About Astronomic Direction Calculations

You can use the Astronomic Direction Calculator to calculate astronomic directions from Sun Shot and Star Shot observations.

In the Astronomic Direction Calculator you can specify the calculation type as either Sun Shot or Star Shot. A Sun Shot calculates the astronomic direction from solar observations by the hour angle method and uses a multiple foresight procedure. A Star Shot calculates the astronomic direction from Polaris or star observations by the hour angle method and uses a single foresight procedure.

Each method requires a source of UT1 time (radio) and a stopwatch to measure the observations accurately. Set the stopwatch to zero (0) at UT1 time. Start the stopwatch at a known interval of UT1 time and write down the value of UT1 time (adding any necessary DUT corrections). When measuring the horizontal angle to the sun or star, record the stopwatch reading (stop time) in the field book along with the angular data.

Single Foresight

This procedure is recommended for Polaris observations, or any sightings that have a large vertical angle inherent in the observation. The single foresight procedure uses the following sequence: direct on the target, direct on Polaris, reverse on target, reverse on Polaris. A stopwatch time is recorded for each pointing on the Polaris. The two stopwatch times and the four horizontal circle readings correspond to one set of data. It is recommended that you perform a minimum of three sets.

Multiple Foresight

This procedure is used as a solar observation method. It uses the following sequence: direct on the target, multiple direct on the sun, multiple reverse on the sun (same number of times as direct), reverse on the target. A stopwatch time is recorded for each pointing on the sun. Pair up each observation on the sun (and its stop time) with its related target observation, and enter it into the editor as a single direct or reverse pair. The editor computes a whole circle bearing (or quadrant bearing) for each pair. You can average any number of these whole circle bearings. Do not average the multiple observations themselves because they are time-dependent and not a linear function of time.

Tutorial Exercise: Calculating a Whole Circle Bearing in the Astronomic Direction Calculator