Every month the Moon passes between Earth and the Sun once, and swings behind Earth once. On that arithmetic there should be an eclipse of each kind every month. There are only a few a year, because the Moon's orbit is tilted.
The Moon's orbit is tilted, which is why eclipses are rare
The Moon's orbit is tilted about five degrees from Earth's, so most new and full moons are not really a straight line. This lesson starts from that tilt and works through the umbra, the penumbra, the ring, the red Moon, and how to keep your eyes.
Most new and full moons do nothing
Every month the Moon passes between Earth and the Sun once and swings behind Earth once. If all three lined up each time, there would be more than two dozen eclipses a year. There are only a few.
The reason is that the Moon's orbit is tilted 5.16 degrees from the ecliptic. In most months the Moon crosses the Sun's neighbourhood a few degrees above or below it and its shadow goes into empty space. The Moon is only half a degree wide, so five degrees is ten of its own diameters — even a near miss is unusual.
The nodes decide whether this month counts
Two planes meet along a line, and the Moon crosses the ecliptic at the two points where its orbit meets that line. Those are the nodes, and an eclipse needs the Moon to be near one of them at the same time as new or full moon.
The Sun passes the line of nodes twice a year, and those two stretches — about thirty-four days each — are the eclipse seasons. A season holds both a new and a full moon, which is why solar and lunar eclipses often come in pairs a fortnight apart. The nodes themselves drift backwards, one full circuit every eighteen and a half years, so the seasons arrive about nineteen days earlier each year.
Umbra and penumbra
The Sun is a disc, not a point, so anything in front of it casts two shadows: the umbra, where the Sun is completely hidden, and the penumbra, where part of it still shows.
Stand in the Moon's umbra and you see totality; stand in the penumbra and you see a bite out of the Sun. The umbral track is only one or two hundred kilometres wide while the penumbra covers thousands, so the people who see totality are always a minority. Lunar eclipses swap the roles: Earth's umbra is more than two and a half Moon-radii wide at the Moon's distance, big enough for the Moon to sit inside it for over an hour.
The Moon can also pass only through the penumbra. It stays lit and dims only slightly, which the eye almost never catches. Those nights are not worth staying up for.
Total or annular depends on how far the Moon is that day
The Moon's orbit runs from 356,500 to 406,700 kilometres, and Earth's orbit is an ellipse as well. The apparent sizes of the two discs therefore each cover a range, and those ranges overlap.
When the Moon is close it looks bigger than the Sun and the umbra reaches the ground: a total eclipse. When it is far it looks smaller, the umbra runs out before it arrives, and a complete ring of Sun is left showing: an annular eclipse. That ring is the photosphere at full brightness, not a weakened Sun, so an annular eclipse needs a filter from beginning to end.
The red Moon
A Moon fully inside Earth's umbra does not vanish; it turns dark red or coppery. Earth blocks the direct sunlight, but the atmosphere acts as a ring-shaped lens and bends light that grazes the limb into the shadow. Blue is scattered away in the air and the red completes the journey to the lunar surface.
Standing on the Moon at that moment, you would see Earth ringed by every sunrise and sunset at once. The exact shade changes with the state of the atmosphere — total eclipses after a large volcanic eruption can go almost black.
Eye safety, and the next one from Taiwan
The Sun does not become safe once ninety percent of it is covered. The remaining tenth still burns the retina within tens of seconds, and the retina has no pain nerves, so nothing hurts while it happens; the damage appears hours later and may not heal. Only ISO 12312-2 eclipse glasses and purpose-made solar filter film qualify, and the film goes on the front of the lens or telescope. Sunglasses, film negatives, smoked glass and CDs do not. Only the totality of a total eclipse can be watched unaided, and the glasses go back on before the Sun returns.
Lunar eclipses are completely safe with the naked eye, binoculars or a camera. From Taiwan the next total lunar eclipse visible from start to finish falls on the night of 31 December 2028, with about 71 minutes of totality. The next solar eclipse visible at all is the partial of 1 June 2030, reaching magnitude 0.36 in Taipei. For a total eclipse over the island itself the wait runs to 11 April 2070, and even then the track only grazes the far south.
A dog, a severed head, and two wolves
In China a solar eclipse was a heavenly dog eating the Sun, a lunar one a toad eating the Moon. The court answered with drums, gongs and firecrackers to frighten the thing off. The Book of Songs already records an eclipse as an omen worth writing down.
India tells it as Rahu. He drank the nectar of immortality, the Sun and Moon reported him, and his head was struck off — but the head had drunk and would not die. It chases the pair of informers still, swallows them when it catches up, and, having no body left, drops them out of the neck again.
Norse myth uses two wolves. Sköll runs down the Sun, Hati runs down the Moon, and a bite is an eclipse.
