If we want the Local Sideral Time, that tells us from how many hours the
Vernal Point is passed at our meridian and what is the Right Ascension
of a
star that is at the meridian itself, we may use the following
formula, that
derives from that of Anselmo:
Local ST = Time_of_the_clock + TZ - Longitude/15 + 2*NrMonth + 4.64
hours
The value 4.64 instead of 4.5 from a better precision.
The TZ and the Longitude ar positive if West.
Yes, you're right, but I think it's simpler to remember the local
corrections for your place, as follows:
I know I am at 4.44 deg W, that is, +20 min from Greenwich, so the last
term in my formula must be
around 5 hours instead of 4.5. I also have to remember that here
UT=LegalTime -1 in winter and LegalTime - 2, so
for my place
Local_ST = LegalTime + 2*NrMonth + 3 or 4 hours
Everybody can make the same calculations for their local longitude.
If we know the Local Apparenty Time ( the time of a sundial) we have :
Local ST = Apparent_Local_Time + TEq_min/60 + 2*NrMonth + 4.64 hours
These formulas can be useful, for instance, to draw on a sundial the
lines
with constant Sideral Time or the line that shaws on the dial when a
star
or a constellation will pass at the meridian.
For example : the line that tells us that in 12 hours the Vernal point
will pass at the meridian or that tells that in 8 hours we will have at
South the constellation of Scorpio; etc.
Oh, yes. For instance, you can draw the line for ST = 6h45m which is the
Right Ascension for Sirius,
the brightest star on the sky (after the Sun). If you look at the
(Southern) meridian when the nodus is on
that line you'll be able to see Sirius (even at daytime!) provided that
you have a clear sky and good sight
(or a pair of binoculars instead).
Best regards,
Anselmo Perez Serrada
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