A solar eclipse is probably the most spectacular celestial event that people can experience. The Moon slowly turns broad daylight into night as it glides in front of the Sun, hiding it from view. If the eclipse is total, at mid- eclipse, the Sun has completely vanished, but its ephemeral corona surrounds the blackened Moon like a crown.

A solar eclipse occurs when the Moon comes between the Earth and the Sun, and casts its shadow on the Earth. For obvious reasons, an eclipse of the Sun can happen only when the Moon is new, and thus lies on the Sun-side of the Earth. If the Moon’s orbit lay in the same plane as that of the Earth, there would be an eclipse every month. However, because the lunar orbit is tilted at an angle of about five degrees, most times the New Moon passes unseen either above or below the Sun in the sky and no eclipse is seen on Earth.

Solar Eclipse Diagram
An eclipse of the Sun occurs when the Earth passes through the Moon’s shadow. The Moon’s shadow has two parts: a fainter outer region (in light gray) called the penumbra and a dark inner shadow (in dark gray) called the umbra.

Solar eclipses are of three types: total, partial and annular. Which kind you see depends on what part of the Moon’s shadow passes over you and the distance between the Earth and the Moon at the time of the eclipse.

Total Solar Eclipse
During a total solar eclipse, the Sun’s extensive outer atmosphere or corona is a breathtaking sight. NSO/AURA/NSF

Total solar eclipses are the result of a fortunate coincidence. Because the Sun and the Moon are almost exactly the same angular size as we see them from Earth, during a total eclipse the Moon covers the bright disk of the Sun, but little or none of the surrounding space. This allows us to see features of the Sun that would otherwise be invisible, except from outer space. These include the glorious corona, which stretches outwards from the Sun in all directions, and solar prominences – large arch-shaped structures observable in the corona.

Total eclipses can be observed only within the region of totality, a very narrow band – up to 169 miles wide – where the Moon’s shadow falls as the Earth rotates. For this reason, total eclipses are rare events in any given geographical area and it is no wonder that many eclipse-tour companies have sprung up, making it easy for any eclipse-chaser to experience the greatest sight of his lifetime.

Much more common are partial solar eclipses, in which the Moon obscures only part of the Sun. Observers located in the much broader outer shadow of the Moon (the penumbra) see such an eclipse. At the edge of the penumbra, the Moon seems to “take a bite out of the Sun”. Further within the penumbra, the Sun’s disk takes the shape of a crescent. Unfortunately, a partial eclipse is not as dramatic as a total eclipse; even with three-quarters of the Sun covered by the Moon, daylight still looks quite ordinary.

Annular Eclipse
An annular eclipse occurs when the Moon is not quite big enough to block all the Sun. NOAA/SEC

A special case of a solar eclipse is known as an annular eclipse. This occurs when the Moon is farther from the Earth than normal, and hence its apparent size is not quite sufficient to cover the Sun completely. Therefore, even though the Sun- Moon alignment is perfect, the Moon will appear slightly smaller in diameter than the Sun and a thin ring of sunlight will remain visible around the dark silhouette of the Moon. The name annular eclipse comes from the Latin annulus, meaning ring.

The next total solar eclipse visible from the United States will occur on August 21, 2017. In the mean time, there will be a few partial solar eclipses to entertain us! Just check the Table of Solar Eclipses for information on upcoming eclipses.

The greatest care must be taken when watching a solar eclipse – but nobody should ever pass up the chance of seeing the splendor of such a celestial event. Never look at the Sun unless you wear special solar eclipse eyeglasses, and even more importantly, make sure you have the proper filters if you plan to observe the Sun through a telescope.