Moon · 7 of 8

Eclipse Seasons

The Moon passes the Sun every month, so why so rarely in front of it?

Why there isn't an eclipse every month: the Moon's path, the two nodes where it crosses the Sun's, and the seasons they make. Drawn for tonight.

You’ll be able to say when the next eclipse season comes, and why there is not an eclipse every month.

12 steps · about 9 min

Before this: The Ecliptic (8 min), Eclipses (10 min)

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Kept for this guide only, until you change it here — your map settings, and every other guide, stay as they are.

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Sources

Meeus, Astronomical Algorithms, 2nd ed. (1998), ch. 25 and 47 — the Sun's and the Moon's positions, the Moon's 5.145° inclination, and the mean rates of the Sun and of the Moon's node (18.6 years) that give the 346.62-day eclipse year
NASA Eclipse Web Site — Solar Eclipses: 2021 to 2030 (the annular eclipse of 6 February 2027 and the total eclipse of 2 August 2027)
NASA Moon Fact Sheet — the Moon's 1737.4 km radius, half a degree across at its mean distance
The 17° ring: new Moons 1990–2040 on this page's own ephemeris; every one that eclipsed the Sun somewhere on Earth fell within 16.8° of a node, every one that missed beyond 17.4°

← GUIDES

The marked body is the Moon. The fine dashed line is its path for the month; the other is the ecliptic, the Sun's. Each month the Moon passes the Sun, at new Moon, and stands opposite it, at full. Were the lines one, every new Moon would eclipse the Sun.

They are not one line. The Moon's orbit tilts 5.15° to Earth's, so for half of each month the Moon runs north of the ecliptic and for half south — at the pin, ten Moon-widths away. So a new Moon usually slips above or below the Sun, and a full Moon past Earth's shadow.

The two lines cross twice, at the nodes. At the ringed one, the ascending node, the Moon climbs north over the ecliptic; at the descending node, half a circle round, it drops back south. Only at a node is the Moon on the Sun's line.

From outside, the Moon's orbit is a tilted hoop. Twice a year the line through its nodes points at the Sun: those weeks are the eclipse seasons.

Now run a month, the camera on the Moon and the ecliptic laid flat. Watch its gap to the line open to five degrees, close at one node, and open the other way. New and full Moon fall wherever the month puts them — in most months, nowhere near a node.

The same month as a graph: the Moon each night, two weeks back to two weeks on, by its distance north or south of the ecliptic — the zero line. The nodes are where the wave crosses it. Most new and full Moons land well off it.

Here is the Sun, on the ecliptic as always — drawn through the ground if it has set. A new Moon happens where the Sun is, so for a solar eclipse it is the Sun that has to be near a node. Thirty degrees from one, the new Moon misses the Sun by 2.6°, five Sun-widths.

The next eclipse of the Sun. The ring is 17° round it, and a node is inside. The Sun takes some 33 days to cross the ring, longer than the 29.5 between new Moons — so each such window, an eclipse season, brings at least one.

Full Moons work the same way, at the other node: here is the next eclipse of the Moon, in Earth's shadow. A full Moon a fortnight either side of a solar eclipse often brings one — eclipses come in pairs.

Now run two years. The crosses slide west, once round in 18.6 years — the turning behind the Moon's 18.6-year swing. So the Sun, moving east, meets each node about 19 days sooner every year.

Three years of the Sun's distance from the nearer node. Each dip below 17° is an eclipse season, not quite half a year apart, and the new Moons inside it are eclipses of the Sun. Drag to the next dip: that is your next eclipse season.

The two paths again, crossing at a node. An eclipse in the news means the Sun is near one.

The Moon's path tilts 5° to the Sun's
Eclipses need the Sun near a node
Two seasons a year, each brings one

Eclipse Seasons

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