This is probably an off-topic, but I've noticed that many people fond of
gnomonics
and even of astronomy do not know this simple rule of thumb to calculate
the sidereal time
(you know, the celestial meridian which lies just above our heads at a
certain moment):
ST(at Prime Meridian) = UT + 2*NrMonth + 4.5
being NrMonth 1.00 for Jan 1st, 1.50 for Jan 15ht, 3.25 for March 8th,
and so on.
Some old dials (not only sundials) show this kind of time and most of
the people do not know
a rough rule to verify them. Addingly, this rule is handy to orientate
ourselves in the night sky.
Notice that at September-Descending-Libra Equinox ST coincides with UT
(more or less).
Best regards,
Anselmo Perez Serrada
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