Jul 27, 2010

Refraction at the horizon hugging eclipse

A photo taken during totality at 21:50:47 UT was matched with a computed horizon for the Balcon de Calafate observation site. The center of the eclipsed sun is determined to be at an elevation of 1.25 deg, zenit distance z=88.75 deg. This value is by 0.36 deg larger than the elevation uncorrected for refraction and slightly smaller in comparison with the Besselian mean refraction table.

Line of sight towards Cerro Murallon

Xavier Jubier’s local circumstances tool predicts h=1.0 deg, or better yet with the 5MCSE web tool
Xavier gave the hint: "Worth to mention also is that when you hover the mouse, leave it a
few seconds, over the eclipse altitude, then the value with refraction is displayed. It gives 1.4 degree, but that is with average weather conditions, not the winter ones you had in Patagonia."

The calculator at eclipse-chasers gives a result of h=1.0 deg too for the height of the sun without refraction for mid-eclipse at 21:49:49UT as does Starry Night Backyard .

The USNO online calculator states in 'Notes on the data"
"The altitude is corrected for refraction assuming standard atmospheric conditions.",
the output is again 1.0 deg however, the value without refraction.

The observed refraction angle is somewhat smaller than predicted from the Besselian table
(mean refraction at 0 deg Celsius, 760mm pressure):
z-obs/deg | R
88 | 19’ 07”
89 | 25’36”
90 | 36’ 38”

Glenn Schneider uses this refraction chart as shown in his 1986 eclipse report.
This gives at the altitude of 979 m a correction of 23' for an elevation of 1.0 deg, resp. 24' for an elevation of 0.9 deg.
The time of the picture is between mid-eclipse and third contact, with an uncorrected height of 55.6' = 0.89 deg. This yields an observed refraction angle of 21.6' which is a little smaller than expected from the graph given by Schneider.

A second photo at 21:50:21, closer to mid-totality will be analyzed shortly.

The digital elevation model (DEM) was obtained from Jonathan Ferranti’s site
and is essentially the latest SRTM data ver 2.1 with voids filled. Visualization was done with Microdem
“MICRODEM is a microcomputer mapping program written by Professor Peter Guth of the Oceanography Department, U.S. Naval Academy. It requires a 32 bit version of Windows (NT/2000/XP or 95/98/ME) and is freeware.”

No comments: