Lunar eclipses are always eclipses of a full Moon. That is,...

Name: Problem 86, Chapter 26: Conceptual Physics
Uploaded: 2019-09-09T23:18:59-08:00
Duration: 1 min 56 s
Channel: Averell Hause
Description: Lunar eclipses are always eclipses of a full Moon. That is, the Moon is always seen full just before and after Earth's shadow passes over it. Why is this? Why can't we ever have a lunar eclipse when the Moon is in its crescent or half-moon phase?

Lunar eclipses are always eclipses of a full Moon. That is, the Moon is always seen full just before and after Earth's shadow passes over it. Why is this? Why can't we ever have a lunar eclipse when the Moon is in its crescent or half-moon phase?

Labs

Want to see this concept in action?

NEW

Explore this concept interactively to see how it behaves as you change inputs.

View Labs

Key Concepts

Lunar Phases

Lunar phases result from the changing relative positions of the Moon, Earth, and Sun. The full Moon phase occurs when the Moon is on the opposite side of the Earth from the Sun, making its face fully illuminated as seen from Earth. This configuration is critical for a lunar eclipse because it aligns the Moon behind the Earth relative to the Sun.

Orbital Alignment and Geometry

Lunar eclipses are a product of the specific alignment of the Sun, Earth, and Moon. An eclipse can only occur when the Earth is directly between the Sun and the Moon, causing the Earth's shadow to fall on the Moon. This precise geometric alignment only happens when the Moon is full, not during other phases like crescent or half-moon.

Earth's Shadow

The Earth casts a shadow that has a darker central region (the umbra) and a lighter outer region (the penumbra). During a lunar eclipse, the Moon must pass through the umbra for a total eclipse to occur. Since the full Moon is positioned directly opposite the Sun, it is the only phase that can be fully immersed in the Earth's shadow, which is why eclipses do not happen during crescent or half-moon phases.

Transcript

Please give Ace some feedback