Eclipse Calendar

A solar eclipse is the Moon passing in front of the Sun, at new moon. A lunar eclipse is the Moon passing into the Earth's shadow, at full moon. The Sun is 400 times wider than the Moon and, by chance, about 400 times farther away, so the two look the same size in our sky and the Moon can just cover the Sun.

There is a new moon and a full moon every month, but the Moon's orbit is tilted about 5° from the Earth's, so most months the Moon passes above or below the Sun or the shadow. Eclipses happen only when a new or full moon falls near one of the two nodes, the points where the Moon's orbit crosses the ecliptic. The Sun lines up with the nodes twice a year, for about five weeks each time: the eclipse seasons, shaded on the timeline. Each season brings two eclipses, sometimes three, a solar and a lunar one about two weeks apart. The nodes slowly turn westward, so the seasons come about 19 days earlier every year.

The Moon's distance changes along its oval orbit. When it is close enough to cover the whole Sun, the eclipse is total along a narrow path; when it is farther and looks slightly smaller, it leaves a ring of fire, an annular eclipse. The Earth's shadow on the Moon is much larger, so a lunar eclipse is seen from the whole night side of the Earth, and during totality the Moon turns copper red, lit by the sunsets and sunrises all around the Earth.

Eclipses repeat after a Saros of 18 years, 11 days and 8 hours (223 lunations), when the Sun, the Moon and the nodes line up the same way again. The eight extra hours shift each repeat a third of the way around the Earth. Eclipses one Saros apart belong to the same series, numbered on each card: the total solar eclipses of 1999 and 2017 are both of Saros 145.

Never look at the Sun, even mostly eclipsed, without certified eclipse glasses or a solar filter. Only during totality itself is it safe to look with the naked eye. A lunar eclipse is safe to watch.

Eclipses are computed in your browser with the methods of Jean Meeus's Astronomical Algorithms: the eclipse times, types and magnitudes from the lunations near the nodes (chapter 54), and the positions of the Sun and the Moon to place them on the Earth and see them from a city (chapters 25 and 47). The results are within a minute or two of NASA's eclipse catalog for 1900 to 2100. Near the edges of a path of totality, use a detailed map.