Hi:
It turns out that because of "seeing" you can not determine meridian
crossing to better than about 5 arc seconds (5 * 55 ms of time). (The
exception is for locations at high elevations or in space.) I've heard
that modern CCD cameras can image sections of the sky and fit the image
to a very good star catalog thus averaging the seeing of hundreds of
stars and get orders of magnitude better time/space resolution.
http://www.prc68.com/I/StellarTime.shtml
Have Fun,
Brooke Clarke
http://www.PRC68.com
Kevin Karney wrote:
Brent
If you measure the transit of a star (or any celestial body) through the
meridian again and again over many years with telescopes that can distinguish
less than 1 second of arc, then it's perfectly possible. Remember 1 second of
time = 15 seconds of arc. These measurements - now done automatically - have
been done for years by using transit telescopes to take photographs of the
stars and then by taking measurements off the photo plates.
Even without sophisticated instruments, surprising accuracies were obtained...
Malik Shah the grandson of Toghril Beg, the founder of the Seljuk dynasty ruled
the city of Isfahan from 1073 AD. His vizier Nizam-ul-Mulk invited Omar Khayyam
to Isfahan, to set up an observatory. Other leading astronomers were also
invited to work at the observatory and for 18 years Omar Khayyam led the
scientists and produced work of outstanding quality. It was a period of peace
during which the political situation allowed Khayyam the opportunity to devote
himself entirely to his scholarly work. During this time Khayyam led work on
compiling astronomical tables and he also contributed to calendar reform in
1079. Khayyam measured the length of the year as 365.24219858156 days, we know
now that the length of the year is changing in the sixth decimal place over a
person's lifetime. It is also outstandingly accurate. For comparison the length
of the year at the end of the 19th century was 365.242196 days, while today it
is 365.242190 days.
Omar Khayyam was a real polymath - a notable astronomer, mathematical and poet.
He famously wrote ....
A Book of Verses underneath the Bough,
A Jug of Wine, a Loaf of
Beside me singing in the Wilderness --
Oh, Wilderness were Paradise enow!
and (especially for us gnomonists)
For in and out, above, about, below,
’Tis nothing but a magic shadow-show,
Play’d in a box whose candle is the Sun,
Round which we phantom Figures come and go
Best regards
Kevin Karney
Freedom Cottage, Llandogo, Monmouth NP25 4TP, Wales, UK
51° 44' N 2° 41' W Zone 0
+ 44 1594 530 595
On 26 Mar 2011, at 13:57, Brent wrote:
It's amazing that someone was able to calculate these
numbers out to 6 decimals. Is that done by some type of
observation or is it mathematics?
How could you possibly measure something like that?
On 3/25/2011 1:14 PM, Kevin Karney wrote:
Nothing is constant in the heavens !
The 'tropical' year (from equinox to equinox) is 365.242190 days
The 'sidereal' year (fixed star to fixed star) is 365.256363 days
The 'anomalistic' year (perihelion to perihelion) is 365.259636 days - cycling
over a period of some 21000 years
(values for 2009 from Astronomical Almanac)
But these are mean values having averaged out the effects of nutation (the
wobbling of the Earth's axis) and various other effects.
---------------------------------------------------
https://lists.uni-koeln.de/mailman/listinfo/sundial
---------------------------------------------------
https://lists.uni-koeln.de/mailman/listinfo/sundial
---------------------------------------------------
https://lists.uni-koeln.de/mailman/listinfo/sundial
