why do we see the 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:. You know that one side of the moon always faces us. So youâd have to be on that side to see any Earth at all. But from any part of the moonâs near side, you could see Earth wax and wane â just as the moon does as seen from our world. Both the Earth and the moon are always half illuminated by the sun. But from either world, at any given time, you can see varying portions of that lighted half â or various phases of the Earth or moon. The phases are always the reverse of each other. When we see the moon as nearly full, any moon people would see a slim crescent Earth. When we see a completely full moon, the moon, Earth and sun are in a line â with Earth in the middle. Then people on the moon wouldnât see Earth at all because itâd be hidden in the sunâs glare. Also consider the moonâs slow rotation. The moon appears to rise and set as seen from Earth not because of the moonâs motion â but because Earth spins once a day on its axis. Because one side of the moon always faces us, from most places on the moon, Earth doesnât appear to rise or set. Instead, from a given point on the moonâs near side, youâd always see Earth hanging in your sky. Meanwhile, the sun would rise and set once each month â each time the moon spun once on its axis, thereby keeping that single face turned toward Earth. So from any spot on the moon, day and night would sweep over you once a month.
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