The earliest writings showing that people paid official attention to eclipses are around 5,000 years old, and the Chaldeans knew the Saros as a period after which lunar eclipses seem to repeat themselves. The word Saros was not used for an eclipse period until English astronomer Edmund Halley adopted it in 1691. The Saros cycle is a period of about 6,585.3 days, or 18 years 11 days 8 hours, after which eclipses of the same kind repeat. Each Saros cycle ends with an extra third of a day, so Earth has turned about 120 degrees farther and the next eclipse is seen a third of the way around the world. Solar and lunar eclipses often come in pairs: two eclipses, one of each kind, usually fall in one eclipse season. Eclipse seasons recur about every 173 days, and the Moon's orbit crosses the plane of Earth's orbit at two points called nodes.
In a total lunar eclipse, the Moon looks red because of Rayleigh scattering in Earth's air. The short wavelengths of light, such as violet and blue, are scattered more strongly by air, while red and orange light passes through. In a lunar eclipse, sunlight bent by Earth's atmosphere reaches the Moon, and the Moon looks red. The color and brightness of a red eclipsed Moon depend on the dust, water droplets, clouds and mist in Earth's air; the Moon in a total lunar eclipse often looks a dull red or orange. The Moon has no light of its own, and it shines because its surface reflects sunlight. There are three types of lunar eclipse: total, partial and penumbral. Eclipses of the Moon can be seen by far more people than eclipses of the Sun, because everyone on the night side of Earth can see them, and seen from the Moon, a total lunar eclipse would show every sunrise and sunset happening on Earth at that moment.
Helium was first seen during the total solar eclipse of 1868 in India, where Janssen pointed a spectroscope at a solar prominence and saw a new line in the Sun's light. Two days later, English astronomer Norman Lockyer independently saw the same spectrum, and he and Edward Frankland suggested naming the new element helium, after Helios, the Greek word for Sun. Helium was the first chemical element found on an object beyond Earth before it was found on Earth. Total eclipses give a rare view of the Sun's corona, which is normally hidden by the bright face of the Sun. The corona is the Sun's outer atmosphere; we cannot normally see it because the Sun's bright surface is in the way, but it shows in a total eclipse.
Eclipses are very useful to scientists, as well as beautiful. In 1917 it was realized that a total solar eclipse would set up the perfect experiment to test relativity, and during the 1919 eclipse astronomers photographed the stars nearest to the Sun. The 1919 test rested on the idea that light bends under the influence of a gravitational field, which the eclipse let astronomers measure. Newton's theory predicted starlight passing the Sun would bend by 0.87 arcseconds, and Einstein's theory predicted 1.74 arcseconds, twice Newton's number. The 1919 eclipse lasted more than six minutes, one of the longest of the twentieth century. The Sobral team in 1919 obtained 19 plates with their astrographic telescope and 8 with a 4-inch camera, while clouds nearly obscured the observations at Principe during the 1919 eclipse. The 1919 eclipse test was a success, and news of the result helped launch Einstein into scientific stardom.
The Sun is much bigger than the Moon and Earth, so each of them casts all three kinds of shadow: umbra, penumbra and antumbra. The Moon's shadow has a dark center called the umbra and a lighter outer part called the penumbra, and as the Moon's shadow stretches away from the Moon, the umbra narrows, so on Earth it covers only a small area compared with the penumbra. In an annular eclipse, the part of the Moon's shadow beyond the end of the umbra is the antumbra; people inside it see a ring of Sun. To see a total solar eclipse you must be inside the Moon's darkest shadow, the umbra. On Earth, the narrow line where a total solar eclipse can be seen is called the path of totality, and totality at one place lasts from about 10 seconds to about seven and a half minutes at most.
Anyone supervising a solar eclipse needs a firm set of rules. Never look at the Sun, even in an eclipse, without eclipse glasses that a grown-up gives you; during a partial eclipse it is never safe to look straight at the Sun without proper eye protection. Eclipse glasses are special glasses made for looking at the Sun safely, and regular sunglasses are not safe for looking at the Sun. Safe eclipse glasses meet the ISO 12312-2 standard, which does not cover ordinary sunglasses, and some sellers put the ISO label on eclipse glasses that were never tested or certified. Do not use adjustable or auto-darkening welding helmets to look at the Sun. Look at an eclipse glasses filter first: scratched, punctured or torn eclipse glasses should be thrown away. Put your eclipse glasses on before you look up at the Sun, and keep your eyeglasses on under them. Always watch children who are using eclipse glasses or other solar filters.
On average, a total solar eclipse happens somewhere on Earth every one or two years. The total solar eclipse of August 21, 2017 crossed the United States from Oregon to South Carolina in a path about 70 miles wide, and the April 8, 2024 total solar eclipse had its longest totality, 4 minutes 28 seconds, at Nazas, Durango, Mexico. The total solar eclipse of April 2024 lasted longer than the one in 2017 because the Moon was closer to Earth in 2024. As of November 2026, the next total solar eclipse that can be seen from the contiguous United States is on August 23, 2044. As of November 2026, a total solar eclipse is due on August 2, 2027, visible in southern Spain and North Africa, and it is expected to last as long as 6 minutes 23 seconds. As of November 2026, the next total solar eclipse after August 2, 2027 is on July 22, 2028.
Words to know
- Saros: eclipse repeat cycle
- antumbra: shadow beyond umbra
- penumbra: lighter outer shadow
- corona: Sun's outer atmosphere
