带食而出的天光变化

作为“夏商周断代工程．天再旦”研究的一部分，讨论带食而出的天光变化．用照相机测光的方法测定正常日出前后的天光变化规律，建立带食而出的天光视亮度变化计算方法，并给出计算“天再旦”现象的范围和强度的方法．为此于1997年3月9日在新疆组织了群众性的日食观测．实测证实带食而出的确可以引起“天再旦”现象，同时与理论计算有很好的符合.

SKY BRIGHTNESS OF HORIZONTAL SOLAR ECLIPSES

A "double dawn" record of 3000 years ago is one of the key items of the "Xia-Shang-Zhou Chronology Project". This ancient record is thought to be a horizontal solar eclipse, although puzzles still exist. A synthetic study of the sky brightness during horizontal solar eclipses is an important measure for solution of the problem.Firstly, we discussed a camera photometer system and its relation to optical luminance standard because we need such a simple system in a mass observation of a solar eclipse. We determined the regulation of the sky luminance in the normal process of dawn shown by Fig. 1 and formulae 3 and 4. Therefore, we can compute the visual sky brightness during horizontal solar eclipses in various situations (seeFig. 2).For this purpose we organized a mass observation at 18 sites in Xinjiang, i.e. the west end of the March 9, 1997 solar eclipse path. About 60 persons sent us 35reports. At several positions in the north of Xinjiang, the "double dawn" deeply impressed the observers when the shaded sun was hidden by mountains or clouds.Observations and measurements show that our computation is well consistent with actual phenomena.We have defined the descent of visual brightness in 10 minutes as the intensity of "double dawn" and so .we can draw the isointensity curves (Fig.3), which depend on eclipse magnitude and solar altitued. Therefore, we may judge whether a "double dawn" will take place at a certain place when a solar eclipse appears.