why do we see different moon phases
Moon phases are determined by the relative positions of the Moon, Earth, and Sun. Before we describe the phases of the Moon, letÁs describe what theyÁre not. Some people mistakenly believe the phases come from EarthÁs shadow cast on the Moon. Others think that the Moon changes shape due to clouds. These are common misconceptions, but theyÁre not true. Instead, the MoonÁs phase depends only on its position relative to Earth and the Sun. The Moon doesnÁt make its own light, it just reflects the SunÁs light as all the planets do. The Sun always illuminates one half of the Moon. Since the Moon is tidally locked, we always see the same side from Earth, but thereÁs no permanent Ádark side of the Moon. Á The Sun lights up different sides of the Moon as it orbits around Earth Á itÁs the fraction of the Moon from which we see reflected sunlight that determines the lunar phase. The new Moon occurs when the Moon, Earth, and Sun all lie along approximately the same line. Since the Sun is behind the Moon from EarthÁs perspective, the side of the Moon that faces Earth is dark. At 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. At first quarter and last quarter, the Moon lies
perpendicular to a line between Earth and the Sun. We see exactly half of the Moon illuminated by the Sun Á the other half lies in shadow. The "quarter" used to name these phases refers to the respective fraction of an orbit that the Moon has completed since new Moon. The illuminated part of the Moon gradually transitions between these phases. To remember the in-between phases youÁll need to understand these terms: crescent, gibbous, waxing, and waning.
Crescent refers to phases where the Moon is less than half-illuminated, while gibbous means more than half is illuminated. Waxing means ÁgrowingÁ or expanding in illumination, and waning means ÁshrinkingÁ or decreasing in illumination. After new Moon, a slice of reflected sunlight becomes visible as a waxing crescent. The lunar crescent grows until first-quarter Moon. As the sunlit portion of the Moon continues to increase to more than half of the MoonÁs face, the Moon turns waxing gibbous. 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. The slice of sunlight continues to decrease until the moon is a waning crescent and then a new Moon. The whole cycle (from new Moon to new Moon) takes about 29. 5 days. If you have a hard time remembering which way the moon phases go, just think: Áwhite on right, getting bright! Á The MoonÁs phases are actually related to orbital motion, and thereÁs a simple and fun observation that shows how theyÁre connected. All youÁll need is a Ping-Pong ball to simulate the MoonÁactually, any small, white sphere would work. Then head outside about an hour before sunset, or around the time of a first-quarter Moon. Find the Moon in the southern part of the sky, then hold the ball up at armÁs length right beside it. YouÁll see that the ball shows exactly the same phase as the Moon. The Sun illuminates both the ball and the Moon from the same direction, and you see them as partly sunlit and partly in shadow, their bright and dark portions mimicking each other perfectly. If the weather stays clear, you can repeat this observation on the next several afternoons.
Each day the MoonÁs orbital motion has carried it farther east, and the sunlit portion of its disk has grown larger. If you hold your ball up near the Moon, youÁll see that its ÁphaseÁ has thickened too. To sneak a preview of the MoonÁs appearance in the days to come, simply move the ball farther east. And if you move it all the way over so your arm points low in the eastern sky, the side of the ball thatÁs facing you will be almost completely illuminated Á nearly a ÁFull Ball,Á so to speak. And, sure enough, a day or two before full Moon, the Moon hangs low in the eastern sky just before sunset and is almost completely illuminated. Try it out! To find out what phase the Moon is tonight, try our. Note that the MoonÁs phase is the same for any location on Earth, but Southern Hemisphere observers will see the Moon Áupside downÁ from the Northern Hemisphere view. Check out this table if youÁd like to know an estimate of the moonrise and moonset for each phase. Keep in mind that this is just an approximation meant to guide the casual observer, and there's no correction for daylight (or summer) time. The exact local time depends on a number of factors, including time zone, season, shape of horizon, atmospheric refraction, among other things. Take your Moon explorations one step further with the only map youÁll ever need:. Why does the moon seem to change its shape every night? Why can I see the moon in the daytime? The answer to both questions is the same. Itâs that the moon is a world in space, just as Earth is. Like Earth, the moon is always half illuminated by the sun; the round globe of the moon has a day side and a night side.
From our earthly vantage point, as the moon orbits around Earth, we see varying fractions of its day and night sides. These are the changing phases of the moon. And the moon is in the daytime sky about half the time. Itâs just that itâs sometimes itâs so near the sun we donât notice it. How can you understand moon phases? Here are four things to remember: moonth ) to orbit the Earth 1. When you see the moon, think of the whereabouts of the sun. After all, itâs the sun thatâs illuminating and creating the dayside of the moon. Moon phases depend on where the moon is with respect to the sun in space. For example, do you see which moon phase is being shown in the illustration above? The answer is, itâs a full moon. The moon, Earth and sun are aligned with Earth in the middle. The moonâs fully illuminated half â its dayside â faces Earthâs night side. Thatâs always the case on the night of a full moon. Donât just take our word for it. Go outside. No matter what phase of the moon you see in your sky, think about where the sun is. Itâll help you begin to understand why the moon you see is in that particular phase. 2. The moon rises in the east and sets in the west, each and every day. It has to. The rising and setting of all celestial objects is due to Earthâs continuous daily spin beneath the sky. Just know that â when you see a thin crescent moon in the west after sunset â itâs not a rising moon. Instead, itâs a setting moon. At the same time, though â 3. The moon takes about a month (one moonth ) to orbit the Earth. Although the moon rises in the east and sets in the west each day (due to Earthâs spin), itâs also moving on the skyâs dome each day due to its own motion in orbit around Earth.
This is a slower, less noticeable motion of the moon. Itâs a motion in front of the fixed stars. If you just glance at the moon one evening â and see it again a few hours later â youâll notice it has moved westward. That westward motion is caused by Earthâs spin. The moonâs own orbital motion can be detected in the course of a single night, too. But you have to watch the moon closely, with respect to stars in its vicinity, over several hours. The moonâs eastward, orbital motion is easiest to notice from one day (or night) to the next. Itâs as though the moon is moving on the inside of a circle of 360 degrees. The moonâs orbit carries it around Earthâs sky once a month, because the moon takes about a month to orbit Earth. So that the moon moves â with respect to the fixed stars â by about 12-13 degrees each day. 4. The moonâs orbital motion is toward the east. Each day, as the moon moves another 12-13 degrees toward the east on the skyâs dome, Earth has to rotate a little longer to bring you around to where the moon is in space. Thus the moon rises, on average, about 50 minutes later each day. The later and later rising times of the moon cause our companion world to appear in a different part of the sky at each nightfall for the two weeks between new and full moon. Then, in two weeks after full moon, youâll find the moon rising later and later at night. We have more details on individual moon phases at the links below. Follow the links to learn more about the various phases of the moon. â and here are the. Bottom line: Why the moon waxes and wanes in phase. Four keys to understanding moon phases. Links to descriptions of the various phases of the moon.
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