Eclipses — Solar and Lunar

Core Idea

An eclipse occurs when the Sun, Moon, and Earth line up so that one body casts its shadow on another. In a solar eclipse, the Moon passes between the Sun and Earth, casting its shadow on Earth's surface and briefly blocking the Sun for observers in that shadow. In a lunar eclipse, Earth passes between the Sun and Moon, and Earth's shadow falls on the Moon, often giving it a reddish color. Eclipses do not happen every month because the Moon's orbit is tilted about 5 degrees relative to Earth's orbit around the Sun, so the three bodies usually do not line up precisely enough.

How It's Best Learned

Use a lamp (Sun), basketball (Earth), and tennis ball (Moon) in a darkened room to demonstrate both types of eclipses by moving the tennis ball into the light path. This hands-on model makes the geometry clear immediately. Show videos of total solar eclipses (the corona becoming visible, daytime darkness) and lunar eclipses (the Moon slowly turning red). Discuss why the Moon appears almost exactly the same size as the Sun in the sky — a coincidence of distance and diameter that makes total solar eclipses possible.

Common Misconceptions

• Solar eclipses are dangerous because the Sun is more powerful during one. The Sun is no different — the danger is that during partial phases, people look directly at the Sun when they normally would not.
• Lunar eclipses only happen during a full moon. This is actually true (they require a full moon), but students often confuse which phase goes with which eclipse. Solar eclipses happen during a new moon.
• Eclipses happen every month. The Moon's orbital tilt of about 5 degrees means that most new moons and full moons pass above or below the alignment needed for an eclipse.

Explainer

Eclipses are among the most spectacular events in astronomy, and they happen because of simple geometry — the Sun, Moon, and Earth occasionally line up precisely enough for one to cast a shadow on another.

A solar eclipse happens when the Moon passes directly between the Sun and Earth during a new moon. The Moon's shadow falls on a small area of Earth's surface, and observers in that shadow see the Sun partially or totally blocked. A total solar eclipse — when the Moon completely covers the Sun — is breathtaking: the sky goes dark in the middle of the day, stars become visible, and the Sun's outer atmosphere (corona) glows around the Moon's dark silhouette. This is only possible because of an extraordinary coincidence: the Sun is about 400 times larger than the Moon but also about 400 times farther away, so they appear almost exactly the same size in our sky. The Moon's shadow on Earth is small (only about 100 miles wide during a total eclipse), which is why total solar eclipses are rare at any given location — typically once every few hundred years.

A lunar eclipse happens when Earth passes between the Sun and Moon during a full moon. Earth's shadow falls on the Moon, and the Moon darkens. But something beautiful happens during a total lunar eclipse — instead of going completely dark, the Moon turns a deep reddish color. This is because Earth's atmosphere bends some sunlight around the planet's edge and filters it. The atmosphere scatters away blue light (the same reason the sky is blue) but lets red light pass through. This dim red light reaches the Moon and illuminates it — the Moon is literally lit by the light of every sunrise and sunset on Earth at once. This is why total lunar eclipses are sometimes called "blood moons."

If the Moon orbits Earth every month, why do not we get eclipses every month? Because the Moon's orbit is tilted about 5 degrees relative to Earth's orbit around the Sun. Most months, the new moon passes slightly above or below the Sun from our perspective, and the full moon passes slightly above or below Earth's shadow. Eclipses only occur when the Moon happens to be at a node — one of the two points where its tilted orbit crosses Earth's orbital plane — at the same time it is in the new moon or full moon phase. This alignment happens 2 to 5 times per year, producing a mix of solar and lunar eclipses.

One final difference: lunar eclipses are visible from anywhere on the night side of Earth (because the Moon itself is changing appearance), so they are relatively common to observe. Solar eclipses are visible only from the narrow path of the Moon's shadow, making them much rarer at any specific location. This is why total solar eclipses draw people from around the world to the path of totality — it is a once-in-a-lifetime event for most places.