Eclipses

An Eclipse, Solar or Lunar, takes place when Sun-Moon-Earth or Sun-Earth-Moon (respectively) align along a straight line or nearly straight line.

In other words, a solar eclipse takes place on the new moon day (Amavasya अमावस्या) or on full moon day (Purnima पौर्णिमा). But not on all new moon or full moon days do we see an eclipse.

Moon’s orbital plane (shown in blue below), is the plane in which the Moon orbits the Earth (shown in red below) and is inclined by about 5 degrees to the orbital plane of Earth. The line of intersection of the planes which pass through the Sun is called the ‘line of nodes’. The point at which the Moon crosses the plane of Earth from below is called Ascending node and the point when it crosses from above or the top is called Descending node. An Eclipse can take place only if Moon is within about 5 deg from the line of nodes.
Rahu (राहू )and Ketu (केतू) of Indian astronomy are nothing but Ascending and Descending nodes respectively.

During the solar eclipse, the shadow of the Moon falls on Earth. The diameter of the Moon is just about one-fourth of Earth its shadow is just about a few hundred kilometer in diameter. Thus a solar eclipse is seen over a very narrow belt over the surface of the Earth.

Whereas the shadow of the Earth falls on Moon or rather it enters the shadow of the Earth. Thus if a lunar eclipse is in progress then from every place where Moon is visible one can see the lunar eclipse.