The science behind eclipses
Twice a year, the geometry lines up and something is visibly happening.
An eclipse is the one astrological event you can photograph. It arrives on a schedule worked out centuries ago, it is visible from a named strip of the Earth, and it either happens or it does not. That makes it a good place to be precise.
What follows is the astronomy, without decoration. Where astrology comes into it is said plainly and kept separate, because the two are more interesting when you can tell which one you are reading.
Next eclipse: The next one is on 6 February 2027: a annular solar eclipse, in Aquarius.
Why eclipses happen
There is a new moon every month and a full moon every month, and almost none of them are eclipses. If the Moon orbited in exactly the same plane as the Earth orbits the Sun, every new moon would blot out the Sun and every full moon would slide into Earth’s shadow. We would have two eclipses a month, every month, and nobody would find them remarkable.
The Moon’s orbit is tilted, by about five degrees. Five degrees is a small angle and a large distance: at the Moon’s remove it is roughly ten Moon-widths of clearance. So most months the new moon passes above or below the Sun, and the full moon passes above or below Earth’s shadow, and nothing happens at all.
The lunar nodes
Two tilted planes cross along a single line, and that line meets the Moon’s orbit at two points. They are the lunar nodes: the only two places where the Moon can be exactly level with the Sun and the Earth rather than above or below.
An eclipse needs two things at once: a new or full moon, and that moon near a node. The nodes drift slowly backwards through the zodiac, taking about eighteen and a half years to go all the way round, and twice a year the Sun lines up with them. Those alignments are the eclipse seasons, which is why eclipses arrive in pairs, roughly six months apart.
Astrology calls that same axis the nodal axis and reads it as a line of direction, of what is being left and what is being moved towards. It is worth being exact about what that means: astrology and astronomy are naming the same two points in the sky, and that is genuinely the case. Naming the same point is not the same as one explaining the other, and this page will not pretend otherwise.
The two families of eclipse
A solar eclipse happens at a new moon. The Moon passes between us and the Sun and puts its shadow on the Earth, so only the strip of the planet inside that shadow sees anything. It is total when the Moon covers the Sun completely, annular when the Moon is far enough away in its orbit to leave a bright ring around the edge, partial when the alignment is off-centre and only a bite is taken out, and hybrid when a single eclipse manages to be annular at the ends of its track and total in the middle.
A lunar eclipse happens at a full moon. The Earth passes between the Sun and the Moon and puts its shadow on her, so everyone on the night side of the planet sees the same thing at the same time. It is total when she is fully inside the dark inner shadow, partial when only part of her is, and penumbral when she only grazes the faint outer shadow, which is subtle enough that you can miss it while looking straight at it.
That asymmetry is worth holding onto. A solar eclipse is a local event with a global schedule. A lunar eclipse is a shared one: half the Earth at once, no travel required.
Why the eclipsed Moon turns red
Earth’s shadow is not empty. Our atmosphere sits in a ring around the planet, and sunlight passing through it gets bent inwards and stripped of its blue, the same scattering that makes the sky blue overhead and the Sun orange at the horizon. What survives the journey is red, and it lands on the Moon.
So the colour on a totally eclipsed Moon is sunlight that has been through our air. It is every sunrise and every sunset happening on Earth at that moment, projected onto a rock a quarter of a million miles away. The deeper she sits in the shadow, and the dustier our atmosphere is that year, the darker the red.
The Saros cycle: eclipses have memory
Three separate lunar rhythms, the month of phases, the month of nodes and the month of distances, come back into near-agreement every 6,585 days and change. That is eighteen years, eleven days and eight hours, and it is called a Saros.
After one Saros the geometry repeats almost exactly, so an eclipse is followed by a near-twin eighteen years later. The eight extra hours mean the Earth has turned a third of the way round underneath it, so the twin lands about a third of the world further west. Follow one Saros series and you watch a family of eclipses walk across the planet over more than a thousand years, being born at one pole, marching across the middle, and dying at the other.
The eclipse of this season is the descendant of one that happened eighteen years ago and the ancestor of one that will happen eighteen years from now. If you were a child at the first, you can watch the same eclipse arrive again as an adult. That is not a metaphor, it is a calendar.
Eclipses and the tides
At a new or full moon the Sun, the Earth and the Moon line up, and the Sun’s pull adds to the Moon’s instead of working across it. Those are spring tides, the highest and lowest of the month. An eclipse only happens at that alignment, so an eclipse always comes with the strongest tides of its cycle. That much is straightforwardly true, and you can check it against a tide table.
It is worth being clear about why the ocean responds and a person does not, because the difference is usually explained wrongly. A tide is not the Moon pulling water upwards. It is the Moon pulling one side of a body slightly harder than the other side, and that difference only becomes large when the body itself is large. The ocean is thousands of kilometres across, so across its width the difference in pull is enough to move it. A human body is not, and across it the difference is so small that the gravitational tug of a person sitting beside you is greater than the Moon’s.
So the tides are real, they are biggest at an eclipse, and they are not happening inside you. Being made of water does not put you in the ocean’s position. What an eclipse actually gives a person is a date, a sky worth going outside for, and a reason to notice where you are, which is enough without borrowing physics that will not hold.
What people notice around an eclipse
Eclipses have a reputation for landing hard, and it is worth separating what is measured from what is reported. What is measured is the geometry: the date, the shadow, the tide. What is reported is that people find eclipse seasons eventful, and that things which were already unstable tend to move during them.
Both can be true without one causing the other. An eclipse season is a marked date on a calendar that many people are watching, and marked dates concentrate attention: you notice what is loose because you looked. That is worth using rather than explaining away. Twice a year the sky gives you a scheduled reason to check what has been sliding, and the checking is the useful part, whatever you believe is behind it.
Frequently asked questions
Why is there not an eclipse every month?
Because the Moon’s orbit is tilted about five degrees from the plane of Earth’s orbit. Most months the new moon passes above or below the Sun, and the full moon above or below Earth’s shadow. An eclipse needs the moon to be near one of the two points where the two planes cross, called the lunar nodes.
How many eclipses are there in a year?
At least four, and up to seven. They come in eclipse seasons roughly six months apart, when the Sun lines up with the lunar nodes, and each season carries at least one solar and one lunar eclipse about two weeks apart.
Why does the Moon turn red during a total lunar eclipse?
Because the only sunlight reaching her has passed through Earth’s atmosphere, which bends it inwards and scatters the blue out of it. What is left is red. The colour on the Moon is every sunrise and sunset on Earth at that moment, projected onto her.
Do eclipses affect the human body through the tides?
No. Eclipses do coincide with the strongest tides of the month, because the Sun and Moon are aligned. But a tide comes from the difference in pull across a body, and that difference only matters when the body is enormous. Across a person it is smaller than the gravitational pull of someone standing next to them.
What is the Saros cycle?
A period of about eighteen years and eleven days after which the Sun, Earth and Moon return to almost the same geometry, so a near-identical eclipse repeats. The extra eight hours in the cycle mean the repeat lands about a third of the way further west around the world.
Last updated . Dates calculated with the Swiss Ephemeris.