All three explain the same thing: why a bite gets taken out of the sky, and why it heals. The model below lets you turn the geometry yourself. The answer is in the tilt of the Moon's orbit.
Eclipse season lab
Two sliders decide everything: how far the line of nodes sits from the Sun, and how far the Moon sits from Earth. Use the task buttons to jump into position, then drag the sliders and watch the verdict change.
The lunar timings are geometry alone and hold for everyone who can see the Moon. The solar totality and annularity figures are the best case — centre line, equator, local noon. Anywhere else the shadow crosses faster and the phase is shorter.
This model is pure geometry with no ephemeris behind it. It answers what a given alignment produces, not which date it happens on, and it cannot say how much of it your own sky will show. For dates, use the almanac.
Try it yourself
The line of nodes is sixty degrees from the Sun, so the Moon passes about five degrees above it and throws its shadow into empty space. This is what most new moons look like.
The nodes point at the Sun and the Moon happens to be near perigee. Its disc is the larger of the two, the umbra reaches all the way to the ground, and anyone standing in it sees totality.
Only the distance changed. Out near apogee the Moon looks smaller, the umbra tapers to a point before it lands, and observers stand beyond the tip — where a full ring of Sun is left uncovered.
Fourteen degrees from the node, the shadow axis misses Earth entirely. Only the penumbra sweeps across, so the best anyone on the planet sees is a bite out of the Sun.
Switch to full moon and the Moon swings behind Earth, deep into the umbra. It does not vanish; it turns dark red, lit by sunlight bent through Earth's atmosphere.
The Moon only clips the outer shadow. It dims slightly, too slightly for the eye to catch — you need before-and-after photographs. It counts as an eclipse, but not one worth losing sleep over.
Why last month's full moon stayed white
There are twelve or thirteen full moons a year and almost all of them pass without incident. If the Moon's orbit lay in the same plane as Earth's, every new moon would bring a solar eclipse and every full moon a lunar one — more than two dozen a year.
They do not, because the orbit is tilted with respect to the ecliptic. In most months the Moon crosses the Sun's neighbourhood a few degrees above or below it, and the shadow goes past Earth into empty space.
The tilt is 5.16 degrees. That sounds small until you remember the Moon is only about half a degree wide: five degrees is ten of its own diameters, which makes even a near miss unusual.
Nodes and eclipse seasons
Two planes meet along a line, and the Moon crosses the ecliptic at the two points where its orbit meets that line. Those nodes are the only places an eclipse can happen: the Moon has to be near one of them and at new or full moon at the same time.
One full turn of the node slider is one eclipse year of 346.62 days. It contains two windows where eclipses are possible, each about thirty-four days long — the eclipse seasons. A season is long enough to hold both a new moon and a full moon, which is why solar and lunar eclipses so often arrive in pairs a fortnight apart.
The eclipse year is shorter than the tropical year because the nodes slide backwards, taking 18.6 years for a full circuit. The Sun meets the next node before it has finished a lap, so the seasons creep about nineteen days earlier each year.
To know whether a month brings an eclipse, ask one thing: at the moment of new or full moon, how far is the Sun from the line of nodes?
Umbra and penumbra
The Sun is a disc, not a point, so anything standing in front of it casts two shadows: the umbra, where the Sun is hidden completely, and the penumbra, where part of it still shows.
Stand in the Moon's umbra and the Sun is gone — that is totality. Stand in the penumbra and the Sun has a bite out of it. Both happen at once during a single eclipse; only your position differs. The umbra draws a track usually one or two hundred kilometres wide, while the penumbra covers thousands.
In a lunar eclipse the roles swap: Earth casts, the Moon catches. Earth is far bigger, so its umbra is still more than two and a half Moon-radii wide out at the Moon's distance. The Moon can sit inside it for over an hour, which the few minutes of a total solar eclipse can never match.
The Moon may also pass only through Earth's penumbra. It stays lit, just slightly dimmer — a penumbral eclipse. The eye rarely catches it; you need photographs taken before and after to see the difference. Those nights are not worth staying up for.
Total or annular, decided between 356,000 and 407,000 kilometres
The Moon's orbit is an ellipse: 356,500 km at its closest, 406,700 km at its furthest. Earth's orbit is an ellipse too, nearest the Sun in January. Between them, the apparent sizes of the two discs each cover a range — and the two ranges overlap.
When the Moon is close it looks larger than the Sun, the umbra reaches the ground, and the eclipse is total. When it is far it looks smaller, the umbra tapers out before it arrives, observers stand beyond the tip, and a complete ring of Sun remains. A few eclipses fall right on the boundary and run total in the middle of the track, annular at both ends.
That ring is not a weakened Sun. It is the photosphere at full brightness, only smaller. An annular eclipse is dangerous from beginning to end, and the filter never comes off.
