Novice astro question: LST

Hi,

Given only longitude and UT of a particular site, how do I obtain local
sidereal time (LST)?

Thanks,

Dennis

"dennis" wrote in message
...
Hi,

Given only longitude and UT of a particular site, how do I obtain local
sidereal time (LST)?


1. Calculate LST (Greenwich) for 0h UT on the date in question. There is a
standard formula for this, given in the Astronomical Almanac. (I am unable
to look this up for you at the moment).

2. For the UT, calculate the number of ST hours, minutes and seconds
elapsed. There is a standard conversion factor for this, but again I can't
give you the exact factors at the moment.

3. Add this ST interval to LST (Greenwich, 0h UT) of step 1. This gives LST
at Greenwich meridian at the UT in question.

4. Convert longitude E or W in degrees, arcminutes, arcseconds, into
longitude in hours, minutes, seconds of time.

5. For W longitudes, subtract result (4) from result (3). For E longitudes,
add. Take result modulo 24 hours. That's your LST (if you haven't made any
mistakes...)

Have fun. I imagine those formulae are given somewhere on the WWW.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

Hi,

Thanks for the reply. Some one suggested the following equations. I had
thought it would be easy to figure out and spent quite a bit of time
attempting to, finally having to post to the newsgroup and got the solution.
thanks.

LST = 100.46 + 0.985647 * d + long + 15*UT

d is the days from J2000, including the fraction of
a day
UT is the universal time in decimal hours
long is your longitude in decimal degrees, East positive.

Add or subtract multiples of 360 to bring LST in range 0 to 360
degrees.
and this formula gives your local siderial time in degrees. You can divide
by 15 to get your local siderial time in hours, but often we leave the
figure in degrees. The approximation is within 0.3 seconds of time for dates
within 100 years of J2000.


"Mike Dworetsky" wrote in message
...


"dennis" wrote in message
...
Hi,

Given only longitude and UT of a particular site, how do I obtain local
sidereal time (LST)?


1. Calculate LST (Greenwich) for 0h UT on the date in question. There is
a
standard formula for this, given in the Astronomical Almanac. (I am
unable
to look this up for you at the moment).

2. For the UT, calculate the number of ST hours, minutes and seconds
elapsed. There is a standard conversion factor for this, but again I
can't
give you the exact factors at the moment.

3. Add this ST interval to LST (Greenwich, 0h UT) of step 1. This gives
LST
at Greenwich meridian at the UT in question.

4. Convert longitude E or W in degrees, arcminutes, arcseconds, into
longitude in hours, minutes, seconds of time.

5. For W longitudes, subtract result (4) from result (3). For E
longitudes,
add. Take result modulo 24 hours. That's your LST (if you haven't made
any
mistakes...)

Have fun. I imagine those formulae are given somewhere on the WWW.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

D
From: "dennis"
D
Subject: Novice astro question: LST
D
Date: Fri, 15 Aug 2003 08:11:25 -0700
D
D Hi,
D
D Given only longitude and UT of a particular site, how do I obtain local
D sidereal time (LST)?
D
D Thanks,
D
D Dennis

With ONLY these two factors, you CAN NOT find your local sidereal
time. Add the date, and you CAN find LST. The UT date gives the
right ascension of the Sun. The hour gives the hour anglle of the Sun.
The sum of Sun's RA and A is the LST at Greenwich. Adding your
longitude (east being plus) to this gives the LST at your location.
You better draw a sketch to get the nagles right way round.

---
þ RoseReader 2.52á P005004


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