formula for RA and declin of real sun, from geog coords and time?

Hi,

I wondered whether someone had a formula to hand for calculating the RA
and declination of the real sun, given the geographical coordinates of a
terrestrial observer and the time (UT or Julian date).

I need one that is accurate to about 3 decimal places of a degree.

Many thanks in advance for any help with this.

Regards,

Neil
--
Neil Fernandez

In article ,
Neil Fernandez wrote:

I wondered whether someone had a formula to hand for calculating the RA
and declination of the real sun, given the geographical coordinates of a
terrestrial observer and the time (UT or Julian date).

I need one that is accurate to about 3 decimal places of a degree.

You'll find it he

http://www.stjarnhimlen.se/comp/ppcomp.html

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/

In article , Paul Schlyter
writes

In article ,
Neil Fernandez wrote:

I wondered whether someone had a formula to hand for calculating the RA
and declination of the real sun, given the geographical coordinates of a
terrestrial observer and the time (UT or Julian date).

I need one that is accurate to about 3 decimal places of a degree.

You'll find it he

http://www.stjarnhimlen.se/comp/ppcomp.html

Many thanks!

Best regards,

Neil
--
Neil Fernandez

Neil Fernandez wrote:

Hi,

I wondered whether someone had a formula to hand for calculating the RA
and declination of the real sun, given the geographical coordinates of a
terrestrial observer and the time (UT or Julian date).

I need one that is accurate to about 3 decimal places of a degree.

Many thanks in advance for any help with this.

Regards,

Neil
--
Neil Fernandez

See: http://www.edu-observatory.org/eo/algorithms.html

Neil Fernandez wrote:

In article , Paul Schlyter
writes

In article ,
Neil Fernandez wrote:

I wondered whether someone had a formula to hand for calculating the RA
and declination of the real sun, given the geographical coordinates of a
terrestrial observer and the time (UT or Julian date).

I need one that is accurate to about 3 decimal places of a degree.

You'll find it he

http://www.stjarnhimlen.se/comp/ppcomp.html

Many thanks!

And good it is!! I implemented the sun position and rise/set times in
APL2. I compared a years results with a New Zealander who was using
data from a site that was supposed to be millisecond accuracy (US Naval
Observatory??). My data and his had a maximum difference over the year
of six seconds. A plot convinced us that the discrepancy was due to
higher order effects rather than any fault in the algorithm. BTW, APL
uses double precision floating point so is working to about fifteen and
a half places.

Ted

In article , Barry Schwarz
writes

On Sat, 1 May 2004 18:00:17 +0100, Neil Fernandez
wrote:

Hi,

I wondered whether someone had a formula to hand for calculating the RA
and declination of the real sun, given the geographical coordinates of a
terrestrial observer and the time (UT or Julian date).

You do realize that the RA and declination do not depend on the
location of the observer. For solar system objects, you need only the
date and time. For stellar objects, they are "constant" if you
discount proper motion.

Did you perhaps mean altitude and azimuth?

Yes, I realised after posting that I did mean altitude and azimuth. (I
got out one of my old astronomy textbooks to find the names for what I
wanted, but scanned it over-hastily).

Neil
--
Neil Fernandez

In article ,
Barry Schwarz wrote:

You do realize that the RA and declination do not depend on the
location of the observer. For solar system objects, you need only
the date and time.

You have a phenomenon called parallax, which for the Moon can shift
its position as seen from a particular place up to a bit more than
one full degree. For those few asteroids passing closer than the
Moon, the parallax is even larger. For Mars and Venus, when closest,
the parallax is about one full minute of arc. And for the Sun it's
almost 9 seconds of arc. The accuracy demands of your application
will determine whether this is something which can be ignored or
something which has to be accounted for.


For stellar objects, they are "constant" if you discount proper motion.

The proper motion of most stars is quite small and can often be
ignored over time scales of a human lifetime. But that doesn't mean
the RA and Decl of stars can be considered constant: due to
precession it changes by some 50 arc seconds per year for ALL stars.
This is considerably more than proper motion (which is some 10 arc
seconds per year for Barnard's Star, which is the star with the
largest known proper motion). Precession is the RA/Decl coordinate
system itself moving, and it must be accounted for for all objects.

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/

In article ,
Barry Schwarz wrote:

On Sun, 02 May 2004 08:44:17 GMT, (Paul Schlyter)
wrote:

In article ,
Barry Schwarz wrote:

You do realize that the RA and declination do not depend on the
location of the observer. For solar system objects, you need only
the date and time.

You have a phenomenon called parallax, which for the Moon can shift
its position as seen from a particular place up to a bit more than
one full degree. For those few asteroids passing closer than the
Moon, the parallax is even larger. For Mars and Venus, when closest,
the parallax is about one full minute of arc. And for the Sun it's
almost 9 seconds of arc. The accuracy demands of your application
will determine whether this is something which can be ignored or
something which has to be accounted for.

Thank you for reminding me. My crude approximation for the moon came
up with 55' which is close enough to your 60+.

When the Moon is closest to the Earth, the lunar parallax will be
somewhat larger than one degree. The mean lunar parallax is somewhat
smaller than a degree though.

But for the sun I came up with 17" which is almost double yours. I
guess I'll have to find some time to look more closely and see where
my error is.

You probably took the Earth's diameter instead of its radius and
divided it by one AU. The solar parallax is a fundamental
astronomical constant and its value is slightly smaller than 8.80 arc
seconds, and it tells you (when the Sun is at its average distance
from us) the maximum difference between the topocentric (= local)
position and the geocentric (= as if viewed by an imaginary observer
at the center of the Earth) position.

When computing the lunar parallax though, you appear to have taken
the Earth's radius and dividing it by the Moon's mean distance, as
one usually does (this yields the parallax in radians - multiply
by 3600*180/pi to convert to arc seconds).

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/

Paul Schlyter, I'd like to thank you for your excellent site.

Personally, I have no interest in canned calculators or on line offers
to do the calculations. I have taken the algorithms that you supplied
and implemented some of them in APL2. This allows me to use them off
line, in OS/2 and gain some understanding at the same time.

Thanks.

Ted

In article ,
(Paul Schlyter) writes:
You have a phenomenon called parallax, which for the Moon can shift
its position as seen from a particular place up to a bit more than
one full degree.

It's actually quite hard to point a large telescope at the Moon. The
blankety-blank thing isn't where the ephemeris coordinates say it
should be (because the published coordinates are geocentric), and
once you find the Moon, it keeps moving out of the field!

Paul might also have mentioned aberration, which shifts coordinates
by up to 20 arcsec or so.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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