Technical data
Chapter 3 Time 14
Finally, the Local Sidereal Time is shown. This is the Local Apparent Sidereal Time for the current location.
Sidereal time is the Right Ascension of any object that is directly overhead (Zenith) at that instance. Or to look at it
another way, Sidereal Time is the time that has elapsed since the Vernal Equinox (or first point of Aries) has passed
overhead. The sidereal time must always be known when attempting to locate objects in the sky based only on their
given coordinates.
Below the time displays, the current Gregorian date is shown along with the current location and its coordinates. These
are shown merely for reference.
The Delta T value is displayed for reference. This value is used when converting from Universal time (UT) to
Barycentric Dynamical Time (TDB). If you have set the current time and selected the Set Dynamical Time option,
this value will be zero (0.0), otherwise it displays the number of seconds added to UT to produce TDB.
The final values displayed are the Julian Date (JD) and the Julian Ephemeris Day (JDE). The Julian date is the
number of days, and fractional days, since January 1, 4713 B.C., at 12:00 Universal Time. The Julian date is the
current date on the Prime Meridian which runs through Greenwich, England and is therefore a function of the
Universal Time (UT1). JDE is the Julian date corrected by Delta T. This value is used in all of the Ephemeris and
Sidereal calculations.
Set Time & Date
Setting the current time and date can be accomplished in two different manners. You may enter either the standard
(Gregorian) time and date values or you may enter the full Julian date. Either method is equally valid. Once you
specify a time and date, the internal Autostar Suite clock is frozen at your desired settings. The internal clock runs
only when you are using the system clock (either from the PC or the observatory). To start the system clock, select
either Gregorian or Julian from the Set Time & Date menu and push the Use System Time & Date button.
The entered value may also be used as the current Dynamical Time by selecting the Set As Dynamical Time button.
Gregorian
The Gregorian time and date format is the format that is normally used throughout the world. It was established at
noon on October 4, 1582 by Pope Gregory. At that instance of time, the current Julian date was 2299160.0. The very
next instance became October 15, 1582 and the Julian date became 2299160.5. This was done to correct for errors that
had accumulated, since the previous calendar did not include leap years. Pope Gregory's new calendar included an
extra day every four years. This is the calendar we still use today.
Julian
The Julian calendar is merely a sequential count of the number of days since 1 January, 4713 B.C. (which is written 1
January, -4712). This value is useful when calculating the Ephemeris data (the changing positions of the planets,
comets, etc.). Most technical almanacs, such as the Astronomical Almanac, exclusively use the Julian date when
specifying a particular time and date.
Set Timezone
The Set Timezone function allows you to change the value for the timezone of the current location. The Daylight
Savings feature can also be enabled or disabled from this dialog box. You should always set the timezone value
correctly, then select Daylight Savings as appropriate.
Set Location
Selecting the Set Location menu item displays a dialog box which allows you to pick an Observatory or City location
from which you will be observing. You may either choose from a list of predefined locations or you may enter your