5.3.9.2  Julian Date 

A full discussion of  "Julian Date" is likewise beyond the scope of this manual.  It is 

interesting to note, however, that "Julian Date" 0,0 was Greenwich Mean Time at noon 

on January 1, 4713 BC.  It is a time scale widely used by astronomers because it is 

unambiguous, employs no leap years, and because a single number represents both date 

and time.  For convenience, Gemini's display of "Julian Date" also includes the day of the 

week for the longitude of Greenwich, England. 

5.3.9.3  Civil Time (Local Civil Time) 

Expressed in 24 hour format, local "Civil Time" is converted to GMT (or UTC) by 

adding or subtracting the time zone (Daylight Saving Time must be accounted for when 

determining the time zone).  For example, New York City in Eastern Standard Time is 5 

time zones earlier than the time in Greenwich England (expressed as GMT or UTC).  

Consequently, 22:05 (10:05 P.M.) local time on May 4 in New York City, is 03:05 UTC 

(GMT) on May 5    the local time in Greenwich England. 

If Gemini's RTC is set with the Hand Controller, UTC must be entered.  In contrast, when 

you set Gemini's time from a PC using a planetarium program, local "Civil Time" must 

be transmitted.  The time zone must therefore be set before setting the time, so that the 

UTC stored in the RTC is correctly calculated. 

5.3.9.4  Local Sidereal (Local Sidereal Time) 

"Local Sidereal" time (LST) at a given location is the right ascension (RA) of the local 

meridian of the celestial sphere, and is related to the hour angle (HA) of the telescope's 

RA setting by the equation HA=LST RA.  It is useful for positioning the telescope, and 

for planning an observation sequence at or near the meridian.  For example, suppose you 

had a list of objects and wanted to observe the objects within 2 hours of the meridian.  If 

the LST were 13:24, you would look for objects with RA coordinates between 11h 24m 

and 15h 24m. 

5.3.10 

Setup 

Hand Controller 

Mount Parameters 

Encoder 

Alarm Timer 

UTC Date/Time 

Credit Info 

Restore Defaults 

Dim Display 

Geogr. Location 

Coordinate Epoch 

Although "Setup" appears as the last menu item, you will often want to begin with it.  All 

parameters entered here are stored in Gemini's battery backed up memory (SRAM) 

together with a checksum.  If the checksum is wrong at the time of powering on, all 

Gemini Users Manual 

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