Why Don't Eclipses Happen Every Month?

Time & Astronomy

The Moon passes between us and the Sun once every 29.5 days, and stands opposite the Sun once every 29.5 days. That is twelve or thirteen opportunities a year for each kind of eclipse, and yet a typical year delivers four. The missing ingredient is a tilt of about five degrees — small enough to sound irrelevant, large enough to turn a monthly event into a rare one.

Key takeaways

The orbit of the Moon is tilted 5.1 degrees against the plane of the orbit of the Earth, so most new and full moons pass above or below the exact alignment.
Five degrees is ten times the apparent width of the Moon, so a near miss is not close at all.
Eclipses are only possible near the two nodes, the points where the two orbital planes intersect.
The Sun passes each node once every six months, creating eclipse seasons about 34 days long, and every season guarantees at least one solar eclipse.
The nodes drift backwards, so the eclipse year is 346.6 days and the seasons arrive about 19 days earlier each calendar year.
Without the tilt there would be roughly 25 eclipses a year, two every month, and totality would be unremarkable.

Two Orbits, Two Different Planes

Draw the path of the Earth around the Sun and you get a flat disc; astronomers call the plane of that disc the ecliptic. If the Moon travelled in the same plane, every new moon would slide exactly across the face of the Sun and every full moon would drop exactly into the shadow of the Earth.

It does not. The lunar orbit is inclined by 5.145 degrees, so for half of each month the Moon rides above the ecliptic and for the other half below it. That figure is easy to underestimate. The Moon and the Sun each appear about 0.5 degrees wide from the ground, so a maximum excursion of five degrees puts the Moon ten of its own diameters away from where it would need to be. On such a month the new moon sails past the Sun with a gap the width of ten Moons, and nothing whatsoever happens.

This is also the answer to the version of the question people usually ask about the full moon. A full moon that misses the shadow of the Earth by five degrees is not a near miss; the shadow is roughly 1.4 degrees wide at that distance, so the Moon passes clear of it by several times its own width. Why the phases themselves occur at all is a separate matter, covered in Moon Phases Explained.

The Nodes: Where the Planes Cross

Two tilted planes always intersect along a line, and that line is what makes eclipses possible. The Moon crosses the ecliptic twice per orbit, at the two points called the nodes: the ascending node where it passes from south to north, and the descending node where it goes the other way. The imaginary line joining them through the Earth is the line of nodes.

Everything hinges on a coincidence of timing. An eclipse needs two conditions at once:

The Moon must be new or full — that is, lined up with the Sun and the Earth as seen from above.
The Moon must simultaneously be near a node — that is, lined up with them as seen from the side.

Each condition on its own is common. The Moon is new every 29.5 days and it crosses a node every 13.6 days. The two rarely coincide, and an eclipse is precisely the coincidence. Ancient astronomers knew the nodes long before they knew why they mattered and named them the head and the tail of the dragon — the creature that was eating the Sun. The technical term for the node-to-node month, the draconic month, still carries the dragon with it.

How Close Is Close Enough

Near a node does not mean exactly at one. Because the Sun and the Moon are discs rather than points, there is a tolerance, known as the ecliptic limit, measured as the angular distance between the Sun and the node at the moment of alignment:

Kind of eclipse	Certain within	Possible out to
Partial solar	About 15.4° of a node	About 18.5°
Total or annular solar	About 9.9°	About 11.8°
Partial or total lunar	About 9.5°	About 12.2°
Penumbral lunar	About 16.5°	About 21°

The exact numbers shift a little from one eclipse to the next, because the Earth and the Moon are both on elliptical orbits and the apparent sizes of the two discs change through the year. The pattern, though, is fixed: the window for a solar eclipse is wider than the window for a lunar one, which is why solar eclipses are in fact more numerous than lunar ones — the opposite of most people’s impression, and only because a solar eclipse is visible from such a small piece of ground. The details of that asymmetry are in Solar Eclipse Explained and Lunar Eclipse Explained.

Eclipse Seasons

As the Earth moves around the Sun, the Sun appears to travel right around the ecliptic once a year, so it passes each node once. While it is within the ecliptic limit of a node, eclipses become possible; while it is anywhere else, they are impossible. Those windows are the eclipse seasons, and each lasts around 34 to 37 days for solar eclipses.

That length is the crucial number. A new moon comes round every 29.5 days, which is shorter than the season, so every eclipse season contains at least one solar eclipse — it is arithmetically impossible to skip. Occasionally a season is long enough to catch two, one at each end. The lunar window is narrower, so an umbral lunar eclipse is likely but not guaranteed in any given season.

