Unit 14: The Earth-Moon-Sun System
The three-body geometry behind the sky's most familiar sights: why the Moon changes shape, why eclipses are rare, and why the seasons happen at all.
7 topics · 24 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Why the Moon Shows Phases
- How it worksHow does sunlight hitting only half the Moon combine with the Moon's changing angle to Earth to create every phase?
- How it worksWalk me through watching the Moon orbit Earth from above while also watching what each phase looks like from the ground below.
- Common mistakeWhy isn't a moon phase caused by Earth's shadow falling on the Moon?
- Going furtherWhat phase is the Moon in if it rises around sunset and sets around sunrise?
- Worked exampleFind how many days into its cycle the Moon is if it's three-quarters of the way from one new moon to the next.
Your turn1 of 2
A full lunar cycle from one new moon to the next lasts about 30 days. If the Moon is three-quarters of the way through that cycle, about how many days into the cycle is it?The Moon's Synchronous Rotation and the Far Side
- How it worksHow does tidal locking sync the Moon's rotation period exactly to its orbital period around Earth?
- Common mistakeWhy isn't the Moon's far side a permanent 'dark side' that never gets any sunlight?
- Going furtherWhat would change about the phases we see if the Moon rotated twice for every orbit instead of once?
Your turn1 of 2
No matter when you look up at the Moon, you always see the identical pattern of craters and dark plains on its face. How does tidal locking sync the Moon's rotation period exactly to its orbital period around Earth?Why Eclipses Don't Happen Every Month
- How it worksHow does the Moon's orbit being tilted about 5 degrees from Earth's orbit usually carry its shadow above or below Earth?
- How it worksWalk me through the 3D geometry of the Moon's tilted orbital plane crossing Earth's orbital plane at only two points a year.
- Ties togetherHow do those two crossing points, called the lunar nodes, decide the handful of dates each year when an eclipse can happen?
Your turn1 of 2
The Moon passes roughly between Earth and the Sun every month at the new moon, yet a solar eclipse happens only a couple of times a year. How does the Moon's orbit being tilted about 5 degrees from Earth's orbit usually carry its shadow above or below Earth?Solar Eclipses
- How it worksHow does the Moon's shadow, only about 100 miles wide, sweep across Earth's surface during a total solar eclipse?
- Worked exampleFind why a total solar eclipse is even possible, given the Sun is about 400 times wider than the Moon but also 400 times farther away.
- Common mistakeWhy isn't a solar eclipse the same kind of event, seen everywhere the same way, as a lunar eclipse?
Your turn1 of 2
The Sun's diameter is about 1.39 million km and it sits about 149.6 million km from Earth. A hypothetical moon has a diameter of 3393 km and orbits at a distance of 397349 km from its planet, which also sits 149.6 million km from the Sun. What is the ratio of the Sun's angular size to that moon's angular size as seen from the planet?Lunar Eclipses
- How it worksHow does Earth's shadow fall across the Moon during a lunar eclipse, unlike the Moon's tiny shadow during a solar eclipse?
- How it worksWalk me through sunlight bending through Earth's atmosphere and filtering to red light that reaches the eclipsed Moon.
- Ties togetherHow does a lunar eclipse being visible from an entire hemisphere at once connect to the size of Earth's shadow versus the Moon's?
Your turn1 of 2
A full moon slowly darkens during an eclipse and turns a deep, coppery red rather than disappearing into blackness. Why does the full moon turn a deep red color during a total lunar eclipse instead of just going dark?What Really Causes the Seasons
- How it worksHow does Earth's tilted axis point the Northern Hemisphere toward the Sun for half the year and away for the other half?
- How it worksWalk me through Earth orbiting the Sun with its axis tilted at a constant 23.5 degrees the entire way around.
- Common mistakeWhy doesn't Earth's changing distance from the Sun cause the seasons?
- Worked exampleFind how much closer Earth actually is to the Sun during Northern Hemisphere winter than during Northern Hemisphere summer.
Your turn1 of 2
Earth's distance from the Sun is about 147 million km during Northern Hemisphere winter and about 151 million km during Northern Hemisphere summer. By what percentage is Earth closer to the Sun in winter than in summer?Why Distance From the Sun Doesn't Explain the Seasons
- How it worksHow does a more direct angle of sunlight in summer deliver more energy per square meter than that same sunlight spread out in winter?
- Going furtherWhat would the seasons look like on a planet with zero axial tilt at all?
- Ties togetherHow does axial tilt changing the sun's angle explain the seasons better than Earth's nearly circular orbit ever could?
Your turn1 of 2
Direct summer sunlight strikes the ground at close to 74 degrees, while the same sunlight in winter strikes at a shallower 33 degrees, spreading over a larger area. Using intensity proportional to the sine of the strike angle, about how many times more intense is the summer sunlight per square meter compared to the winter sunlight?