Total solar eclipses can only happen during a new moon. But not every new moon results in an eclipse.
What Is a New Moon?
New moon is a phase of the moon that occurs each month. When the moon is positioned between Earth and the sun, it is not visible in the evening sky. The side of the moon that is facing Earth is dark and the moon is not reflecting any light toward Earth.
The phase right before new moon is that of a waning crescent, when the moon is seen before sunrise; and the phase after new moon is a waxing crescent, when the moon is regaining its light and is seen as a sliver after sunset.
The phase of new moon is important in numerous religions. It marks the beginning of the month for many calendars. In Islam, the first sighting of the moon after new phase is how they mark the beginning of the holy month of Ramadan.
From the moment of new moon for about 20 hours afterward, it is very difficult to observe the moon. The thin crescent sets too close to the sun and is too thin to reflect much light to be easy to observe.
New Moon Triggers a Total Solar Eclipse
The new moon phase occurs once every month, but an eclipse does not occur every month. The moon orbits the Earth in a plane that is very close to the ecliptic, but not exact. There are two nodes, or points, in which the moon crosses the plane of the ecliptic.
When the moon is in one of these nodes and reaches new phase, a solar eclipse will occur. Earth, the moon, and the sun will all lie in a line. A fancy word for this alignment is "syzygy". The moon briefly passes directly in front of the sun as seen from Earth and the moon's shadow is cast upon the land, blotting out the sun.
When the moon is in the plane with Earth and the sun and is in full phase, a lunar eclipse occurs. At this time, the moon, Earth, and sun is in a line so that the Earth's shadow is cast upon the moon, blocking the light that it reflects from the sun.
The moon and sun appear about the same size in the sky. Although the moon is much smaller, it is much closer. Both solar system bodies take up a half degree in Earth's sky. Solar eclipses are therefore fleeting, as the moon just covers the sun, whereas lunar eclipses last longer, because it involves the nearby Earth casting its much larger shadow onto the moon.
Source: NASA Eclipse Web Site
