The Phases of the Moon: Understanding the Changes in its Appearance over Time

SPACE — The Moon, like Earth, is a sphere, and is always half illuminated by sunlight. As the moon moves around the Earth, we can see a full portion of its illuminated surface, and at other times only a small portion is visible.

The phase of the moon describes how much of the moon’s disk is illuminated by the sun from our perspective on earth. Of course, the phases of the moon reveal the passage of time in the night sky. Some nights a month, we can see the moon looks full and bright, and other days sometimes only a glimmer of silvery light. These changes in the appearance of the moon are called the phases of the moon.

As the moon orbits Earth, it cycles through eight different phases. The four main phases of the moon occur approximately every week, with the full moon being the most dazzling.

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New Moon (New moon): The moon is between the Earth and the sun, and the side of the moon that faces us does not receive direct sunlight. It is illuminated only by dim sunlight reflected from Earth.

Waxing Crescent (crescent): As the moon moves around the Earth, the side we can see is gradually brightened by direct sunlight. Earth’s shadow covers most of the moon’s surface, giving it a sickle-like shape.

First Quarter (First quarter): The moon is 90 degrees from the sun in the sky and half illuminated from our perspective. Astronomers call this the first quarter because the moon has traveled about a quarter of the way around Earth since the new moon started.

Waxing gibbous: The area directly lit by the sun is steadily increasing. More than half of the moon’s surface is exposed to sunlight.

Full Moon (Full Moon): The moon is 180 degrees from the sun and is as close to the surface of sunlight as we can see on earth. The sun, Earth, and moon are aligned, but the moon’s orbit isn’t in the exact same plane as Earth’s orbit around the sun, so it rarely forms a perfect line. However, when they form a perfect line, we will experience a lunar eclipse, which is when the Earth’s shadow crosses the face of the moon.

Waning gibbous: More than half of the moon’s surface appears to receive sunlight, but the amount is decreasing. This is the opposite of the crescent moon.

Last Quarter (Last quarter): The Moon has moved another quarter around the Earth, to its third quarter position. The sun’s light now illuminates the half of the moon’s face as seen from Earth.

Waning Crescent (Crescent crescent): Less than half of the moon’s surface appears to receive sunlight. The surface illuminated by the sun is decreasing.

Finally, the moon returns to its new moon position. That seat, the moon is between the Earth and the sun. Usually, the moon passes above or below the sun from our perspective. But sometimes, the moon passes directly in front of the sun, and it causes us to experience a solar eclipse.

Next, Moon phases in April 2023…