Fig. 4

a Different situations for the positional relationship between the Sun, the Earth, and the atmosphere; b situation when the solid Earth and the atmosphere partially occult the solar disk. A1 and A2 are the area of the Sun occulted by the atmosphere and solid Earth. Oe is the center of the Earth’s image, and Os is the center of the Sun’s image. H1 and H2 are the upper and lower boundaries for the overlapping part between the Sun’s image and the atmosphere’s image; H2 and H3 are the upper and lower boundaries for the overlapping part between the Sun’s image and the solid Earth’s image