This also explains why eclipses arrive in clusters rather than being sprinkled through the year. Once the Sun is near a node, the full moon a fortnight before or after the new moon is often still inside the limit. That is why a solar eclipse is so frequently paired with a lunar one about two weeks away, and why a run of three eclipses in five weeks is followed by five months of nothing at all.

Why the Seasons Come Every 173 Days, Not Every 183

If the line of nodes were fixed in space, the Sun would return to the same node every 365.24 days and eclipse seasons would fall on much the same dates every year. Instead the line of nodes drifts backwards, dragged round by the gravity of the Sun, completing a full circuit in 18.6 years.

Because the nodes are sliding towards the oncoming Sun, the Sun meets a node sooner than it otherwise would. The interval between successive passes through the same node — the eclipse year — is 346.62 days, nearly nineteen days short of a calendar year, and half of it is the 173.31 days between one eclipse season and the next.

The visible consequence is that eclipse seasons walk backwards through the calendar at about nineteen days a year. A season in mid-August is in late July three years later and in the spring after a decade. Any impression that eclipses favour a particular time of year is an artefact of a short memory: over 18.6 years they visit every part of the calendar in turn. It is the same kind of slow drift that makes lunar and solar calendars disagree, and one of the reasons civil timekeeping eventually abandoned the sky in favour of standard time.

Four Different Months, and the Beat Between Them

The Moon has more than one month, depending on what you measure it against, and eclipse prediction is essentially the study of how these periods interfere:

Month	Length	Measured from	Controls
Synodic	29.531 days	New moon to new moon	Whether the Moon is new or full
Draconic	27.212 days	Node to node	Whether an eclipse is possible at all
Anomalistic	27.555 days	Perigee to perigee	Whether the eclipse is total or annular
Sidereal	27.322 days	Star to star	Where the Moon sits against the constellations

An eclipse needs the first two to agree, and its character depends on the third. Every so often all three come back into step at once: 223 synodic months, 242 draconic months and 239 anomalistic months all take almost exactly 6,585.3 days. That is the saros of 18 years and 11 days, the cycle Babylonian astronomers extracted from centuries of records, and the reason a very similar eclipse recurs on a schedule you can set a calendar by. Its awkward extra third of a day is why the repeat lands a third of the way around the world, as described in What Is an Eclipse?

What a World Without the Tilt Would Look Like

It is worth picturing the alternative. With the two orbits in a single plane, every new moon would produce a solar eclipse and every full moon a lunar one: about 25 eclipses a year, forever. There would be a band of totality sweeping the Earth every month, so most people would see several total eclipses in a lifetime without leaving home, and the event would carry roughly the cultural weight of a heavy shower.

The real sky gives us four to seven eclipses a year, most of them partial or penumbral, and a total solar eclipse over any particular town about once every 375 years. Five degrees of tilt is the entire difference between the commonplace and the extraordinary — and, incidentally, the reason predicting eclipses was worth the centuries of arithmetic that people from Babylon to Athens and Alexandria put into it.

None of this changes how you use a prediction once you have one. The catalogues give the circumstances in UTC, and turning that into a time to stand outside in London, New York or Tokyo is the subject of How Astronomical Events Are Converted to Local Time.

Frequently Asked Questions

Why is there not an eclipse at every new moon?

Because the orbit of the Moon is tilted about 5 degrees to the plane in which the Earth orbits the Sun. At most new moons the Moon passes above or below the Sun by several times its own apparent width, so its shadow misses the Earth entirely.

What are the nodes of the lunar orbit?

They are the two points where the tilted orbit of the Moon crosses the plane of the orbit of the Earth. An eclipse can only happen when a new or full moon occurs close to one of them, which is why eclipses cluster into two seasons each year.

How long is an eclipse season?

Roughly 34 to 37 days for solar eclipses. Since a new moon comes round every 29.5 days, at least one solar eclipse is guaranteed in every season, and occasionally two fall inside the same window.

Why do eclipse dates shift earlier each year?

Because the line of nodes drifts backwards, completing a circuit in 18.6 years. The Sun therefore returns to the same node every 346.62 days rather than every 365.24, so eclipse seasons arrive about 19 days earlier each calendar year.

How many eclipses are there in a year?

Between four and seven of all kinds. Four is the normal count: two solar and two lunar, one pair per eclipse season. Years with seven require an unusual arrangement of eclipse seasons around the ends of the calendar year.