"Earth's shadow falls on the Moon" is a real phenomenon — it's just called a lunar eclipse, and it only happens a couple of times a year, when the Sun, Earth, and Moon line up almost perfectly. Ordinary phases have nothing to do with it. The Moon always has a sunlit half and a dark half, exactly like Earth does at any given moment. As the Moon orbits Earth, the angle between the Sun, the Moon, and you keeps changing — so you see different amounts of that sunlit half from night to night.
Astronomers divide the roughly 29.5-day cycle into eight named phases. Starting from New Moon (the Moon between Earth and the Sun, its dark side facing us, essentially invisible), the illuminated sliver grows through Waxing Crescent, reaches half-lit at First Quarter, keeps growing through Waxing Gibbous, and peaks at Full Moon (Earth between the Sun and Moon, the whole near side lit). From there it shrinks back down through Waning Gibbous, Last Quarter, and Waning Crescent, before returning to New.
You don't need to remember eight names to know whether the Moon is growing or shrinking tonight — just look at which side is lit. For a Northern Hemisphere observer: if the right side is lit, the Moon is waxing (growing toward full); if the left side is lit, it's waning (shrinking toward new). The line between the lit and dark halves is called the terminator.
This is the exact same gap you already met in Project 1.1.3, just applied to a different orbit. The Moon's sidereal month — one full 360° orbit relative to the stars — takes about 27.3 days. But the synodic month — the time from one full moon to the next — takes about 29.5 days, roughly two days longer.
The reason is identical to the solar/sidereal day gap: while the Moon orbits Earth, Earth is also orbiting the Sun. By the time the Moon has gone all the way around once relative to the stars, the Sun's direction as seen from Earth has shifted slightly too — so the Moon has to travel a little bit farther in its orbit before it lines back up the same way relative to the Sun, which is what actually determines the phase.
Because the Moon moves noticeably along its own orbit — creeping eastward roughly its own width every hour — it rises about 50 minutes later each day than it did the day before. That daily delay, stacked over the whole cycle, means each phase has a rough, predictable rise and set time:
| Phase | Rises around... | Sets around... |
|---|---|---|
| New Moon | Sunrise | Sunset |
| First Quarter | Noon | Midnight |
| Full Moon | Sunset | Sunrise |
| Last Quarter | Midnight | Noon |
Once you know tonight's phase and the 29.5-day cycle length, predicting a future phase is mostly arithmetic. New to Full is about half the cycle — roughly 14.75 days. The tool below lets you scrub through an entire cycle to see how the numbers line up.
Drag through a full 29.5-day cycle starting from a new moon.
Tonight the Moon is a Waxing Crescent, about 4 days past new. How many nights until the next Full Moon?