Eclipses

Big Idea

 An eclipse happens when one object moves into the shadow of another, blocking light.

 Solar eclipse: the Moon blocks the Sun and its shadow falls on Earth.

 Lunar eclipse: Earth blocks sunlight and Earth’s shadow falls on the Moon.

 Cool coincidence: the Sun is about 400× wider than the Moon and about 400× farther away, so they

look about the same size in our sky.

Why We Don’t Get Eclipses Every Month

 The Moon orbits Earth about once per month, and Earth orbits the Sun once per year.

 If the Moon’s orbit were perfectly aligned with Earth’s orbit plane, we would get an eclipse at every

New Moon (solar) and Full Moon (lunar).

 But the Moon’s orbit is tilted by about 5°, so most months the Moon passes slightly above or below

the Sun (for solar eclipses) and misses Earth’s shadow (for lunar eclipses).

 Eclipses tend to happen during “eclipse seasons,” when the Sun is near the points where the

Moon’s orbit crosses Earth’s orbit plane.

Solar Eclipse Shadows: Umbra and Penumbra

 Umbra: the darkest inner shadow. If you are inside it, the Sun can be completely blocked (total

solar eclipse).

 Penumbra: the lighter outer shadow. If you are inside it, only part of the Sun is blocked (partial

eclipse).

 A total solar eclipse is local because the Moon’s umbra makes a narrow path on Earth.

Types of Solar Eclipses

1. Partial solar eclipse — you are in the penumbra, so only part of the Sun is covered.

2. Total solar eclipse — the umbra reaches you and the Sun’s bright face is fully covered (this is called

totality).

3. Annular solar eclipse — the Moon is farther from Earth and looks slightly smaller than the Sun, so

a bright “ring of fire” remains.

What You Can See During a Total Solar Eclipse

 Totality: when the Moon completely covers the Sun’s bright face (photosphere).

 Corona: the Sun’s faint outer atmosphere becomes visible during totality (usually washed out by

the bright Sun).

 Baily’s beads: tiny bright points of sunlight that shine through valleys on the Moon’s edge right

before/after totality.

 Diamond ring effect: when one very bright bead remains, making the eclipse look like a glowing

‘diamond’ on a ring.

How long does totality last?

 Totality can be very short — from seconds to about 7.5 minutes (maximum).

Lunar Eclipses (Why the Moon Can Turn Red)

 During a lunar eclipse, the Moon moves into Earth’s shadow.

 A lunar eclipse can be seen by anyone on the night side of Earth who can see the Moon (so it’s

visible to a much larger area than a solar eclipse).

 The Moon can look orange/red because Earth’s atmosphere scatters away more blue light and

bends more red/orange light into Earth’s shadow.

How long can a lunar eclipse last?

 A total lunar eclipse can last much longer than a total solar eclipse — often around an hour or

more, and it can approach about two hours of totality.

Safety Rules (Very Important!)

 Never look directly at the Sun during a partial or annular solar eclipse without proper solar

viewing protection (eclipse glasses/filters).

 It is only safe to view a total solar eclipse with your eyes during the brief time of totality (when the

Sun’s bright face is fully covered).

 Homemade filters (CDs/DVDs, exposed film, etc.) are not safe.

 Lunar eclipses are safe to watch with the naked eye.

Extra Facts & History

 Ancient Greeks used the curved Earth-shadow on the Moon during lunar eclipses to reason that

Earth is a sphere.

 Scientists can measure that the Moon is slowly moving away from Earth (about 3.8 cm per year)

using laser ranging. Over very long times, this changes how eclipses look.

Quick Revision Questions

4. 1) What is the difference between a solar eclipse and a lunar eclipse?

5. 2) Why don’t we get a solar eclipse every New Moon?

6. 3) What are the umbra and penumbra?

7. 4) What is an annular eclipse?

8. 5) Why does the Moon sometimes look red during a lunar eclipse?

9. 6) Why is a total solar eclipse only visible along a narrow path on Earth?

10. 7) When is it safe to look at a solar eclipse without special eye protection?

Answers

 Solar: Moon blocks Sun and its shadow falls on Earth. Lunar: Earth blocks sunlight and its shadow

falls on the Moon.

 The Moon’s orbit is tilted about 5°, so the Moon usually passes above/below the Sun from our

viewpoint.

 Umbra = darkest shadow (can cause total eclipse). Penumbra = lighter shadow (partial eclipse).

 When the Moon looks smaller than the Sun, leaving a bright ring (“ring of fire”).

 Earth’s atmosphere scatters blue light and bends red/orange light into the shadow, making the

Moon look reddish.

 The Moon’s umbra is small where it hits Earth, so only places in that narrow shadow path see

totality.

 Only during totality of a total solar eclipse; otherwise you need proper solar filters.