The Question
Every month, the Moon goes through a predictable cycle of shapes—from a thin crescent to a full circle and back again. These phases have been used to mark time since the earliest human civilizations. But the Moon doesn't actually change shape. So what is causing this monthly transformation in the sky?
Detailed Explanation
The Moon does not produce its own light—it shines by reflecting sunlight. At any given moment, exactly half of the Moon's surface is illuminated by the Sun. The phases of the Moon are simply the result of our changing viewing angle of this illuminated half as the Moon orbits the Earth over the course of about 29.5 days (the synodic month). When the Moon is between the Earth and the Sun (new moon), the illuminated half faces away from us, and we see only the dark side—the Moon is invisible. As the Moon moves in its orbit, we begin to see a sliver of the illuminated side—the waxing crescent. When the Moon has moved a quarter of the way around its orbit, we see half of the illuminated side—the first quarter moon. As it continues, we see more and more of the illuminated side (waxing gibbous) until the Moon is on the opposite side of the Earth from the Sun. Now the illuminated half faces directly toward us—the full moon. As the Moon continues its orbit, we see progressively less of the illuminated side (waning gibbous, last quarter, waning crescent) until it returns to the new moon position. The word "waxing" means growing (the illuminated portion we see is increasing), and "waning" means shrinking. The Moon always shows the same face to Earth because it is tidally locked—it rotates on its axis at exactly the same rate as it orbits the Earth.
Going Deeper
Solar and lunar eclipses are special events that occur when the Sun, Earth, and Moon are perfectly aligned. A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting its shadow on the Moon. During a total lunar eclipse, the Moon turns a deep red or orange color—a "blood moon"—because the only light reaching it has been filtered through Earth's atmosphere, which scatters away the blue light and lets the red light through (the same reason sunsets are red). A solar eclipse occurs when the Moon passes between the Earth and the Sun, blocking the Sun's light. Because the Moon and Sun appear almost exactly the same size in the sky (a remarkable coincidence—the Sun is 400 times larger but also 400 times farther away), the Moon can perfectly cover the Sun's disk during a total solar eclipse, revealing the Sun's corona—its outer atmosphere—which is normally invisible. The Moon's orbit is slightly tilted relative to the Earth's orbit around the Sun, which is why eclipses don't happen every month. The Moon's orbit intersects the plane of the Earth's orbit at two points called nodes, and eclipses can only occur when the Moon is near a node during a new or full moon. The pattern of eclipses repeats on an 18-year, 11-day cycle called the Saros cycle, which was known to ancient astronomers.
Did You Know?
The Moon is slowly moving away from the Earth at a rate of about 3.8 centimeters per year. Billions of years ago, the Moon was much closer and appeared much larger in the sky. In the far future, the Moon will be too far away to completely cover the Sun during a solar eclipse, and total solar eclipses will no longer be possible. We live in a special time in Earth's history when the Moon and Sun appear almost exactly the same size—a coincidence that has no known physical significance but has profoundly shaped human culture, religion, and science. Another fascinating fact is that the Moon has a very thin atmosphere called an exosphere, composed of atoms blasted off its surface by solar wind and micrometeorite impacts. It is so thin that the atoms rarely collide with each other—it is essentially a vacuum with a few stray atoms floating around.
