wunder beta🌙 Why the Moon changes shape
See why the Moon seems to change shape as sunlight, orbit, viewpoint, earthshine, eclipses, and lunar motion work together.
What you’ll learn
- The light never changes sidesExplain why sunlight and viewpoint, rather than a changing lunar body, create the phases.The Sun always illuminates half the Moon, while orbit and perspective change the visible fraction.
- Reading the eight phasesIdentify the eight named phases and connect each appearance to its position in the cycle.New, crescent, quarter, gibbous, and full phases describe a repeating sequence of illumination.
- From full to waningDistinguish ordinary waning phases, earthshine, and eclipses.After full Moon the bright portion wanes; earthshine and eclipses are additional lighting effects, not the cause of phases.
- What the cycle can tell youUse phase geometry to predict visibility and separate phase, distance, rotation, and libration.The cycle acts as a sky clock while orbital distance and synchronous rotation explain other apparent effects.
Questions this course answers
What causes the Moon’s phases?
The Sun always illuminates half of the Moon, but its orbit changes how much of that bright half faces Earth.
Why is a first-quarter Moon called quarter?
At first quarter, the Moon has traveled about a quarter of the cycle and appears half lit from Earth.
Why does a lunar eclipse not happen at every full Moon?
The Moon usually passes above or below the exact Earth-Sun plane during full Moon.
What is earthshine?
Earthshine is sunlight reflected from Earth onto the lunar night side and back toward observers.
Why do we usually see the same side of the Moon?
Synchronous rotation keeps nearly the same lunar hemisphere facing Earth.
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