The changing appearance of the Moon as it orbits Earth is described by the lunar phases, a continuous cycle that repeats approximately every 29.5 days. This shift in visible illumination is caused by the changing angles between the Sun, Earth, and Moon. The specific term used to describe the period when we see more and more of the Moon’s sunlit surface is “waxing.”
Defining the Term for Increasing Light
The word “waxing” means to grow or increase, and in astronomy, it precisely defines the half of the lunar cycle where the illuminated fraction of the Moon’s face expands nightly. This period begins immediately after the New Moon, when the Moon is essentially invisible from Earth, and continues until the Full Moon. The New Moon occurs when the Moon is positioned between the Earth and the Sun, causing the side facing us to be unlit.
As the Moon moves along its orbit, it shifts away from the Sun’s position, allowing sunlight to catch more of the surface we can see. The illuminated portion grows larger each successive night. This increasing visibility is a result of the Moon’s orbital mechanics and the reflection of solar light. The entire waxing period spans about 14.75 days, culminating in the Full Moon when nearly 100% of the Earth-facing side is illuminated.
Understanding the Opposite Cycle
The counterpart to the increasing light cycle is known as “waning,” which refers to the period when the visible illuminated portion of the Moon decreases. The waning cycle begins immediately after the Full Moon and concludes with the return to the New Moon phase. During this time, the Moon’s position relative to the Sun and Earth causes less of its sunlit side to be visible to us each night.
The word “waning” signifies a decrease or decline, reflecting the visual change as the bright circle of the Full Moon gradually appears to shrink. This reduction in visible light continues until the Moon is once again situated between the Earth and the Sun, returning to the New Moon phase. The duration of the waning period is also approximately 14.75 days.
Naming the Specific Shapes of the Moon
Within the overall waxing cycle, three distinct shapes are named to describe the degree of illumination.
The first is the Waxing Crescent, which appears after the New Moon and describes the phase where less than half of the Moon’s face is visible as a thin, curved sliver. This shape is seen when the Moon is between 1% and 49% illuminated.
Following the crescent phase is the First Quarter, which occurs when exactly half of the Moon’s face is illuminated, resembling a “half-moon.” This means the Moon has completed one-quarter of its full orbit, resulting in a 90-degree angle between the Sun, Earth, and Moon.
The final shape before the peak is the Waxing Gibbous, which occurs when more than half but not all of the Moon is lit. The term “gibbous” means hump-backed, describing the bulging shape that is fuller than a half-circle but not yet a complete disc. This phase spans the period where the illumination is between 51% and 99%.