A solar eclipse can only occur during a new Moon, the phase of the Moon when it's positioned directly between the Earth and the Sun. Because the Moon's orbit around the Earth is tilted, not every new Moon results in an eclipse. This NASA animation shows how everything needs to line up just right for an eclipse to occur.
On August 21, 2017, millions of Americans and visitors from around the world witnessed a total solar eclipse in the U.S. This animation from NASA's Goddard Space Flight Center explains the basics, including how to view an eclipse safely and how rare this event was.
This animation from NASA shows the path of totality that swept across the U.S. on August 21, 2017. Anyone who was located within the narrow center shadow witnessed a total solar eclipse. Areas outside the path of totality experienced a partial solar eclipse only.
During a total solar eclipse, anyone located within the narrow center shadow of the Moon, called the umbra, experiences the total phase of the eclipse, or "totality." Within the much wider outer shadow, the penumbra, the Sun is only partially obscured. This NASA animation shows the 2017 eclipse from the Moon's perspective.
In this visualization of the path of the 2017 eclipse, NASA's Goddard Space Flight Center shows that the Moon's shadow isn't a smooth oval, but a polygon with a jagged, irregular edge. This is caused both by the peaks and valleys along the lunar limb as well as the topography along the path of totality on Earth.
This animation from NASA shows the path of totality for the total solar eclipse that occurred on March 9, 2016, when parts of Indonesia and the Federated States of Micronesia were plunged into darkness. People in Hawaii and most parts of Alaska saw a partial solar eclipse.
On September 27, 2015, millions witnessed a supermoon lunar eclipse when the Moon passed into the Earth's shadow during its closest orbital approach. This animation from NASA's Goddard Space Flight Center explains what a supermoon is, how a lunar eclipse occurs, and how rare this combination really is.