Click here to find Fred Espenak's eclipse page at the NASA Goddard Space Flight Center.
Solar Eclipses: A solar eclipse occurs when the Moon passes directly in front of the Sun, blocking out its view over a small region on the Earth. Remember that the angular diameters of the Sun and the Moon as viewed from Earth are almost identical.
Lunar Eclipses: A lunar eclipse occurs when the Moon moves through the Earth's shadow, thereby blocking sunlight from falling on the Moon. In order for there to be a total eclipse, the Moon must go through the umbra of the Earth's shadow. If the Moon partly goes through the umbra, there will be a partial eclipse. If the Moon only goes through the penumbra, then the Moon's light is only slightly dimmed, and the eclipse is hardly noticeable here on Earth.
The Moon's orbit is tipped by 5 degrees with respect to the Earth's orbit. Therefore, eclipses do not occur every month. Instead, there are two eclipse seasons every year, about 6 months apart. Eclipses can occur only then the Sun, Moon and Earth line up perfectly, that is, when the line of interesection of the Earth's orbit plane and the Moon's points directly at the Sun. This is called the line of nodes. Movement of the line of nodes causes the eclipse seasons to vary from one year to the next.
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