During a solar eclipse seen from Earth the sun is obscured by the Moon fully or partially. Sun and moon appear to an observer on Earth with approximately the same apparent diameter, and so the moon’s disk can cover the solar disk completely. During a total solar eclipse the on Earth falling umbra track of the moon is a maximum of a few hundred kilometers wide. The moonīs penumbra measures thousands of kilometers, so that from more than one quarter of the earth’s surface a partial eclipse of the sun ca be observed.
During a solar eclipse the sun, moon and earth have to be in a line. Because the moon’s orbit is inclined to the ecliptic plane, it does not occur every time at new moon, but only when the moon then is also close to one of the two intersection points of the lunar orbit and the ecliptic plane. Until the sun passes one of these two lunar nodes again, it takes about 173 days.
During a total eclipse the apparent diameter of the moon covers the sun completely. Sometimes the moon disk is relative to the solar disk too small, so that the sun remains visible around the moon. Then it is called an annular eclipse. A partial eclipse means, that from the observation point on Earth not the umbra but the half-shadow of the moon is seen.