How Eclipses
Happen
Discover why the Sun, Moon, and Earth sometimes align to create the most spectacular events in our sky: solar and lunar eclipses.
Earth & Space Science
What Is an Eclipse?
An eclipse occurs when one celestial body moves into the shadow of another. In our solar system, eclipses happen when the Sun, Earth, and Moon line up in a nearly straight path called syzygy.
The Three Key Players
★ The Sun
Our star: a massive ball of hot gas about 109 times Earth's diameter. It is the light source in every eclipse.
★ The Earth
Our home planet orbits the Sun once per year. Earth casts a long shadow into space behind it.
★ The Moon
Earth's natural satellite orbits us about once every 29.5 days. It can block the Sun or enter Earth's shadow.
Shadows in Space
Every object in sunlight casts a shadow. That shadow has two parts:
Umbra
The darkest, innermost part of the shadow where all sunlight is blocked.
Penumbra
The lighter, outer part of the shadow where only some sunlight is blocked.
The Moon's Tilted Orbit
The Moon's orbit around Earth is tilted about \(5^\circ\) relative to Earth's orbit around the Sun. This is why eclipses don't happen every month: the Moon usually passes slightly above or below the Sun-Earth line.
How a Solar Eclipse Works
A solar eclipse occurs during a new moon when the Moon passes directly between the Sun and Earth, casting its shadow on Earth's surface.
Types of Solar Eclipses
Total
The Moon completely covers the Sun. The sky goes dark and the Sun's corona becomes visible. Only from within the umbra.
Partial
Only part of the Sun is covered. Observers are in the penumbra. The Sun looks like a "cookie with a bite."
Annular
The Moon is farther from Earth (smaller apparent size), so a bright "ring of fire" remains around the Moon.
How a Lunar Eclipse Works
A lunar eclipse occurs during a full moon when the Earth passes directly between the Sun and Moon, casting Earth's shadow onto the Moon.
Types of Lunar Eclipses
Total
The entire Moon enters Earth's umbra. The Moon glows red/copper for up to 1 hour 40 minutes.
Partial
Only part of the Moon enters the umbra. A dark "bite" appears on one edge of the Moon.
Penumbral
The Moon passes through Earth's penumbra only. The dimming is so subtle it is hard to notice.
Solar vs. Lunar Eclipses
| Solar Eclipse | Lunar Eclipse | |
|---|---|---|
| Moon Phase | New Moon | Full Moon |
| What's Shadowed? | Part of Earth | The Moon |
| Who Sees It? | Small area on Earth (narrow path) | Anyone on Earth's night side (half the planet) |
| Duration | Minutes (totality up to ~7.5 min) | Hours (totality up to ~100 min) |
| Safe to Watch? | Requires special glasses | Safe with naked eyes |
| Shadow Caster | The Moon | The Earth |
Why Not Every Month?
1
The Moon's orbit is tilted \(5^\circ\). Most months, the Moon passes above or below the Sun-Earth line, missing the shadow entirely.
2
Eclipses require alignment at a node. The Moon must cross Earth's orbital plane at the same time it is in new moon or full moon phase.
3
Eclipse seasons shift each year. The nodes slowly rotate, creating eclipse windows roughly every 6 months: typically 2-5 eclipses per year.
How to Identify an Eclipse
1
Check the Moon phase. New Moon → possible solar eclipse. Full Moon → possible lunar eclipse.
2
Check the alignment. The Sun, Earth, and Moon must be nearly in a straight line (near a node).
3
Identify the shadow. Solar: Moon's shadow on Earth. Lunar: Earth's shadow on the Moon.
4
Determine total vs. partial. If the object is fully in the umbra → total. If only in the penumbra → partial (or annular for solar).
Example: A Total Solar Eclipse
Observer A
In the umbra: sees a total solar eclipse. The sky goes dark, the corona appears.
Observer B
In the penumbra: sees a partial solar eclipse. Part of the Sun is still visible.
Observer C
Outside the shadow: sees no eclipse at all. A normal sunny day.
Example: A Total Lunar Eclipse
Example: An Annular Solar Eclipse
Common Mistakes
✕ Confusing the order
In a solar eclipse the order is Sun → Moon → Earth. In a lunar eclipse it is Sun → Earth → Moon. Students often reverse these.
✕ Wrong Moon phase
Solar eclipses happen at new moon, lunar eclipses at full moon. A common error is mixing up which phase goes with which eclipse.
✕ Eclipses every month
The Moon's \(5^\circ\) orbital tilt means alignment at the nodes is rare. Eclipses do not happen every new or full moon.
Summary at a Glance
| Feature | Solar Eclipse | Lunar Eclipse |
|---|---|---|
| Alignment order | Sun → Moon → Earth | Sun → Earth → Moon |
| Moon phase | New Moon | Full Moon |
| Shadow caster | Moon | Earth |
| Visibility | Narrow path on Earth | Entire night side of Earth |
| Types | Total, Partial, Annular | Total, Partial, Penumbral |
| Eye safety | Special glasses needed | Safe to view |
| Why not monthly? | Moon's orbit tilted \(5^\circ\): alignment at nodes required | |
Question 1 of 5
During a solar eclipse, which object casts a shadow on Earth?
In a solar eclipse, the Moon passes between the Sun and Earth, blocking sunlight and casting its shadow on Earth's surface.
Question 2 of 5
A lunar eclipse can only occur during which Moon phase?
A lunar eclipse requires the Earth to be between the Sun and Moon, which only happens at full moon when the Moon is on the opposite side of Earth from the Sun.
Question 3 of 5
Why don't eclipses happen every month?
The Moon's orbit is tilted about \(5^\circ\) relative to Earth's orbital plane. Most months, the Moon passes above or below the Sun-Earth line, so no shadow falls on either body. Eclipses happen only when the Moon is near a node.
Question 4 of 5
An observer in the penumbra during a solar eclipse would see:
The penumbra is the lighter, outer part of the Moon's shadow where only some sunlight is blocked. An observer here sees a partial solar eclipse: the Moon covers part of the Sun, but not all of it.
Question 5 of 5
What causes the Moon to appear red during a total lunar eclipse?
Earth's atmosphere acts like a lens, bending (refracting) sunlight around the planet. Shorter blue wavelengths scatter away, but longer red wavelengths pass through and illuminate the Moon with a copper-red glow: the famous "Blood Moon."