An eclipse is one body moving into another body's shadow. A solar eclipse is the Moon's shadow landing on Earth; a lunar eclipse is the Moon entering Earth's shadow.
Step 1: Something to Notice
Watch first. The explanation comes later.
There is a new moon every 29.5 days, and at a new moon the Moon is between us and the Sun. Yet a total solar eclipse is visible from any given place roughly once every 375 years.
The alignment happens every month. Why does the eclipse not?
Step 2: Find Out
Set the tilt to 0 and run a month. Then set it back to 5.1 and run the same month again.
Step 1 — Predict
With the tilt set to zero, how many eclipses would there be in one month?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: Find Out
Turn the tilt down slowly and find the value where eclipses start happening.
Step 1 — Predict
The Moon has to come within about one Earth-radius of the line. Roughly what tilt would allow that at this point in the orbit?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 4: So Here Is Why
Now the explanation, after you have seen it happen.
The orbit is not flat
The Moon's orbit is tilted about 5.1° from the plane of Earth's orbit around the Sun. So at most new moons the Moon passes above or below the Sun, not in front of it.
Orbital tilt The angle between the Moon's orbit and Earth's orbit around the Sun.
Two crossing points
The two orbits cross at two points, called nodes. An eclipse needs a new or full moon that happens near a node.
Node One of the two points where the Moon's orbit crosses Earth's orbital plane.
Eclipse seasons
That double condition is why eclipses arrive in clusters about six months apart rather than monthly.
Rarer still where you live
The Moon's shadow is only about 100 miles wide where it touches Earth, so a total solar eclipse is visible from any one town roughly once in 375 years. A lunar eclipse is visible from the whole night side at once, which is why you have probably seen more of those.
Why not every month?
The Moon's orbit is tilted about five degrees from Earth's orbital plane, so most months the shadow passes above or below. Eclipses need the three bodies nearly in a line, which happens only occasionally.
Two kinds, and which is rarer to see
A lunar eclipse is visible from the whole night side of Earth. A solar eclipse casts a narrow shadow track, so any one place sees one rarely — which is why totality is such an event.
Step 5: A Common Mistake
Lots of people think
“If a new moon is directly between Earth and the Sun, there should be a solar eclipse every month.”
Step 6: Why Totality Is a Coincidence
The same idea somewhere new.
The Moon is about 400 times smaller than the Sun and currently about 400 times closer, so the two appear almost exactly the same size from Earth. That is why a total solar eclipse just barely covers the Sun's disc and no more. It is not a law of anything — the Moon is drifting away from us at about 3.8 cm a year, and in roughly 600 million years the last total eclipse will happen.
Step 7: Think It Through
Practice makes it stick.
The Miss
Problem 1 of 2
At a typical new moon the Moon passes about 4.7 Earth-radii off the Sun-Earth line. Earth's radius is about 4,000 miles. About how many thousand miles off the line is that? Give your answer in thousands of miles.
thousand miles
Eclipse Seasons
Problem 2 of 2
Eclipse seasons happen when the Sun lines up with one of the two nodes. There are two nodes. About how many months apart are consecutive eclipse seasons?
months
Alignment and Shadow
1 of 4
A solar eclipse can only happen at which phase?
2 of 4
A lunar eclipse can only happen at which phase?
3 of 4
If the Moon's orbit had no tilt at all, how many eclipses would there be each month?
4 of 4
Match each condition to what it rules out.
Tap a card on the left to start.
Step 8: Quick Check
Show what you know.
Question 1 of 1
Why is there not a solar eclipse at every new moon?
Step 9: Explain It in Writing
Claim, evidence, then reasoning.
The question
A classmate reasons: "At every new moon the Moon is between Earth and the Sun, so there should be a solar eclipse every month." Their reasoning is careful and their premise is correct. Explain what they are missing, using evidence from the model.
Words to useorbital tiltnew moonEarth-radiinodeshadowalignment
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- An eclipse is one body entering another body's shadow.
- The Moon's orbit is tilted 5.1°, so most new moons pass above or below the Sun.
- An eclipse needs a new or full moon that also happens near a node.
- That double condition is why eclipses come in seasons, not months.