An eclipse occurs when three celestial bodies become aligned. When the Sun, Moon and Earth align such that the moon is in the middle a solar eclipse occurs and when the alignment is such the Earth is between the Sun and Moon a lunar eclipse occurs.
A solar eclipse occurs when the moon moves across the disc of the sun and cast its shadow across the face of the Earth. The inner part of the shadow is called the umbra. None of the sunlight can penetrate the umbra and so to observers from Earth within the umbra the sun appears completely covered by the moon. This is known as a total eclipse. The outer part of the moon’s shadow is called the penumbra. To observers on the Earth on which the penumbra falls the moon appears to partially cover the sun’s disk and this is called a partial solar eclipse.
The distance between the Sun and the Earth is about 390 times the distance between the Earth and the Moon, and the Sun’s diameter is approximately 400 times the Moon’s diameter. Due to this ratio the Sun and the Moon when viewed from the earth appear to be the same size. It is for this reason that during a total solar eclipse the moon can totally block out the light from the Sun.
The animation below shows a solar eclipse.
A lunar eclipse occurs when the Moon passes behind the Earth such that the Earth blocks out the sunlight and casts it shadow over the Moon.
A lunar eclipse only occurs when there is a full Moon. This is because a full Moon only occurs when the moon is directly opposite the Sun. In this instance if the Moon passes through the Earth’s shadow a lunar eclipse takes place.
A total lunar eclipse will be observed when the entire Moon passes through the umbra, the inner region of the shadow where the Earth blocks out all the sunlight reaching the Moon. A partial lunar eclipse is observed when part of the moon passes the umbra shadow. As the Moon moves across the penumbra, the outer shadow where the Earth partially blocks the sunlight, a penumbral lunar eclipse is observed.
A lunar eclipse does not occur at every full Moon because the Moon’s orbit is tipped 5 degrees to the Earth’s orbit around the sun. This means the moon passes either above or below the Earth’s shadow most of the time.