Home > BRG Sidereal Clocks
Model: SD625Brand: BRG Precision
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High quality 2.5" 6 digit sidereal clock by BRG Precision. One of the leadest manufacturers of sidereal and other precision clocks. Also comes with the mains power supply.
All BRG Series time displays are capable of displaying Universal and Local Sidereal Time. Time zone displays may be configured to display Sidereal Time on one of the zones, or a single display may be configured to alternate between multiple time zones, including Sidereal Time. The Sidereal Time displayed is calculated from the clock's internal UTC Time. Therefore, if the clock obtains its' time from an atomic clock (using GPS, Ethernet, IRIG-B, PC, or other methods), then the Sidereal Time displayed will be highly accurate and will not deviate over time. The addition of the optional GPS Atomic Time Receiver allows the display to obtain the local latitude and longitude for use in the internal calculations. This option is highly recommended. Manual input of the required information involves a set of control buttons on the back of the unit with a numeric menu system.
These clocks come in either 2.5" or 4 LEDs in a plastic enclosure with a a TCXO Oscillator and auto-brightness included. Options for the single position displays will be –OCXO, -EN, -AFS, and –AFS-GP. The last option will include auto-brightness as well.
What is Sidereal Time?
The time most people use in their everyday life is Solar Time. The fundamental unit of Solar Time is a Day; the time i takes the Sun to travel 360 degrees around the sky, due to the rotation of the Earth. However the Earth doesn't actually spin around 360 degrees in one Solar Day. The Earth is in orbit around the Sun, and over the course of one day, it moves about one Degree along it's orbit (360 degrees/265.25 Days for a full orbit = about one Degree per Day). So, in 24 hours, the direction toward the Sun changes by about a Degree. Therefore, the Earth has to spin 361 degrees to make the Sun look like it has traveled360 degrees around the sky.
Astronomers are concerned with how long it takes the Earth to spin with respect to the "fixed" stars, not the Sun. So, they need a time scale that removes the complication of Earth's orbit around the Sun, and just focuses on how long it takes the Earth to spin 360 degrees with respect to the stars. This rotational period is called a Sidereal Day. On average, it is 4 minutes shorter than a Solar Day, because of the extra 1 degree the Earth spins in a Solar Day.
Side Time (literally means "star time") is useful for determining where the stars are any given time. Sidereal Time divides one full spin of the Earth into 24 Sidereal Hours; similarly, the map of the sky is divided into 24 Hours of Right Ascension. This no coincidence; Local Sidereal Time (LST) indicates the Right Ascension on the sky that is currently crossing the Local Meridian. So, if a star has a Right Ascension of 05h 32m 24s, it will be on your meridian at LST=05:32:24. More generally, the difference between an object's RA and the Local Sidereal Time tells you how far from the Meridian the object is. For Example, the same object at LST=06;32;24 (one Sidereal Hour later), will be one Hour of Right Ascension west of your meridian, which is 15 degrees.
Therefore, Local Sidereal Time is the most useful form of Sidereal Time since it gives the right ascension of a transiting celestial object at a given location. Of course, BRG offers clocks that display both Universal and Local Sidereal Time.
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