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The Lunar Cycle

Humans throughout history have been treated to the same month-long lunar light that modern humans enjoy. In this show, known as the lunar cycle, the moon appears to grow larger every day for 14 days until no part of its face is hidden, and then it shrinks for another 14 days until it's completely invisible. The show happens because the moon revolves around the Earth while the sun remains stationary, as seen from Earth's surface.

Orbital Dynamics
- The moon completes an orbit of the Earth once every 27.32 days as seen from the stars, but from the Earth, the time between identical phases is longer -- 29.53 days. This is true because the Earth is also orbiting the sun. Each day, when the moon is visible, it's in a slightly different position in the sky, and people see the sunlight reflected from its surface from a different angle. From the Earth's surface, you see the most reflected sunlight when it's on the opposite side of the Earth from the sun and the least when it's on the same side.
The Waxing Phase
- The moment when the moon is between the Earth and the sun and is invisible to people on Earth is the new moon. As seen from the stars, the moon moves 13.2 degrees in its orbit every day, and each day, an increasing amount of its reflected light is visible. As it approaches its first quarter, which occurs about a week after new moon, it's in the waxing crescent phase, and for one week after that, it's waxing gibbous. Because of the growing amount of light, gardeners have traditionally considered the new moon to be the best time to plant crops.
The Waning Phase
- As the luminance of the moon fades in intensity after the moment of the full moon -- when it's fully illuminated -- it passes through the waning gibbous and waning crescent phase. The moon is on the opposite side of the Earth from the sun when it's full, so it rises at sunset. Each night after that, it rises about 27 minutes later. Consequently, the waning moon is often visible during the day on clear days with little cloud cover. Because the light of the moon is fading, gardeners have often considered the waning phase to be the best time for pruning, transplanting and harvesting crops.
Lunar Phases and Eclipses
- Solar eclipses are possible when the moon is new, and lunar eclipses happen when it's full. They don't happen every month because the plane of the moon's orbit is tilted by 5 degrees. The new and full moon coincide with the passage of the moon through one of its two nodes -- which are the times when it lines up with the sun and the Earth -- only once or twice every year. Solar and lunar eclipses happen in pairs because the moon is close to a node for more than 14 days, which is the time span between new and full moon.

Astronomy