What Is This Calculator For?
This calculator allows the user to input any date and the Right Ascension of the object of interest. It then calculates and displays the Greenwich Sidereal Time at midnight GMT, the Hour Angle, and Transit Time across the meridian.
How To Use The Calculator
Input the Year, Month, and Day, as well as the Right Ascension (Epoch J2000) for the object of interest, then click the Compute button. Computed Results are the Greenwich Sidereal Time at midnight GMT, the Hour Angle off the meridian at that time, as well as its Transit Time across the meridian. Sidereal Time corresponds to the hour of Right Ascension on the Meridian. Hour Angle is the number of hours from reaching meridian transit; it is negative if prior to meridian transit, is zero at meridian transit, and is positive if past meridian transit. The final value given is the time of transit across the meridian. The Reset button will set Year, Month, and Day to the current date without affecting the value entered for Right Ascension of the object.
Meridian transit for any sky object is when the object is at its highest in the sky and best to view because the observer is looking through less atmospheric turbulence. The meridian is an imaginary line that runs from pole to pole directly overhead.
Use this calculator for not only computing Transit Times of deep sky objects, but also for the planets, comets, asteroids, and even meteor shower radiants!
This calculator lacks a minor correction for longitude and time zone of the observer. Accuracy is thus limited to +/- 8 minutes depending on how close the observer is to their prime time zone meridian. Ignoring this correction, all values can be interpreted to be the same as local standard time. Add 1 hour for daylight savings time.
The Default Example
The default example computes results for Omega Centauri (NGC5139), which has a RA of 13 hours 26 min 48 sec (13.44667 hours). From the computed results we see that the Sidereal Time on Jan. 1, 2002 at mignight GMT is 6.56696 hours. The Hour Angle of -6.87971 means the object is 6.8 hours away from meridian transit, which will occur at 6:52 GMT