden]

Here is how to convert lunar coordinates to Right Ascension,
Declination on the sky, given the UT time and date, and location of
the observer.

Link to JPL Horizons ephemeris generator http://ssd.jpl.nasa.gov/?horizons
Link to JPL Horizons ephemeris generator web interface
http://ssd.jpl.nasa.gov/horizons.cgi

Suggested Inputs are:
Ephemeris Type: OBSERVER
Target Body: g:83.1,-87.5,0@301 (LCROSS "sweet
spot" in Faustini Crater: -87.5 (S), 83.1 E 83.1,-87.5)
Observer Location: (Select your city/observatory from list or Search)
Time Span: Start=2008-12-06 04:00 UT
End= 2008-12-06 10:00 UT
Step=5 min
Table Settings = 1,3,8,10 (click table quantities and hit "Use
Selected Settings")
Display/Output = default or plain text or download/save

Note: The NASA Infrared Telescope Facility (IRTF) uses Astrometric
Coordinates [Table Settings 1].
Amateurs may need to use Apparent Coordinates [Table Settings 2]

Note2: g:83.1,-87.5,-2.76@301 (may more accurate because it
includes the altitude)
The coordinates wrt IAU Moon ellipsoid at the S pole (1737.4 km
radius).

Note3: Mauna Kea [568] Observer Location for NASA IRTF

Description of lunar coordinate input parameters is taken from
http://ssd.jpl.nasa.gov/?horizons_tutorial :
"To select a point on the surface of a target body (limited to those
with an IAU rotational model), use this form (units are degrees and
km):
{g: E.Long, latitude, h@}BODY [geodetic/planetographic coords]
{c: E.Long, DXY, DZ@}BODY [cylindrical coordinates]
For example: "g: 348.8, -43.3, 0 @ 301" specifies the crater Tycho on
the Moon (body 301), at geodetic (planetographic) coordinates 348.8
degrees east longitude, -43.3 degrees latitude (south), and zero km
altitude with respect to the IAU reference triaxial ellipsoid."

From the description at the bottom of the generated ephemeris:
"TARGET POINT "NEAR-SIDE" AND ILLUMINATION FLAGS
Coordinates on the target body's surface were specified, so the
solar/lunar
presence markers (observing location) are followed by a blank and then
two
additional symbols relating to the target surface location.

The first symbol indicates if the target surface location is on the
side of the target facing the observer:

'N' Surface location on the target body is on the near-side,
facing
TOWARD the observer. Visible, however, ONLY when target
location
is also above the horizon at the observing site.
'-' Surface location on the target body is on the far-side,
facing AWAY FROM the observer. NOT visible.

The second symbol indicates if the target surface location is
illuminated
by any portion of the Sun's extended disk:

'L' Surface location on target body faces Sun (is lit)
'-' Surface location on target body does NOT face Sun (is NOT
lit)"
Interpretation of the above Information:...NL indicates Faustini is on
the near-side and is lit by the Sun, whereas N- means the Sun is not
lighting the crater, and -L means the Sun is lighting the crater rim
but the libration puts the crater out of the Earth's field of view.

[Jim Mosher]

Thanks so much for taking the time to write this up, Dr. Wooden!

I have used JPL Horizons for several years but never realized you
could specify a particular point of interest on a target body using
the web interface -- I thought that to do that you had to use "telnet"
interface (which has never worked for me), or the e-mail interface
(which I find cumbersome). I see that with slightly different inputs
you can also use the web interface to ask for the position of the Sun
as seen from Faustini, and so get its altitude and azimuth as seen at
the crater center.

In truth, given the simple spherical coordinate system, the observed
positions and lighting of individual features on the Moon are
completely constrained by, and, with a little spherical trigonometry,
can be computed from, the Sub-Observer, Sub-Solar and North-Polar
positions available in the "normal" Horizons output (and the printed
Astronomical Almanac, for that matter), but it's good to see the
calculations carried out explicitly.

As you suspected, most amateurs will, I think, find it more practical
to point by offsets from nearby observable surface features (as
opposed to using RA-Dec setting circles), but with a professional
instrument it may well be different.

One small correction: at least with the web interface it's not legal
to specify "UT" in both the start and end times. I think the "UT"
notation needs to be omitted from the end time; and probably it isn't
necessary on the start time either, since UTC is assumed by default.

-- Jim

P.S.: Faustini could be described as being between Shackleton and
Amundsen in the USGS/IAU nomenclature map:

http://planetarynames.wr.usgs.gov/images/Lunar/lac_144.pdf

Given the uncertainties in selenographic coordinates from different
sources is it possible to describe where the LCROSS "sweet spot" is
located in geographic terms? I.e., is it at the center of Faustini?
or is it offset from the center?

If offset from the crater center, by what fraction of the crater
radius is it offset and in what direction (towards Amundsen, towards
Nobile, away from Nobile, etc.)?

Likewise is it possible to describe the locations of targets "Crater
A" and "Crater F" in relation to the named features depicted on:

http://planetarynames.wr.usgs.gov/images/Lunar/lac_1.pdf ?

Thanks!