The Sun is about 400 times wider than the Moon and 400 times farther from Earth, causing the two to appear to be almost exactly the same size in our sky. This relationship is also responsible for the phenomenon of the total solar eclipse, an eclipse of the Sun in which the disk of the Moon fully covers that of the Sun, blocking the Sun's light and causing the Moon's shadow to fall across the Earth. A total solar eclipse can be viewed only from a very narrow area on Earth, or zone of totality, where the dark central shadow of the Moon, or umbra, falls. From this perspective one can view the Sun's delicate corona— tendrils of charged gases that surround the Sun but are invisible to the unaided eye in normal daylight. This is also the only time when stars are visible in the day sky. Those viewing the eclipse from where the edges of the Moon's shadow, or penumbra, fall to Earth will see only a partial solar eclipse. The orbits of the Earth around the Sun and of the Moon around the Earth are not perfect circles, causing slight variations in how large the Sun and Moon appear to us and in the length of solar eclipses. The maximum duration of a total solar eclipse when the Earth is farthest from the Sun and the Moon is closest to the Earth is seven and a half minutes.