UT1 is based on the actual rotation of the earth. It is obtained by adding the DUT correction to UTC. The DUT correction is given by faint double ticks following the minute tone. For each double tick heard during the first seven seconds after the minute, an additional 0.1 seconds is added to UTC time heard at the minute tone to obtain UT1. If double ticks are heard between nine seconds and fifteen seconds, then an additonal 0.1 seconds must be subtracted from the UTC time heard at the minute tone. Thus UT1 is computed as
UT1 = UTC + DUT
Using the UT1 time and the appropriate date, the GHA and declination for the beginning of the day of the observation and the following day are obtained from ephemeris tables. The actual GHA and declination at the time of the observation are interpolated from the tables using the equations.