Friday, May 05, 2006

Twilight Colours during the total Sun Eclipse

A total Sun eclipse occured on 29th March 2006. The weather was very good in all southern Turkey. The sequence photo was taken by Jukka Ruoskanen on a beach close to the town called Side in Turkey. Few minutes before totality high clouds came, and a halo was seen. The halo, of course, vanished with the sunlight and reappeared again after the total phase - the high clouds responsible for the halo can be seen in some of the photos. The sky colours were truly amazing with deep bluish hue towards the zenith and a great "sunset-like" appearence all over the horizon. The other noteworthy points were a significant temperature drop and the peculiar light some minutes before second contact. At that time the shadows were really sharp too.

The second (16 mm) wide-angle picture of the sky during the totality of the 2006 eclipse is taken by Günther Können in Colakli near Side at the south coast of Turkey, straight on the central line. The horizontal field of view is 135 degrees. At the 4 o'clock position fron the overexposed corona, Venus is visible. The limiting magnitude during totality is +3, about the same as during twiligt with the sun 7 degrees below horizon. The light of the sky occurs because of leaking of light via the horizon, from regions where the sun is not completely eclipsed.

8 comments:

Les Cowley said...

Beautiful images! We were also near Side but to the east of the town. The thin clouds were to our west and (thankfully!) no halos were visible. Any high haze was quite thin. The sky at the zenith before second contact was a steely blue-dark grey that somehow made the almost pinpoint remnant of the sun whiter. Pre second contact shadows were noticeably sharper on one side but blurred on the other. The tops of shadows looked slightly reddened but I do not understand the reason.

Les Cowley said...

Laurent Laveder was also near Side and has many eclipse images in his gallery (http://www.photoastronomique.net/)including one of the halo.

Günther Können said...

I saw that halo and even took a picture of it with the 16 mm lens -- just 10 minutes or so before totality.
Another thing: did you know that the 'inky-black' solar disk during totality is just an optical illusion? In fact, the radiance of the 'black disk' is just the same as the sky at -say- 7 degrees away from the sun (hence, limiting maginitude +3). This is good visible on wide angle pictures with the corona overexposed. The 'black disk' emerges because of the contrast in radiance with the corona.

Les Cowley said...

The 'black sun' could even be brighter than the sky a few degrees away because the Moon's disk is lit by earthlight. See http://www.digitalsky.org.uk/solar/tse-20060329/tse.html

Günther Können said...

There is an aspect in Ruoskanen's picture that I fail to understand. At Side, longitude 31E, the sun should have culminated (passing through South) on 10:00 UT. The eclipse lasted from 9:38 till 12:13 UT. So its height over the horizon should have passed through a maximum during the initial partial phase. Why is the solar track all the way descending on the picture?

anjolene__ said...

these pictures are simply breath-taking... (: nice job

Jukka Ruoskanen said...

The sequence photo of this posting and another one in here are genuine sequence photos taken with a fixed camera as multiple exposures into a single piece of slide film. The Sun path appears linearly descending in the photo because of gnomonic mapping function of a rectangular wide-angle lens (20mm lens in this case). To demonstrate this I quickly made a coordinate grid with Matlab and superimposed it with the original photo. The behaviour of the celestial coordinates seen through a wide angle lens can be seen here. The dots of the grid are separated by 5° in az and el directions. The grid also explains the appearence of the Sun path - the path does indeed reach the culmination point as expected, but due to mapping it can not be easily seen by just looking at the photo.

The difference in mapping functions of a rectilinear wide angle lens and a fisheye lens can be seen in the two attached photos of this posting. In the fisheye photo taken by Mr. Konnen the horizon is strongly curved whereas in the sequence photo it is more or less straight. The horizon remains as a straight line when using rectilinear wide angle lenses. In a fisheye projection the horizon gets strongly curved unless it is passing through the centerpoint of the photo.

Günther Können said...

Overcast versus clear-sky eclipses

The 2006 eclipse was my second total solar eclipse, the first being the 1999 one, which I observed under overcast skies in Luxemburg. Comparing the clear-sky and overcast eclipses, I got a distinct impression that two aspects of a total eclipse are actually most spectacular under overcast condition, namely the drop in sky radiance at the start of totality and the return of light after totality. In that sense, an overcast eclipse would indeed be a phenomenon in its own right, although that is rarely mentioned. I never saw quantitative observations of the sky radiance during overcast eclipses in the literature. My question to the audience is if anyone can confirm or contradict my observation.