What Is an Eclipse?

Time & Astronomy

An eclipse is what happens when three bodies fall into a straight line and one of them ends up in the shadow of another. From the surface of the Earth there are only two worth setting an alarm for: the Moon passing in front of the Sun, and the Moon sliding into the shadow of the Earth. They are built from the same geometry, but almost nothing else about them matches — not how long they last, not how many people can see them, and not whether it is safe to look.

Key takeaways

A solar eclipse happens at new moon, when the Moon passes between the Sun and the Earth and drops its shadow onto us.
A lunar eclipse happens at full moon, when the Earth passes between the Sun and the Moon and the Moon enters our shadow.
There is no eclipse every month because the orbit of the Moon is tilted about 5 degrees, so the alignment is usually near-miss rather than exact.
Every year brings between four and seven eclipses of all kinds, clustered into two eclipse seasons roughly six months apart.
A total solar eclipse is visible from a strip a few hundred kilometres wide; a total lunar eclipse is visible from the entire night side of the planet at once.
Eclipse predictions are always published in UTC, so the local clock time — and sometimes the local date — has to be worked out for each city.

Two Eclipses, Two Completely Different Events

Both eclipses need the Sun, the Earth and the Moon in a line. What changes is the order of the three, and that one detail decides everything else about the experience.

	Solar eclipse	Lunar eclipse
Order of bodies	Sun – Moon – Earth	Sun – Earth – Moon
Phase of the Moon	New moon	Full moon
What sits in shadow	A strip of the Earth	The Moon
Who can watch it	A path at most about 270 km wide	Everyone on the night side of the Earth
How long it lasts	Totality of 2–4 minutes, never more than about 7.5	Totality of up to about 1 hour 45 minutes
Time of day	Daytime, by definition	Night, by definition
Safe to look at	Only during totality; filters at every other moment	Yes, always, with no equipment at all
How often from one spot	A total eclipse roughly every 375 years	A total eclipse every few years

That last row is the one that surprises people. Eclipses of the two kinds happen at broadly similar rates across the planet as a whole, but a lunar eclipse hangs in the sky above half the Earth while a solar eclipse paints a narrow line across it. Wait at home long enough and the Moon will be eclipsed above your head many times over a lifetime; wait for the Sun to be eclipsed above the same rooftop and you would need several human lifetimes in a row. Each is unpacked in its own guide: Solar Eclipse Explained and Lunar Eclipse Explained.

Why There Is Not an Eclipse Every Month

The Moon completes a full cycle of phases every 29.5 days, so it passes between us and the Sun once a month and stands opposite the Sun once a month. If everything lay in one flat plane, that would mean a solar eclipse and a lunar eclipse every four weeks, which would make them about as remarkable as a full moon.

The saving detail is that the orbit of the Moon is tilted by roughly 5.1 degrees against the plane in which the Earth goes round the Sun. Most months the Moon passes above or below the line joining us to the Sun, misses the shadow entirely, and nothing happens. The two points where the tilted orbit crosses that plane are called the nodes, and an eclipse is only possible when a new or full moon happens to arrive close to one of them.

The nodes themselves drift slowly backwards around the orbit, which is why the alignment windows do not sit at fixed points in the calendar. Those windows — eclipse seasons — come round every 173.3 days, a bit less than half a year, and last about a month each. Every eclipse season contains at least one solar eclipse and usually a lunar one about a fortnight either side of it, because a new moon and a full moon are always two weeks apart. That is the reason eclipses arrive in pairs and small clusters rather than evenly spaced through the year.

Add it up and a calendar year holds a minimum of four eclipses and a maximum of seven, counting the faint ones nobody notices. The four-eclipse years are the common case: two solar and two lunar, spread across two seasons. The full mechanism — the ecliptic limits, the slowly drifting nodes and the 173-day rhythm they produce — is worked through in Why Don’t Eclipses Happen Every Month?

The Coincidence That Makes Totality Possible

There is no physical law requiring the Moon to cover the Sun neatly. It does so because of an accident of scale that has no explanation beyond luck. The Sun is about 400 times wider than the Moon, and it also happens to sit about 400 times further away. The two therefore look almost exactly the same size from the ground — each about half a degree across, roughly the width of a pea held at arm’s length.

Because both orbits are slightly elliptical, that near-match tips one way or the other through the year. When the Moon is closer to us than average it looks a little larger than the Sun and can hide it completely, producing a total eclipse. When it is further away it looks slightly too small, and a ring of blazing sunlight is left around it: an annular eclipse, which is a very different sight and never grows dark.

The coincidence is also temporary. Tidal forces are pushing the Moon away from us at about 3.8 centimetres a year, so its apparent size is shrinking. In a few hundred million years it will no longer be able to cover the solar disc at all, and total solar eclipses will stop happening for good. Anyone alive now is watching them in the narrow window when they still work.

Eclipses Repeat: The Saros Cycle

Eclipses are not random. Babylonian astronomers noticed more than two thousand years ago that a very similar eclipse returns after 6,585.32 days — 18 years, roughly 11 days and 8 hours. That interval, now called the saros, works because three separate lunar cycles nearly coincide over it: the cycle of phases, the cycle of the Moon returning to a node, and the cycle of its varying distance from the Earth.

