On August 21 2017 a total solar eclipse will be visible across North America. Here are links to some of the best resources to explain why a solar eclipse occurs. From the why will flow the where and when.
Anyone would know that, in a solar eclipse, the Moon blocks out the light of the Sun. But the reason it happens is less obvious than may at first appear, and this explains why they are not seen more frequently.
The Moon revolves around the Earth in a plane that is inclined by roughly 5 degrees to the plane in which the Earth revolves around the Sun. This means that, most of the time, the Moon is above or below the direct path between the Earth and the Sun, and cannot block the light. The points where the two planes intersect are called the nodes. An eclipse occurs when the Moon moves through one of the nodes at a time in the calendar when the node is oriented between the Earth and the Sun.
The Moon happens to have roughly the same apparent size as the Sun, seen from Earth, so blocking the light. The magic number is 400. The Sun’s diameter is about 400 times that of the Moon, but it is about 400 times further away. The shadow falls on a swathe of the Earth’s surface, but only obscures the Sun fully from a small part of the path. This means that a total eclipse at any given place is a rare event.
You can see the path for the 2017 eclipse clearly in NASA Eyes on the Eclipse.
NASA’s Eyes is a web and desktop application that shows celestial phenomena interactively. Eyes on the Eclipse is a sub-section for the solar eclipse. You can select different views to see how the eclipse occurs. This one (below) simulates two years of the Earth/Moon/Sun system, showing how the Moon’s shadow lines up periodically with the Sun and Earth.
The exact geographic path of the eclipse on Aug 21 2017 is shown on this NASA interactive map: Total Solar Eclipse Interactive Map.
Another NASA site provides a highly detailed simulation of the appearance of the Moon’s shadow on Earth, based on topographical details of both Earth and Moon: Tracing the 2017 Solar Eclipse.