The Eclipse Spiral
Every mark is a real eclipse, solar or lunar, placed on the zodiac at its degree, and the picture grows outward with time, so the latest eclipses are on the outside. The eclipses line up along two spirals, the paths of the Moon's north and south nodes, which slide backwards through the signs once every 18.6 years. Each eclipse season lights up a pair of opposite signs, and the pair keeps moving.
| Eclipses a year | 4 to 7, about 4.5 |
|---|---|
| Node cycle | 18.6 years, backwards |
| One saros | 223 lunations, 18 years 11 days |
| Years with 7, this century | 2038 and 2094 |
What you are looking at
The wheel is the tropical zodiac, with 0° Aries on the left and the signs running counterclockwise, as on a birth chart. Each mark is one eclipse: a solar eclipse sits at the Sun's degree and a lunar eclipse at the Moon's. Distance from the centre is time. The first eclipse after your start date is near the middle and the last is near the rim, so the default 54 years wind outward in about three turns.
The teal and violet curves are the Moon's north and south nodes. Every eclipse sits close to one of them, which is the whole reason eclipses happen when they do. Black suns in a ring of light are total solar eclipses, rings of fire are annular ones, and small gold dots are partial. Red discs are total lunar eclipses, coral dots partial ones, and faint pearl dots penumbral eclipses, when the Moon only grazes the Earth's outer shadow.
The node curves are not perfectly smooth, and that is real. The true node does not move at a steady pace: it runs at up to about twice its average speed and then almost stands still, about every six months. Switch on the saros links to join each eclipse to its twin one saros later. The links cross the spiral almost straight outward, because a saros of 18.03 years is just short of the nodes' 18.6 year turn.
The math
The Moon's orbit is tilted about 5° to the Earth's. Most months the new moon passes above or below the Sun and the full moon misses the Earth's shadow. An eclipse needs a new or full moon close to a node, one of the two points where the Moon's path crosses the ecliptic, so eclipses come in seasons about six months apart, when the Sun stands near a node. A solar eclipse falls within about 18° of a node, a lunar eclipse in the Earth's dark inner shadow within about 12°, and a faint penumbral one within about 18°.
The nodes are not fixed. The pull of the Sun turns the Moon's orbit slowly backwards, and the nodes slide round the zodiac in 18.6 years. Because the eclipses follow them, the eclipse degrees step backwards too, and drawn against time they make a spiral turning the opposite way to the planets. Over the century from 2001 to 2100 there are between 4 and 7 eclipses every year, about 4.5 on average, and only 2038 and 2094 have seven.
A saros is 223 lunations, 6,585.3 days or about 18 years and 11 days. After one saros the Sun, the Moon and the node return to almost the same arrangement, so an eclipse repeats with nearly the same type, about 11 degrees further along the zodiac. The total solar eclipse of 8 April 2024 followed the total eclipse of 29 March 2006 one saros earlier, and the next in that series is the total eclipse of 20 April 2042.
How it is computed
The eclipses come from astronomy-engine, the same library the app's on-device engines use and check against the Swiss Ephemeris. It searches every new moon for a solar eclipse seen anywhere on Earth and every full moon for a lunar eclipse, and classifies each by the geometry of the shadows. It reports a hybrid solar eclipse as total or annular. Each eclipse is placed at the apparent tropical longitude of the Sun (solar) or the Moon (lunar) at its peak, the degree an ephemeris prints.
The node curves are the Moon's apparent longitude at the instants it crosses the ecliptic, northbound for the north node and southbound for the south node, found with astronomy-engine's node search. For the 19 year span every crossing is used; for longer spans every other month, which still draws the node's twice-yearly steps. The saros links join eclipses of the same kind 223 lunations apart, within a day.
Questions
Why don't we get an eclipse every month?
Because the Moon's orbit is tilted about 5° to the Earth's. At most new and full moons the Moon passes above or below the line from the Sun to the Earth. Only when a new or full moon falls near one of the Moon's nodes do the three line up closely enough for an eclipse.
Why do eclipses move backwards through the signs?
Eclipses happen near the Moon's nodes, and the nodes slide backwards around the zodiac once every 18.6 years. So each year's eclipse seasons arrive about 19 degrees earlier in the zodiac than the year before, and the eclipses follow.
What is a saros?
A period of 223 lunations, about 18 years and 11 days, after which the Sun, the Moon and the nodes return to nearly the same arrangement. Eclipses one saros apart look alike: the total solar eclipses of 29 March 2006, 8 April 2024 and 20 April 2042 belong to one saros series.
What do astrologers make of eclipses?
In astrological tradition eclipses are read as powerful new and full moons, often taken as marking turning points, especially where they fall on sensitive degrees of a birth chart, and the signs of the current eclipse pair are read as the themes of that season. That is interpretation; the piece only shows where and when the eclipses happen.