Solar and lunar eclipses are typical examples of light propagating along a straight line in the same homogeneous medium. When the moon moves between the sun and the earth, there is not a solar eclipse every time. There are two conditions for a solar eclipse. First, eclipses always occur on the new moon (the first day of the lunar calendar). Not all new days are bound to have a solar eclipse, because the orbit of the moon (the white track) and the orbit of the earth (the ecliptic) are not on the same plane. There is an angle of 5 ° 9 'between the plane of the ecliptic and the plane of the ecliptic. Second, both the sun and the moon move near the intersection of the ecliptic and the ecliptic, and the sun has a certain angle from the intersection (solar eclipse limit).

Since the orbits of the moon and the earth are not round, and the distance between the sun and the moon and the earth is close and far, the shadow formed by the sun's light obscured by the moon can be divided into cost shadow, pseudo umbra (formed when the moon is far from the earth) and penumbra on the earth. The total solar eclipse can be seen when the observer is within the umbra; The annular eclipse can be seen in the range of pseudo umbra; Only partial solar eclipses can be seen in the penumbra.