The moon often appears different from one evening to the next, with its size, shape, and even color seeming to shift. These variations are not random, but result from predictable astronomical phenomena and optical effects.
The Moon’s Changing Phases
The most noticeable transformation in the moon’s appearance is its changing shape, known as its phases. The amount of its sunlit surface visible from Earth changes as it orbits our planet. Sunlight consistently illuminates one half of the moon, but our perspective shifts as the moon travels through its approximately 29.5-day cycle.
When the moon is between the Earth and the sun, its sunlit side faces away from us, resulting in the “new moon” phase, where it is largely invisible. As it progresses in its orbit, a sliver of its illuminated surface becomes visible, growing into a crescent. This visible portion continues to increase through the quarter and gibbous phases until the entire sunlit side faces Earth, creating the “full moon.” After the full moon, the illuminated portion gradually diminishes, returning through gibbous, quarter, and crescent phases until it once again becomes a new moon.
Varying Distance from Earth
Beyond its changing phases, the moon can appear larger or smaller due to its elliptical, or oval-shaped, orbital path. This means the moon is closer to Earth at certain points in its orbit, and farther away at others.
The closest point in the moon’s orbit to Earth is called perigee, while the farthest point is known as apogee. When a full moon occurs near perigee, it can appear noticeably larger and brighter than usual, a phenomenon sometimes referred to as a “supermoon.” Conversely, when the moon is near apogee, it appears smaller in the sky. The difference in apparent size between these two extremes can be around 14 percent, and its brightness can vary by approximately 30 percent.
Impact of Earth’s Atmosphere
The Earth’s atmosphere also affects the moon’s color and brightness. When the moon is low on the horizon, its light travels through a greater amount of atmospheric particles. These particles scatter shorter wavelengths of light, such as blue and violet, more effectively than longer wavelengths like red and orange.
This scattering effect is similar to what causes sunsets to appear red or orange. Consequently, when the moon is near the horizon, more of its blue light is scattered away, allowing the red and orange light to pass through, giving the moon a reddish or yellowish hue. Furthermore, atmospheric conditions such as humidity, haze, or clouds can make the moon appear dimmer or less distinct, as they absorb or scatter moonlight.
The Moon Illusion
The moon often appears strikingly large near the horizon, yet much smaller when higher in the sky. This “moon illusion” is an optical trick of the brain, not a physical change in the moon’s actual size. The moon’s angular size, or how much space it takes up in the sky, remains nearly constant regardless of its elevation.
Scientists propose several theories to explain this perceptual anomaly. One idea suggests our brains compare the moon to foreground objects like trees and buildings, making it seem larger when these familiar objects are present. Another theory posits that our brains perceive the sky as a flattened dome, making objects near the horizon appear farther away and larger than objects at the zenith. This illusion highlights how perception influences what we see in the night sky.