Answer key: Moon Phases Worksheet
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Goal: Model why the Moon’s appearance changes during a 29.5-day cycle, and name and predict the eight phases.
Completed results table
| Setting | Illuminated | Phase |
|---|---|---|
| Day 0 | 0% | New Moon |
| Day 3.7 | 14.7% | Waxing crescent |
| Day 7.38 | 50% | First quarter |
| Day 11.1 | 85.6% | Waxing gibbous |
| Day 14.77 | 100% | Full Moon |
| Day 18.5 | 85% | Waning gibbous |
| Day 22.15 | 50% | Last quarter |
| Day 25.8 | 14.9% | Waning crescent |
| Today’s date: ____________ | Answers vary | Answers vary |
Answers
How many days does it take to go from one new Moon to the next? From new Moon to full Moon?
About 29.53 days for the whole cycle (a synodic month). New Moon to full Moon takes half of that, about 14.77 days.
At first quarter, what percentage of the Moon’s face is lit? Why is it called a “quarter” Moon?
50%: half of the face we see is lit. It is called first quarter because the Moon is a quarter of the way through its cycle (about 7.4 days after new Moon).
Days 3.7 and 25.8 have almost the same lit percentage. How are the two phases different?
Both are crescents about 15% lit (14.7% and 14.9%). Day 3.7 is a waxing crescent: the lit part is growing, and from the northern hemisphere the right side is lit. Day 25.8 is a waning crescent: the lit part is shrinking and the left side is lit.
Predict: tonight the Moon is at first quarter. What will the phase be in about one week? In about two weeks? Check with the slider.
In about a week (day 14.77) it will be a full Moon; in about two weeks (day 22.15) it will be at last quarter.
Many people think the phases are caused by Earth’s shadow. Use the model to explain what really causes them.
The Sun always lights half of the Moon. As the Moon orbits Earth, we see different amounts of that sunlit half. Earth’s shadow only falls on the Moon during a lunar eclipse, which can only happen at full Moon.
The Moon orbits Earth in 27.3 days. Why does the cycle of phases take longer (29.5 days)?
While the Moon goes around Earth, Earth also moves along its orbit around the Sun. The Moon needs about two more days to get back to the same position relative to the Sun, so the phase cycle is longer than one orbit measured against the stars.