The Moon passes between Earth and the Sun every single month. So why doesn't the Sun go dark every single month?
Before you investigate, here's the setup. Two types of eclipse exist — and they work in completely opposite ways.
Scientists can predict eclipses centuries in advance. The positions are exact. Can you find them?
New moon + crossing the orbital plane = solar eclipse. Most months the Moon passes above or below this line — and the shadow misses Earth completely.
Full moon + crossing the orbital plane = lunar eclipse. Both conditions have to be true at exactly the same time.
Here's what you found. The two eclipses look similar from a distance — but the geometry, the phase, and the experience are completely different.
| Solar Eclipse | Lunar Eclipse | |
|---|---|---|
| What happens | Moon blocks sunlight from reaching Earth | Earth's shadow falls on the Moon |
| Moon phase | New Moon — Moon between Sun and Earth | Full Moon — Moon on far side of Earth |
| Alignment | Sun → Moon → Earth | Sun → Earth → Moon |
| Seen from | Only the narrow path of totality on Earth | Anywhere the Moon is visible in the sky |
New moon happens every 29.5 days — the exact phase a solar eclipse needs. So why don't eclipses happen every month? A single 5-degree tilt in the Moon's orbit changes everything.
The Moon's tilted orbit crosses Earth's orbital plane at exactly two points, called nodes — the only places where an eclipse is geometrically possible. Both conditions must be true at the same time:
A new moon happens every ~29.5 days, but the Moon is near a node for only a few days each orbit. That rare overlap is exactly why eclipses feel special.
The Moon casts two shadow zones during a solar eclipse — and they produce completely different experiences. Which one you're in determines everything.
The umbra is the innermost, darkest cone of the Moon's shadow. Stand inside it and the Moon completely covers the Sun — the sky goes dark, stars appear, and the Sun's corona glows around the Moon's edge. The umbra can be as narrow as a few kilometers and rarely exceeds about 270 km wide on Earth's surface.
The penumbra is the outer, lighter shadow zone. Here, the Moon covers only part of the Sun — you see a partial solar eclipse, a crescent Sun still bright enough to need eye protection. The penumbra spreads wider than the umbra across Earth's surface. Most people observing a solar eclipse are in the penumbra and see only a partial eclipse.
Every term below appeared in this lesson. Click any pill to jump to where it was explained.
10 questions covering solar and lunar eclipses, shadows, orbital tilt, and nodes. Answer every question, then submit.
Ready to push your eclipse knowledge deeper? These optional extensions are waiting for you.