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Quiz #1 RecapScientific MethodMotions of Sun, Moon and PlanetsThe Zodiacal ConstellationsSlide 5If the Earth’s axis was not tilted…North/South Hemisphere SeasonsLength of Continuous Sunlight (Day) or Darkness (Night) at the PolesSeasons by Hemisphere December Solstice ExampleEquinoxesThe Seasons 1The Seasons 2The Seasons 3The Seasons 4Lunar Cycle – First Week WaxingLunar Cycle – Second Week WaxingSolar and Lunar EclipsesFull Moon Rises at Sunset, Full Moon Sets at SunriseThe Moon’s Orbital Plane is TippedFull Moon is on the Night Side of the EarthCan see any lunar phase at some time during the day except fullQuiz #1 RecapPSC 101020110915Scientific Method•Hypothesis: initial idea/observation on physical phenomenon (or, an answer to the question how does something work)–Must be testable•Theory: widely accepted explanation for a physical phenomenon•Hypothesis evolves into a theory after– Rigorous testing in controlled experiments– Communicating results to peers (publishing)– More testing by independent researchers/teams– An accepted theory will be able to make accurate predictions about physical phenomenaMotions of Sun, Moon and Planets•The apparent motion of the Sun, Moon, and planets relative to the background stars (through the Zodiacal constellations) as measured from night-to-night is a consequence of the facts that •The Moon orbits the Earth and•The Earth and planets orbit the Sun and •These orbits lie in approximately the same plane in space (within the band of the Zodiac).•Our line of sight to any of the planets, Sun, and Moon changes relative to the background stars and constellations as the Earth, Moon, and planets orbit the Sun.•Since all orbits are in approximately the same plane, the Sun, Moon and planets all cross in front of the same group of constellations, all on the ecliptic plane: the Zodiac.The Zodiacal Constellations •The path in the sky that the Sun, Moon, and planets follow throughout the year, and year after year is called the ecliptic.•The constellations that the ecliptic passes through are called the Zodiacal Constellations.•As with constellations anywhere in the sky, only those Zodiacal constellations in a direction away from the sun are visible at any given time of year in the night sky.The Zodiacal Constellations •The Zodiac constellations are those constellations that the Sun, Moon, and planets pass through during the course of a year, and repeating year after year (yellow dots on figure indicate the ecliptic path).If the Earth’s axis was not tilted…•Every day would be an equinox•For any single spot on the Earth, sunlight angle of incidence would not change from day-to-day•There would be no seasons•Figure shows sunlight angle of incidence on an equinox for an observer at latitude 38 (Washington, D.C.)North/South Hemisphere Seasons•Seasons in the Southern Hemisphere are opposite those in the Northern Hemisphere. Therefore, on the day of Quiz #1, 15 Sep. 2011, the season in South Africa was winter (soon to be spring)North Spring Summer Fall WinterSouth Fall Winter Spring SummerLength of Continuous Sunlight (Day) or Darkness (Night) at the Poles•6-months•http://astro.unl.edu/naap/motion3/animations/sunmotions.htmlSeasons by HemisphereDecember Solstice Example •Northern Hemisphere–1st day of winter–Compare the amount of light vs. dark at the observer’s latitude on the globe at right: Longest night time of the year–Sun lowest in the sky throughout the day: winter•Southern Hemisphere•1st day of summer•Longest day time of the year•Sun highest in the sky throughout the dayEquinoxes•The Equinoxes days with equal daylight and darkness occur in March and SeptemberThe Seasons 1 •We experience seasons on the Earth– because the Earth’s rotation axis (the North-South line through the Earth) is tilted (by 23.5°) with respect to the direction of its motion as it orbits the Sun, or in other words•because the Earth’s rotation axis is tilted with respect to the plane of the Earth’s orbit by 23.5°.The Seasons 2and the consequences of this tilt are: •day time and night time are not of equal length in time for every day throughout the year •when days are longer (and the Sun is high in the sky throughout the day, and the sunlight is more concentrated on the ground), we have summer•when nights are longer (and the Sun is low in the sky throughout the day, and the sunlight is less concentrated on the ground ), we have winterThe Seasons 3additional consequences of this tilt are: •the sun rises and sets at different positions on the horizon from day to day throughout the year–when the Sun rises at the northern most point along the eastern horizon, that day is the June solstice –when the Sun rises at the southern most point along the eastern horizon, that day is the December solsticeThe Seasons 4additional consequences of this tilt are: •The seasons are not the same on the Northern and Southern hemispheres of the Earth–Northern Hemisphere: spring, summer, fall, winter–Southern Hemisphere: fall, winter, spring, summer•Therefore, when summer ends in the U.S. (and fall begins in the U.S.), spring will start in South Africa.Lunar Cycle – First WeekWaxingOne week after themoon is new, the moonphase is First Quarter.Lunar Cycle – Second WeekWaxingAfter one more week, the moon is full. Therefore, two weeks elapse between the new moon and the full moon.Solar and Lunar EclipsesSolar Eclipse: Lunar phase is new (compare with Lunar Cycle: First Week slide).Lunar Eclipse: Lunar phase is full(compare with Lunar Cycle: 2nd Week slide).Full Moon Rises at Sunset, Full Moon Sets at Sunrise19The Moon’s Orbital Plane is Tipped•Eclipses do not occur every month because the Moon’s orbital plane around the Earth is not aligned with the Earth’s orbital plane around the •Since the Moon is far and small it’s easy for its shadow to miss the Earth if the two aren’t closely aligned with the Sun.•An alignment happens where the orbital planes cross each otherEarth MoonThe Earth-Moon system to scaleFull Moon is on the Night Side of the EarthCan see any lunar phase at some time during the day except fullHowever, the different phases are visible only certain times during the lunar cycle. Those times are the same from one lunar cycle to the


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PGCC PSC 1010 - Lecture Notes

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