Timekeeping on other celestial bodies than Earth I have looked for months for any site capable of calculating solar altitude w.r.t an observer located on the surface of a generic celestial bodies, but with no luck as of now. Even NASA does not provide such information in its Horizons page/software, although technically Horizons contains all the needed data to calculate &quot;local solar time&quot;; indeed, Horizons allows asking for such an output... but currently output is jut &quot;n/a&quot; for bodies other than Earth:
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 Date__(UT)__HR:MN, , , L_Ap_Sid_Time,  L_Ap_SOL_Time,  L_Ap_Hour_Ang,
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$$SOE
 2020-Aug-09 10:51,*,x,          n.a.,           n.a.,           n.a.,
 2020-Aug-09 11:51,*,x,          n.a.,           n.a.,           n.a.,

Url to ask for quantities 7, 34 and 42 (note: only 42 is declared as &quot;Earth surface only&quot;).
https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&amp;COMMAND=%27-85%27&amp;CENTER=%27coord@301%27&amp;OBJ_DATA=%27yes%27&amp;MAKE_EPHEM=%27yes%27&amp;TABLE_TYPE=%27OBSERVER%27&amp;REF_PLANE=%27ECLIPTIC%27&amp;COORD_TYPE=%27GEODETIC%27&amp;SITE_COORD=%2723.4729700,0.67417634,0%27&amp;START_TIME=%272020/8/9%2010:51%27&amp;STOP_TIME=%272020/8/10%2010:51%27&amp;STEP_SIZE=%271h%27&amp;QUANTITIES=%277,34,42%27&amp;FIXED_QUANTITIES=%27Custom%27&amp;REF_SYSTEM=%27J2000%27&amp;OUT_UNITS=%27KM-S%27&amp;VECT_TABLE=%273%27&amp;VECT_CORR=%27NONE%27&amp;CAL_FORMAT=%27CAL%27&amp;ANG_FORMAT=%27HMS%27&amp;APPARENT=%27AIRLESS%27&amp;TIME_TYPE=%27UTC%27&amp;TIME_DIGITS=%27MINUTES%27&amp;RANGE_UNITS=%27AU%27&amp;SUPPRESS_RANGE_RATE=%27no%27&amp;SKIP_DAYLT=%27no%27&amp;EXTRA_PREC=%27yes%27&amp;CSV_FORMAT=%27yes%27&amp;VEC_LABELS=%27yes%27&amp;ELM_LABELS=%27yes%27&amp;TP_TYPE=%27ABSOLUTE%27&amp;R_T_S_ONLY=%27NO%27&amp;CA_TABLE_TYPE=%27STANDARD%27
So I tried to setup my own page ( http://win98.altervista.org/space/exploration/moon/moontime.html ), which makes its calculations starting from &quot;colongitude&quot; and moon terminator longitude, but it completely disregards observer latitude and altitude.
Any suggestion about pages which perform these calculations?

My GUI for NASA Horizons:
http://win98.altervista.org/space/exploration/NHUGUI.html
To specify a location on another body, write &quot;coord@body&quot; in &quot;CENTER&quot; field, without changing &quot;COMMAND&quot; field, which will be ignored, and write the triplet &quot;lon, lat, alt&quot; in field SITE_COORD.
To find predefined sites on a body use *@body , for example *@301 will list Moon landing aites.
The HORIZONS web interface allows for you to change the observer location to other bodies in the solar system.  Select &quot;Observer location [change] and read the instructions carefully.  I was able to generate an ephemeris showing the Sun's position for the Viking lander location by entering in &quot;Viking 1@499&quot;.  Mars is designated by &quot;@499&quot; and it allows you to enter in any Mars surface coords (Mars lat/long) before the @499 to generate the ephemeris.  There is a search function to find the code for nearly anything in the solar system; it will always be @ something.
To determine when the Sun rises and sets on Mars, you'll also need to change the columns returned to include &quot;Apparent AZ/EL&quot; (azimuth and elevation over the horizon).  Also, you'll want to have it run for the accuracy you're looking for, such as every 5 mins or every 1 min, etc. Here is a screenshot using 10 mins for the step time.