Astronomy 2e · Earth, Moon, and Sky
Eclipses of the Sun and Moon
On this page 9 sections
In 30 seconds
An eclipse happens when one astronomical body moves into the shadow of another: a solar eclipse The Moon's shadow falls on Earth, hiding the Sun Full entry →, when the Moon's shadow falls on Earth and hides the Sun, and a lunar eclipse Earth's shadow falls on the Moon, darkening it Full entry →, when Earth's shadow falls on the Moon and darkens it. Perfect alignment would give an eclipse every month, but the Moon's orbit is tilted about 5° to Earth's orbital plane (a commonly taught reference value). Exact alignments therefore happen only a few times a year, during eclipse seasons.
Why this matters
Ancient Babylonian, Chinese, and Greek astronomers predicted eclipses using recurring cycles, and the circular shadow Earth casts on the Moon gave early evidence that Earth is round. A total solar eclipse in 1919 offered one of the first experimental confirmations of general relativity, when starlight passing near the Sun was seen to bend. Today, total eclipses still give one of the best views of the Sun's faint corona, and eclipses recorded in ancient texts help historians date events.
The college version
Core Concepts
Shadows: umbra and penumbra
Every shadow has two parts. The umbra Cone of total shadow where the Sun is fully blocked Full entry → is the central cone where the light source is completely blocked; the penumbra Region of partial shadow around the umbra Full entry → surrounds it, where the source is only partially blocked. Test this with a lamp and your hand: the dark inner shadow is the umbra, the fuzzy outer ring the penumbra. Because the Moon's umbra tapers with distance, whether an eclipse is total, partial, or annular depends on how far the Moon is from Earth.
Lunar eclipses: Earth's shadow on the Moon
A lunar eclipse occurs when the Sun, Earth, and Moon line up with Earth in the middle — only at full moon:
- Total: the whole Moon enters Earth's umbra. It does not vanish; it usually turns deep red-orange, because sunlight refracting through Earth's atmosphere bends red light into the shadow while blue light is scattered away. This "blood moon" is an atmospheric effect, not a property of the Moon.
- Partial: only part of the Moon enters the umbra, leaving a bright crescent.
- Penumbral: the Moon passes only through the penumbra; the dimming is subtle and easily missed.
Earth's shadow is far wider than the Moon, so total lunar eclipses can last over an hour and are visible from the whole night side of Earth.
Solar eclipses: the Moon's shadow on Earth
A solar eclipse occurs when the Sun, Earth, and Moon line up with the Moon in the middle — only at new moon:
- Total: the umbra reaches Earth's surface. Observers inside the narrow path of totality see the Sun fully covered; the sky darkens, stars appear, and the corona glows. Totality lasts at most a few minutes.
- Partial: observers in the penumbra see part of the Sun bitten away.
- Annular: the Moon is near the far side of its orbit, appears slightly smaller than the Sun, and its umbra does not quite reach the ground; observers see a bright "ring of fire."
The path of totality is a narrow band, at most a few hundred kilometers wide, which is why any given place sees a total solar eclipse only about once every few centuries.
Why not every new moon and full moon?
The Moon's orbit is tilted about 5° from the ecliptic, so at most new and full moons the Moon passes above or below the Sun–Earth line. Eclipses occur only when the Moon is near a node One of two points where the Moon's orbit crosses the ecliptic Full entry → — one of the two points where its orbit crosses the ecliptic — while the Sun is near a node too. The Sun passes through each node region roughly every six months, producing two eclipse seasons per year, each about a month long. A typical year has between two and seven eclipses of all types (commonly taught range).
The Saros cycle
A nearly identical eclipse recurs about 18 years and 11 days later. This Saros cycle The ~18-year-11-day period after which similar eclipses repeat Full entry → comes from the near-synchronization of the Moon's phase, node, and distance periods. Because the cycle is not exact and includes about 8 extra hours, each repeat occurs roughly 120° of longitude west of the previous one. Ancient astronomers used such cycles to predict eclipses long before orbits were understood.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Solar eclipse | Lunar eclipse | Solar = Moon's shadow on Earth (new moon, narrow path, eye protection needed); lunar = Earth's shadow on Moon (full moon, half the planet, safe to watch) |
| Umbra | Penumbra | Umbra is total shadow; penumbra is partial. In the penumbra of a total eclipse you never see totality |
| Total solar eclipse | Annular solar eclipse | Annular: Moon appears too small to cover the Sun, so a ring remains and totality never happens (Moon near apogee) |
| "Blood moon" as an omen | Reddish Moon from refraction | The red color is sunlight bent through Earth's atmosphere with blue scattered — normal optics |
| Every new/full moon gives an eclipse | Eclipses need node alignment | The ~5° tilt makes exact alignment rare; only about 2–7 eclipses occur per year (commonly taught range) |
| Sunglasses are enough for eclipse viewing | Only certified solar filters are safe | Ordinary sunglasses block too little light and no infrared; permanent eye damage is possible |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine shining a flashlight at a ball: it makes a dark spot (umbra) and a fuzzy shadow (penumbra) on the wall. A solar eclipse is when the Moon's dark spot lands on Earth and blocks the Sun for people standing in it. A lunar eclipse is when Earth's dark spot lands on the Moon, so the Moon looks dark or reddish. The Moon's path is tilted, so the shadows line up only a few times a year — and never stare at the Sun to watch, even with sunglasses!
