The earliest writings showing that people paid official attention to eclipses are around 5,000 years old, and Babylonian astronomers could predict lunar eclipses, and later solar eclipses, with fair accuracy. On average, a total solar eclipse happens somewhere on Earth every one or two years. The Moon's orbit around Earth is tilted by about five degrees, and it crosses the plane of Earth's orbit at two points called nodes. An eclipse season lasts about 35 days and comes again just short of six months later, and eclipse seasons recur about every 173 days. 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. Solar and lunar eclipses often come in pairs: two eclipses, one of each kind, usually fall in one eclipse season.

A solar eclipse happens at the new moon, when the Moon moves between Earth and the Sun, but most months the new moon passes above or below the Sun, so its shadow misses Earth and there is no solar eclipse. A lunar eclipse happens at the full moon, when Earth's shadow falls on the Moon. 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. In a lunar eclipse, sunlight bent by Earth's atmosphere reaches the Moon, and the Moon looks red; the Moon in a total lunar eclipse often looks a dull red or orange. 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.

Anyone helping others watch a solar eclipse should begin with one rule. 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. Do not use adjustable or auto-darkening welding helmets to look at the Sun. Some sellers put the ISO label on eclipse glasses that were never tested or certified. 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.

Total eclipses give a rare view of the Sun's corona, which is normally hidden by the bright face of the Sun. Eclipses are very useful to scientists, as well as beautiful. Helium was first seen during the total solar eclipse of 1868 in India, and during that eclipse Janssen 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. 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. The diamond ring effect looks like a bright gem on a ring around the Moon, at the first and last glimpse of the Sun.

On Earth, the narrow line where a total solar eclipse can be seen is called the path of totality. Totality at one place lasts from about 10 seconds to about seven and a half minutes at most. The April 8, 2024 total solar eclipse had its longest totality, 4 minutes 28 seconds, at Nazas, Durango, Mexico. The 1919 eclipse lasted more than six minutes, one of the longest of the twentieth century. 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. In the 1919 test, Newton's theory predicted starlight passing the Sun would bend by 0.87 arcseconds, and Einstein's theory predicted that starlight passing the Sun would bend by 1.74 arcseconds, twice Newton's number.

Words to know

• Saros: eclipse repeat cycle
• nodes: orbit crossing points
• penumbral: faint shadow type
• corona: Sun's outer atmosphere