Why a totally eclipsed Moon turns red
A Moon fully inside Earth's umbra does not disappear. It goes dark red or coppery, roughly ten thousand times fainter than a full moon, and stays visible.
The light gets there by bending. Earth blocks the direct beam, but the atmosphere works as a ring-shaped lens, refracting sunlight that grazes the limb into the shadow. Blue is scattered away in the air and the red survives the trip to the lunar surface. Turn it around: standing on the Moon at that moment, you would see Earth outlined by a complete ring of sunrise and sunset.
The shade differs from eclipse to eclipse with the state of the atmosphere. Total eclipses after a large volcanic eruption often go almost black. Astronomers describe this with the Danjon scale, L0 for nearly invisible up to L4 for a bright coppery orange.
Eye safety: the only part of this lesson that can blind you
At every stage of a partial or annular eclipse the Sun can damage your retina, and the retina has no pain receptors — by the time you notice, it is done. Use eclipse glasses certified to ISO 12312-2, or a solar filter fitted to the front of the lens or telescope. Sunglasses, film, smoked glass, CDs and X-ray sheets are not filters. Lunar eclipses are entirely safe with the naked eye, binoculars or a camera.
The Sun does not become safe once ninety percent of it is covered. The remaining tenth still burns the retina within tens of seconds, and because the retina has no pain nerves nothing hurts while it happens. The damage shows up hours later as a blur in the centre of your vision, and it may not heal.
Only two things qualify: eclipse glasses certified to ISO 12312-2, and purpose-made solar filter film. The film goes on the front of the telescope or lens, never behind the eyepiece — once the objective has concentrated the light, any filter downstream will burn through. Sunglasses, stacked film negatives, smoked glass, CDs and X-ray sheets all fail.
Only the totality of a total solar eclipse can be watched with the naked eye, and the glasses must go back on before the Sun reappears. Totality begins and ends with Baily's beads and the diamond ring, which are sunlight streaming through valleys on the Moon's limb. The diamond ring is the signal to look away. Annular eclipses have no such phase; they need protection throughout.
The same applies to cameras and phones. Without a filter in front, the sensor picks up permanent burnt pixels. Lunar eclipses need no protection at all.
When is the next one from Taiwan
Solar and lunar eclipses happen about equally often; what differs is how many people can see one. A lunar eclipse is visible from wherever the Moon is above the horizon, half the planet at once. A solar eclipse track is one or two hundred kilometres wide, and any given spot waits centuries for its turn.
So the ones Taiwan actually gets are lunar eclipses and partial solar eclipses. The dates below come from official forecasts; check the almanac again before you plan around them.
Want to know which day?
This lesson covers the mechanism. Actual dates, types and visibility live on the almanac and eclipse pages.
Total or annular, decided between 356,000 and 407,000 kilometres
Body
Distance
Apparent radius
Moon at perigee
356,500 km
0.279°
Moon at apogee
406,700 km
0.245°
Sun at perihelion
147,098,000 km
0.271°
Sun at aphelion
152,098,000 km
0.262°
FAQ
Why isn't there an eclipse every month?
They do not, because the orbit is tilted with respect to the ecliptic. In most months the Moon crosses the Sun's neighbourhood a few degrees above or below it, and the shadow goes past Earth into empty space. Two planes meet along a line, and the Moon crosses the ecliptic at the two points where its orbit meets that line. Those nodes are the only places an eclipse can happen: the Moon has to be near one of them and at new or full moon at the same time.
What is the difference between a total and an annular solar eclipse?
When the Moon is close it looks larger than the Sun, the umbra reaches the ground, and the eclipse is total. When it is far it looks smaller, the umbra tapers out before it arrives, observers stand beyond the tip, and a complete ring of Sun remains. A few eclipses fall right on the boundary and run total in the middle of the track, annular at both ends. That ring is not a weakened Sun. It is the photosphere at full brightness, only smaller. An annular eclipse is dangerous from beginning to end, and the filter never comes off.
Why does the Moon turn red during a total lunar eclipse?
A Moon fully inside Earth's umbra does not disappear. It goes dark red or coppery, roughly ten thousand times fainter than a full moon, and stays visible. The light gets there by bending. Earth blocks the direct beam, but the atmosphere works as a ring-shaped lens, refracting sunlight that grazes the limb into the shadow. Blue is scattered away in the air and the red survives the trip to the lunar surface. Turn it around: standing on the Moon at that moment, you would see Earth outlined by a complete ring of sunrise and sunset.
The model on this page computes geometry only: apparent sizes, shadow-cone radii and the resulting classification. It has no ephemeris, predicts no eclipse date or time, and calculates no local contact times or magnitude. The dates in the table come from the official forecasts listed above.