Eclipse & Saros Explorer

How eclipses work

An eclipse happens when the Sun, Earth and Moon fall almost exactly in line. 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. They do not happen every month because the Moon's orbit is tilted about five degrees, so most months the shadow misses. Eclipses come only when a new or full moon falls near a lunar node, the points where the tilted orbit crosses our path around the Sun. That is the whole story told on the eclipses lesson.

The Saros, and why eclipses repeat

After 223 lunar months, about 18 years and 11 days, 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. A 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, lasting twelve to fifteen centuries in all.

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.

Because one Saros runs about a third of a day, some eight hours, past a whole number of days, each eclipse in a series arrives about eight hours later and its path lands roughly a hundred degrees of longitude to the west of the one before. Three Saroses, the exeligmos of about 54 years, bring 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 exception is the optional place-name lookup when you tap Use my location). 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.