The Sidereal Clock Pro displays local and Greenwich sidereal times and several other time
All the displayed times are computed from the Sidereal Clock's own timebase which is synchronized
to an external NTP
time server, a networked precision time reference. You may expect a precision within +/- 200ms
or better if you have a stable data connection.
The local times, sidereal and solar, are also computed from your current geographic longitude,
obtained by GPS or, if not available, by Network Location.
To switch between the different times, touch and hold any of the time
displays until a menu pops up. Then choose the desired time standard for this display.
Touch the Hold button at the bottom of the screen to stop all the time
displays. Touch it again to resume. Could be handy to write down the exact time of an event.
Select Menu/Longitude to see the geographic longitude currently in
use, or Menu/Settings to set the longitude manually.
Select Menu/Analog Clock to switch to the 24 hours Analog Clock.
Touch and hold the analog clock to switch between Local Mean
Sidereal Time and Local Apparent Solar Time.
Select Menu/Settings and uncheck Use NTP to disable the the
NTP timebase. When the NTP timebase is disabled, or when the NTP server is unreachable, all
the times are computed from your device's internal clock, which is continuously updated by
- Local Standard Time
The local standard time is your watch time, which is defined for your time-zone.
- Local Mean Solar Time
While the standard time is the same in any place inside your time zone, the solar time is computed for your particular geographic longitude. This is your true local time.
- Local Apparent Solar Time
The length of a solar day is actually not constant. It varies throughout the year, and the effect of these variations produces seasonal deviations of up to 16 minutes from the mean solar day, which is exactly 24 hours. The apparent solar time scale takes in account these variations,
applying the daily deviations (known as equation of time) to the solar mean time, to get the true local position of the Sun. This is the time you would read on a sundial.
- Local Mean Sidereal Time
The sidereal time is based on the apparent motion of the distant stars rather than the Sun. One sidereal day is approximately 23 hours, 56 minutes, 4.091 seconds.
By definition, the right ascension of any object currently crossing the local meridian is equal to the current local sidereal time, and that's why it is widely used by astronomers.
Coordinated Universal Time
UTC is the universal time reference. It is the mean solar time at the Royal Observatory in Greenwich (GMT) and consequently the standard time at time-zone zero (GMT+0).
- Greenwich Mean Sidereal Time
The sidereal time at longitude zero, which corresponds to the Royal Observatory in Greenwich.
- Julian Day
The julian day, or julian date, is the interval of time in days and fractions of a day since January 1, 4713 BC Greenwich noon.
Julian day number is the integral part of the Julian Day.
- Modified Julian Day
The Modified Julian Date is found by rounding downward the Julian Date, by subtracting the value 2400000.5. The MJD was introduced by the Smithsonian Astrophysical Observatory in 1957 to record the orbit of Sputnik via an IBM 704 (36-bit machine) and using only 18 bits until August 7, 2576.
(see the Alternatives box)
- Equation of Time
The equation of time is the difference between apparent solar time and mean solar time for each day of the year. It varies between (approx) -14m06s and +16m30s through the year.
The mean sidereal time analog clock
- A 24 hours analog clock that shows the local mean sidereal time.
- Tip: Facing north, hold the clock in front of your eyes so that the clock axis points
to the celestial north pole (the place in the sky where the Polaris star is) and then
turn the clock until its hour hand points up (to the local meridian) and the shadowed W-E
markers are paralel to the horizon. Now, the clock surface is paralel to the equatorial plane and
all clock hour markers are pointing to the respective RA meridians.
If you are in the southern hemisphere, point the clock axis north and below the horizon in an angle
equal to your latitude.
The apparent solar time analog clock
- A 24 hours, zero hour down analog clock that shows the local apparent solar time.
- Tip: Facing south, hold the clock in front of your eyes so that the clock axis points
to the celestial south pole and the 12 hour marker points up (the shadowed E-W markers are
paralel to the horizon).
Now, the clock surface is paralel to the equatorial plane and the clock's hour hand should
be pointing to the meridian that contains the Sun. Remember that the Sun may be up to 23.5
degrees above or below the equator, depending on the current date.
If you are in the northern hemisphere, point the clock axis south and below the horizon in an angle
equal to your latitude.