Eclipses

Solar eclipses of the type we now experience started to happen about 150 million years ago. About 150 million years into the future, these will cease and there will always be a band of photospheric light visible, blocking out the light from the chromosphere and corone which are so characteristic of present-day eclipses. The present-day eclipses will occur during about 2% of our planet's life.

The maximum number of solar and lunar eclipses possible in one year is 7.

Eclipse can only occur at, or near, to the nodes. These nodes are where the path of the Moon's intersects that of the Sun's. An eclipse can occur whenever the Sun is within a 30-37° area of the ecliptic centered on a node. This variation in the number of degrees is due to the variation in size of the Moon and Sun (i.e. whether these bodies are at aphelion or perihelion or somewhere in between at the relevant time). During the time, it is inevitable that the Moon will 'touch' the Sun at least once at each node, so there will be a minimum of two solar eclipses per year (we are not restricting ourselves to total eclipses in this discussion). Depending on alignment, the Sun and Moon could 'touch' twice at every 'nodal' period, giving a maximum of four solar eclipses per year.

These nodes move slowly westwards (regression of the nodes) because of the Sun's gravitational pull.

Lunar Eclipses

When a lunar eclipse is seen it is visible from a complete hemisphere of the Earth (unlike a solar eclipse). It can be either a total eclipse, when the Moon is in the Earth's umbra, or a partial eclipse, when the Moon is in the Earth's penumbra.

Totality can last for up to 1 hour 44 minutes.

By virtue of the bending of some Sunlight thru the Earth's atmosphere, the Moon is always visible during a lunar eclipse. Because absorption is stronger in the blue, the Moon will tend to take on a reddish color.

full phase during eclipse

used by Greeks to deduce that Earth was a sphere., mentioned by Aristotle (300s Bc). (by 200BC, they had a fairly accurate measure of the size of this sphere, see Eratosthenes).