Look up on a few nights in a row and the Moon seems to change shape: a thin sliver, then a half, then a full circle, then back again. People have tracked this cycle for thousands of years; many calendars, including the 13 moons of many First Nations calendars, are built around it. The Moon does not make its own light. It shines because it reflects sunlight, and the Sun always lights exactly half of it, just as the Sun lights half of Earth (the daytime side). What changes is how much of that sunlit half faces us. The same three bodies, the Sun, Earth and Moon, also explain two other things you can observe: eclipses and tides.
Why the Moon has phases
The Moon orbits Earth about once a month. From one new moon to the next takes about 29.5 days. When the Moon is between Earth and the Sun, its lit half faces away from us and we see almost nothing: a new moon. As it moves around its orbit we see more and more of the lit half. While the lit part is growing, the Moon is waxing; while it is shrinking, the Moon is waning. Less than half lit is a crescent; more than half lit is gibbous. At first quarter and third quarter (also called last quarter) we see exactly half of the Moon's face lit; the name "quarter" means the Moon is a quarter of the way (or three quarters of the way) around its orbit. The Moon also spins exactly once per orbit, so the same side always faces Earth. The side we never see is the far side, and it gets just as much sunlight as the near side.
| Phase | Where the Moon is | Rises about | Best seen |
|---|---|---|---|
| New moon | Between Earth and the Sun | Sunrise | Not visible (lost in the Sun's glare) |
| First quarter | A quarter of the way around; right half lit (from Canada) | Noon | Afternoon and evening |
| Full moon | On the opposite side of Earth from the Sun | Sunset | All night |
| Third (last) quarter | Three quarters of the way around; left half lit (from Canada) | Midnight | Late night and morning |
Solar and lunar eclipses
An eclipse happens when one body moves into the shadow of another. A shadow has two parts: the umbra, the dark centre where the light source is completely blocked, and the penumbra, the lighter outer part where it is only partly blocked. In a solar eclipse, the Moon passes between the Sun and Earth, so this can only happen at new moon. People inside the Moon's small umbra see a total solar eclipse: the sky darkens and the Sun's corona appears. People in the penumbra see a partial eclipse. On April 8, 2024, a total solar eclipse crossed parts of southern Ontario, including Niagara Falls and Kingston. In a lunar eclipse, Earth passes between the Sun and Moon, and the Moon moves into Earth's shadow, so this can only happen at full moon. During a total lunar eclipse the Moon often turns a coppery red, because Earth's atmosphere bends some red sunlight into the shadow.
- Order: Sun, Moon, Earth
- Only at new moon
- The Moon's shadow falls on Earth
- Total eclipse seen only along a narrow path, for no more than about 7.5 minutes
- Needs certified eclipse glasses except during totality
- Order: Sun, Earth, Moon
- Only at full moon
- Earth's shadow falls on the Moon
- Seen from the whole night side of Earth; totality can last over an hour
- Safe to watch with your eyes
Tides
Tides are the regular rise and fall of the ocean surface. The Moon's gravity pulls on all of Earth, but it pulls a little harder on the side facing the Moon and a little less on the far side. This difference stretches the oceans into two bulges: one facing the Moon and one on the opposite side. As Earth spins, a coastline passes through both bulges, so most places get two high tides and two low tides about every 24 hours and 50 minutes. High tides come about 12 hours 25 minutes apart, and about 50 minutes later each day, because the Moon moves along its orbit while Earth spins.
The Sun raises tides too, but only about half as strongly as the Moon, because even though the Sun is far more massive, it is much farther away. At new moon and full moon, the Sun, Earth and Moon are lined up and their tidal pulls add together. This gives spring tides, with the highest highs and lowest lows. (The name has nothing to do with the season; it comes from the water "springing" up.) At first and third quarter, the Sun and Moon pull at right angles and partly cancel, giving neap tides, with the smallest difference between high and low water. The Bay of Fundy, between New Brunswick and Nova Scotia, has the highest tides in the world: its funnel shape makes the water rise and fall by up to about 16 m.