Convert a POSIXt time to a sidereal time, using the method in Chapter 7 of reference 1. The small correction that he discusses after his equation 7.1 is not applied here.
A sidereal time, in hours in the range from 0 to 24.
Meeus, Jean, 1982. Astronomical formulae for Calculators. Willmann-Bell. Richmond VA, USA. 201 pages
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