What is a total solar eclipse?

When does a total solar eclipse occur and why it happens?

by Lorenzo Ciotti
What is a total solar eclipse?
© Scott Olson / Staff Getty Images

The spectacular Great North American Eclipse was visible in a band of shadow that crossed North America and a small part of Central America. The major eclipse lasted 4 minutes and 28 seconds near Nazas, Durango, Mexico.

The path of this eclipse crossed the path of the previous total solar eclipse on August 21, 2017, with the two paths intersecting in southern Illinois, at Makanda, just south of Carbondale.

Eclipse© Handout / Handout Getty Images

The cities of Benton, Carbondale, Chester, Harrisburg, Marion and Metropolis in Illinois, Cape Girardeau, Farmington and Perryville in Missouri, as well as Paducah, Kentucky, will lie within an approximately 9,000-square-mile intersection of the routes of totality of the 2017 and 2024 eclipses.

But what is a solar eclipse?

A solar eclipse occurs when the Moon, whose orbit is inclined by about five degrees with respect to the ecliptic, intersects the latter at a point called the node. If the Moon is located between the Sun and the Earth in the hypothetical intersection of the two planes, then it projects its shadow onto a portion of the Earth's surface, from which, in fact, a solar eclipse will be witnessed.

When the Sun-Earth-Moon are perfectly aligned, the Moon projects a precise and narrow cone of shadow on the Earth's surface of the hemisphere exposed to the Sun along a narrow geographical band of the Earth, while in the surrounding areas the Moon's shadow will be much more extensive and, however, weaker and only partial, in a cone of twilight. This is the generic case of a solar eclipse, which is commonly called a central one.

Eclipse© George Frey / Stringer Getty Images

Total solar eclipse occurs only if the Moon is at such a distance from the Earth that it appears to have an angular diameter slightly larger than that of the Sun. If this does not occur, i.e. the Moon shows an apparent angular diameter smaller than that of the Sun, you will observe a suggestive bright ring, however not appreciable for the observation of the solar corona.

In a total central solar eclipse, the Moon projects a long path of obscuration onto the Earth which, however, only affects a very narrow portion of the Earth, along the entire path of the eclipse and, however, on average only about a hundred kilometers wide.