The awkward eight hours are the interesting part. In eight hours the Earth turns a third of the way round, so the repeat eclipse falls about 120 degrees of longitude further west. An eclipse over Asia returns over Europe and Africa, then over the Americas, and only after three saros cycles — 54 years and about 33 days, a period the Greeks called the exeligmos — does it come back to roughly the same part of the world. The rotation of the Earth is what turns a tidy astronomical cycle into a globe-hopping one, in much the same way it forces the whole apparatus of time zones on us.

Eclipses belonging to one saros form a numbered series that runs for twelve to fifteen centuries and contains seventy or eighty eclipses, drifting gradually from pole to pole before it dies out. Several dozen series are running concurrently at any moment, which is why the pattern is invisible to casual observation but obvious in a catalogue.

Not Every Alignment Is an Eclipse

Astronomers separate three closely related events, and the distinction is worth knowing because the words get swapped around freely in headlines:

Eclipse — one body passes into the shadow of another, or blocks it from view at close to the same apparent size. The two familiar cases are the solar and lunar ones.
Transit — a small body crosses a much larger one, appearing as a dot on the disc. Mercury and Venus transit the Sun as seen from Earth. Venus does so in pairs eight years apart, separated by gaps of more than a century; the last pair was in 2004 and 2012, and the next is not until 2117.
Occultation — a nearer body hides a much more distant one completely, as when the Moon drifts in front of a star or a planet.

Nor is the Earth the only place with eclipses. A spacecraft at Jupiter sees the shadows of the large moons sweep across the cloud tops constantly. What makes the Earth unusual is the size coincidence described above: our eclipses are the ones where the covering disc fits the covered disc almost exactly.

Eclipse Times Are Always Published in UTC

Every eclipse catalogue, every map and every set of contact times is quoted in Coordinated Universal Time. There is no alternative that would work: an eclipse is a single physical event crossing dozens of national time zones, and expressing it in any one of them would make the numbers meaningless everywhere else. Astronomers actually compute the geometry in a uniform timescale free of leap seconds and then convert it to UTC for publication, which is why some sources also mention a small correction called delta T. The distinction between UTC and the older GMT is covered in UTC vs GMT.

Turning the published time into something useful means two different jobs, depending on the kind of eclipse:

A lunar eclipse has one timetable for the whole planet. The Moon enters the shadow of the Earth at a single instant, and everyone who can see the Moon sees it happen simultaneously. Convert the UTC times to your own offset once and you are done.
A solar eclipse has a different timetable for every location. The shadow sweeps across the surface, so the moment the Sun is covered depends on where you stand, sometimes varying by an hour or more along the path. Published tables therefore list contact times city by city.

The conversion regularly moves the event onto a different calendar date. A lunar eclipse peaking at 02:30 UTC happens late on the previous evening in New York and in the small hours in London, while for Tokyo it falls in mid-morning and is not visible at all. This is the same date arithmetic that catches travellers out on long flights, described in Understanding Flight Times Across Time Zones and, in its most extreme form, at the International Date Line. The conversion itself, step by step and with the traps that ruin it, is in How Astronomical Events Are Converted to Local Time.

The Next Eclipses

A selection of the events worth planning around, with dates given in UTC:

Date (UTC)	Type	Where it is best seen
12 August 2026	Total solar	Greenland, Iceland, northern Spain
28 August 2026	Partial lunar	The Americas, Europe, Africa
6 February 2027	Annular solar	Chile, Argentina, the South Atlantic
2 August 2027	Total solar	Southern Spain, North Africa, Luxor, Saudi Arabia
26 January 2028	Annular solar	Ecuador, Peru, Brazil, then Spain and Portugal at sunset
22 July 2028	Total solar	Australia, including Sydney, and southern New Zealand
31 December 2028	Total lunar	Europe, Africa, Asia, Australia
26 June 2029	Total lunar	The Americas, Europe, Africa
25 November 2030	Total solar	Southern Africa and southern Australia

Check the local clock time for the city you will be watching from before the day itself — the date in the table is the UTC date, and for observers well east or west of Greenwich the event can fall on the day before or after. The current offset for any city is on its local time page, including the ones, such as Madrid, that will be on summer time when the eclipse arrives. The same single-instant logic governs solstices and equinoxes, while sunrise and sunset work the other way round and are local from the start.

Frequently Asked Questions

What is an eclipse in simple terms?

It is a body moving into the shadow of another. In a solar eclipse the Moon comes between the Sun and the Earth and its shadow falls on us; in a lunar eclipse the Earth comes between the Sun and the Moon, and the Moon passes through the shadow of the Earth.

Why is there no eclipse every month?

Because the orbit of the Moon is tilted about 5 degrees relative to the orbit of the Earth around the Sun. Most new and full moons pass above or below the exact line-up, so an eclipse is only possible during the two eclipse seasons each year when the alignment falls close to a crossing point of the two orbits.

How many eclipses happen each year?

Between four and seven, counting solar and lunar events of every kind. Four is the usual number: two solar and two lunar. Many of them are partial or penumbral and pass unnoticed by anyone not looking for them.

Why are eclipse times given in UTC?

Because an eclipse crosses many time zones and belongs to none of them, so a single global reference is the only sensible way to publish it. Convert the UTC time to the offset of your own city, remembering that the result can land on the previous or the following calendar date.

Do solar and lunar eclipses come in pairs?

Very often, yes. A new moon and a full moon are always about a fortnight apart, so if the alignment is good enough for a solar eclipse it is frequently still good enough for a lunar one two weeks earlier or later. Some eclipse seasons produce three events in a row.