The Moon’s appearance in our night sky undergoes a regular, predictable transformation known as lunar phases. These changing shapes, from a slender crescent to a full, luminous disk, repeat approximately every 29.5 days, a cycle that has captivated observers for millennia.
The Source of Moonlight
The Moon does not generate its own light. The glow we see is sunlight reflecting off its surface. The Sun always illuminates one half of the Moon, creating a day and night side, similar to Earth. The amount of this sunlit portion visible from Earth changes, giving rise to the different phases.
The Moon’s Orbital Journey
The Moon’s changing appearance results from its continuous orbit around Earth. As the Moon travels, its position relative to both Earth and the Sun constantly shifts. This alters the angle at which we view the Moon’s illuminated half from Earth.
When the Moon is positioned between the Sun and Earth, the side facing us is largely unlit, resulting in the new moon. As it progresses in orbit, more of its sunlit surface becomes visible. Conversely, when Earth is between the Moon and the Sun, we see the Moon’s entire illuminated side, appearing as a full moon. The varying fractions of the Moon’s illuminated side visible create the distinct phases.
Observing the Lunar Cycle
The lunar cycle includes eight primary phases, each representing a different stage of illumination as seen from Earth. It begins with the new moon, largely invisible because its illuminated side faces away from Earth. The waxing crescent then appears as a slim, growing sliver of light. The first quarter moon shows half of its face lit, signifying it has completed a quarter of its orbit.
Illumination increases during the waxing gibbous phase, where more than half of the Moon is visible but not yet full. At the full moon, the entire side facing Earth is brightly illuminated. After the full moon, the lit portion begins to shrink, entering the waning gibbous phase, where it is still more than half lit but decreasing.
The last quarter moon, also a half-moon, shows the opposite side illuminated compared to the first quarter. The cycle concludes with the waning crescent, a diminishing sliver of light, before returning to the new moon.
Distinguishing Phases from Eclipses
A common misunderstanding is that Earth’s shadow causes the Moon’s phases. However, lunar phases result from our changing perspective of the Moon’s sunlit surface as it orbits Earth. The Moon is always half-illuminated by the Sun; what changes is how much of that illuminated half we can see.
Eclipses are distinct, less frequent celestial events involving the alignment of the Sun, Earth, and Moon. A solar eclipse occurs when the Moon passes between the Sun and Earth, casting a shadow on Earth and blocking the Sun’s light. A lunar eclipse happens when Earth passes between the Sun and Moon, casting Earth’s shadow onto the Moon. The Moon’s orbit is tilted by approximately five degrees relative to Earth’s orbit around the Sun, which prevents these perfect alignments from occurring every month.