Case study · US

Full Moon, Yosemite

Yosemite Valley

Photographers wait for a full Moon to clear Half Dome. They are waiting on geometry, not weather.

A waning Moon — the same phase everywhere that night. — Phase is a viewing angle. A valley does not change the Moon.NASA / Wikimedia, public domain

Locator · OpenStreetMap / OpenFreeMap · 37.74°, -119.57°

Full Moon plus a node crossing would be a lunar eclipse. Most months, the Moon misses Earth’s shadow by 5° of orbital tilt. The same phase is up in Snowdonia that night. Weather is local. Phase is not.

Open this state in the lab

Exam-style question

Why is a full Moon necessary but not sufficient for a lunar eclipse?

Teacher hint: Full = opposite the Sun. Eclipse also needs the Moon near a node of its 5° tilt.

Process labs

Moon phases and Sun–Earth–Moon geometry

Sources

NASA / NPS