Worked example
Suppose a total solar eclipse is announced for your region. The day unfolds geometrically:
- At new moon, the Moon is between Earth and the Sun, near a node — alignment is good enough for an eclipse.
- First contact: the Moon's edge begins covering the Sun. You are in the penumbra, seeing a partial eclipse. Wear certified eclipse glasses for every moment you look at the Sun.
- Baily's beads and diamond ring: as the last sliver of Sun shrinks, sunlight streams through valleys on the Moon's limb; a final bright flash — the diamond ring — signals the start of totality.
- Totality: the umbra reaches you. The corona appears as a pearly halo, stars become visible, and the temperature drops. Only now, for these few minutes, is it safe to look without filters.
- Third/fourth contact: the Sun reappears; glasses go back on immediately, and the partial phase plays out in reverse.
Compare with a lunar eclipse the same year: step outside at night during full moon and watch Earth's curved shadow creep across the Moon for over an hour — no location, filters, or equipment needed. That is why lunar eclipses are the ones to watch casually.
Key takeaways
- Solar eclipse needs new moon; lunar eclipse needs full moon.
- Umbra = total shadow (totality); penumbra = partial shadow. Annular eclipses occur when the Moon is far enough that its umbra misses Earth.
- Moon's orbit tilted ~5° to the ecliptic; eclipses happen only near a node during an eclipse season (about twice a year).
- A total lunar eclipse looks reddish because Earth's atmosphere refracts sunlight and scatters blue light — a normal optical effect.
- The Saros cycle (~18 years 11 days) predicts when similar eclipses repeat.
- Safety: never look directly at the Sun, through a telescope, or with ordinary sunglasses during any partial solar eclipse. Use certified solar eclipse glasses (e.g., meeting ISO 12312-2) or a pinhole projector. Only the brief minutes of totality are safe without filters; filters go back on the instant the Sun reappears.
- Total solar eclipses occur somewhere on Earth roughly every 18 months (commonly taught value); a given location waits centuries.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
What phase must the Moon be in for a solar eclipse? For a lunar eclipse?
Show answer
Solar: new moon (Moon between Sun and Earth). Lunar: full moon (Earth between Sun and Moon).
Why don't we get two eclipses every month?
Show answer
The Moon's orbit is tilted about 5° to the ecliptic, so at most new/full moons it passes above or below the Sun–Earth line. Eclipses require the Moon to be near a node during an eclipse season.
What is the difference between a total and an annular solar eclipse?
Show answer
In a total eclipse the umbra reaches Earth and fully covers the Sun. In an annular eclipse the Moon is near apogee, appears smaller than the Sun, and leaves a bright ring.
Why does the Moon look reddish during a total lunar eclipse?
Show answer
Sunlight passing through Earth's atmosphere is refracted into the shadow, and blue light is scattered away, so mostly red light reaches the Moon.
If total solar eclipses happen somewhere on Earth roughly every 18 months, why might your hometown wait centuries for one?
Show answer
The Moon's umbra covers only a narrow band of Earth (at most a few hundred kilometers wide), so any single location enters the path of totality only rarely — about once every few centuries on average.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- umbra
- Cone of total shadow where the Sun is fully blocked
- penumbra
- Region of partial shadow around the umbra
- solar eclipse
- The Moon's shadow falls on Earth, hiding the Sun
- lunar eclipse
- Earth's shadow falls on the Moon, darkening it
- annular eclipse
- Moon appears smaller than the Sun, leaving a bright ring
- node
- One of two points where the Moon's orbit crosses the ecliptic
- eclipse season
- The ~month-long period, twice a year, when eclipses are possible
- Saros cycle
- The ~18-year-11-day period after which similar eclipses repeat
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.