Eclipse & Saros Explorer

How eclipses work

For the models, accuracy and data behind these figures, see the methodology and sources page.

An eclipse happens when the Sun, Earth and Moon fall almost exactly in line. There are two kinds:

• A solar eclipse comes at new moon, when the Moon passes between us and the Sun and its shadow touches Earth.
• A lunar eclipse comes at full moon, when the Moon slides into Earth's shadow.

Eclipses do not happen every month because the Moon's orbit is tilted about five degrees, so most months the shadow misses. They come only when a new or full moon falls near a lunar node, one of the two points where the tilted orbit crosses the plane of our path around the Sun. The full story, with a visual you can move, is on the eclipses lesson.

The Saros, and why eclipses repeat

After 223 lunar months (about 18 years, 11 days and 8 hours), the Sun, Moon and node return to nearly the same arrangement and produce a strikingly similar eclipse. That period is the Saros, and the chain of eclipses it links is a Saros series.

Each series lives a long life of about 12 to 15 centuries. A solar series is born as a small partial eclipse near one of Earth's poles, grows over centuries into a run of central total or annular eclipses, then fades back to partials at the other pole, marching steadily in latitude all the while.

A lunar series makes the matching journey through Earth's shadow: in from slight penumbral eclipses, through partials, to a run of totals, and back out again.

What the Saros numbers mean

Every series carries a catalog number, and the one this tool shows is the standard number observatories use, computed straight from each eclipse's date (it matches the published NASA catalog). Solar and lunar eclipses are numbered separately, so a solar Saros 126 and a lunar Saros 126 are unrelated families.

The numbers run roughly in the order the series began through history, and they also track which of the Moon's two nodes a series belongs to, so neighboring numbers alternate between the ascending and descending node. Two eclipses that share a number are the long-separated near-twins of one family.

Why each eclipse steps west

One Saros runs about eight hours, a third of a day, past a whole number of days. Each eclipse in a series therefore arrives about eight hours later in the day than the one before, by which time the Earth has turned roughly 120 degrees farther to the east. The eclipse lands about a third of the way around the world to the west of the one before it.

The point of greatest eclipse usually shifts a little less than the full 120 degrees, somewhere around 100 to 120 degrees of longitude. That is because the leftover is really closer to 7 hours 43 minutes than to a full 8, about 116 degrees of Earth's rotation, and because the exact gap between two members varies by an hour or two either way, since the Moon's orbit is not a perfect circle.

Three Saroses add up to almost exactly a whole number of days. That triple period, the exeligmos of about 54 years, brings the eclipse back to nearly the same longitude, which is why your own region tends to see kindred eclipses around 54 years apart. You can place the Saros among the other rhythms of the sky on the Cycles by Length page.

How this tool works

Eclipse times, types and locations come from the open-source Astronomy Engine running entirely in your browser, so the times and positions are computed on your device rather than fetched from a server (the only exceptions are the optional place lookups, when you tap Use my location or search a city by name).

The Saros family is found by stepping forward and back by whole Saros cycles and taking the nearest eclipse each time. Positions are accurate to well under a degree, and the tool is reliable for roughly the years 1700 to 2200.