You'll see that the ball shows exactly the same phase as the Moon. Find the Moon in the southern part of the sky, then hold the ball up at arm's length right beside it. Then head outside about an hour before sunset, or around the time of a first-quarter Moon. All you'll need is a Ping-Pong ball to simulate the Moon-actually, any small, white sphere would work. The Moon's phases are actually related to orbital motion, and there's a simple and fun observation that shows how they're connected. If you have a hard time remembering which way the moon phases go, just think: “white on right, getting bright!” The Ping Pong Perspective Holding a white ball at arm's length in the direction of the Moon shows how lunar phases depend on where the Moon is in the sky with respect to the Sun. The whole cycle (from new Moon to new Moon) takes about 29.5 days. The slice of sunlight continues to decrease until the moon is a waning crescent and then a new Moon. Then, after the full Moon the sunlit fraction begins to decrease again (though it still takes up more than half the face of the Moon) to make a waning gibbous and then a third-quarter Moon. As the sunlit portion of the Moon continues to increase to more than half of the Moon's face, the Moon turns waxing gibbous. The lunar crescent grows until first-quarter Moon. Waxing means “growing” or expanding in illumination, and waning means “shrinking” or decreasing in illumination.Īfter new Moon, a slice of reflected sunlight becomes visible as a waxing crescent. Crescent refers to phases where the Moon is less than half-illuminated, while gibbous means more than half is illuminated. To remember the in-between phases you'll need to understand these terms: crescent, gibbous, waxing, and waning. The illuminated part of the Moon gradually transitions between these phases. The "quarter" used to name these phases refers to the respective fraction of an orbit that the Moon has completed since new Moon. We see exactly half of the Moon illuminated by the Sun - the other half lies in shadow. Since the Sun is behind the Moon from Earth's perspective, the side of the Moon that faces Earth is dark.Īt full Moon, the three bodies also lie approximately in a line, but this time, the Moon is on the opposite side of Earth, so the Sun illuminates the whole side facing us.Īt first quarter and last quarter, the Moon lies perpendicular to a line between Earth and the Sun. The new Moon occurs when the Moon, Earth, and Sun all lie along approximately the same line. Since the Moon is tidally locked, we always see the same side from Earth, but there's no permanent "dark side of the Moon." The Sun lights up different sides of the Moon as it orbits around Earth – it's the fraction of the Moon from which we see reflected sunlight that determines the lunar phase. The Sun always illuminates one half of the Moon. The Moon doesn't make its own light, it just reflects the Sun's light as all the planets do. Instead, the Moon's phase depends only on its position relative to Earth and the Sun.įind tonight's Moon phase with our Interactive Phases of the Moon Tool! These are common misconceptions, but they're not true. Others think that the Moon changes shape due to clouds. Some people mistakenly believe the phases come from Earth's shadow cast on the Moon. We see the Moon go through a changing cycle of phases each month due to its orbital motion around Earth and the changing geometry with which we view it.īefore we describe the phases of the Moon, let's describe what they're not.
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