2Scientific MethodHypothesis: initial idea/observation on physical phenomenon (or, an answer to the question how does something work)Must be testableTheory: widely accepted explanation for a physical phenomenonHypothesis 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 phenomena
3Motions 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 thatThe Moon orbits the Earth andThe Earth and planets orbit the Sun andThese 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.
4The 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.
5The 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).
6If the Earth’s axis was not tilted… Every day would be an equinoxFor any single spot on the Earth, sunlight angle of incidence would not change from day-to-dayThere would be no seasonsFigure shows sunlight angle of incidence on an equinox for an observer at latitude 38 (Washington, D.C.)
7North/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 SpringSummerFallWinterSouth
8Length of Continuous Sunlight (Day) or Darkness (Night) at the Poles 6-months
9Seasons by Hemisphere December Solstice Example Northern Hemisphere1st day of winterCompare the amount of light vs. dark at the observer’s latitude on the globe at right: Longest night time of the yearSun lowest in the sky throughout the day: winterSouthern Hemisphere1st day of summerLongest day time of the yearSun highest in the sky throughout the day
10EquinoxesThe Equinoxes days with equal daylight and darkness occur in March and September
11The 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 wordsbecause the Earth’s rotation axis is tilted with respect to the plane of the Earth’s orbit by 23.5°.
12The Seasons 2 and the consequences of this tilt are: day time and night time are not of equal length in time for every day throughout the yearwhen 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 summerwhen 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 winter
13The Seasons 3 additional consequences of this tilt are: the sun rises and sets at different positions on the horizon from day to day throughout the yearwhen the Sun rises at the northern most point along the eastern horizon, that day is the June solsticewhen the Sun rises at the southern most point along the eastern horizon, that day is the December solstice
14The Seasons 4 additional consequences of this tilt are: The seasons are not the same on the Northern and Southern hemispheres of the EarthNorthern Hemisphere: spring, summer, fall, winterSouthern Hemisphere: fall, winter, spring, summerTherefore, when summer ends in the U.S. (and fall begins in the U.S.), spring will start in South Africa.
15Lunar Cycle – First Week Waxing One week after themoon is new, the moonphase is First Quarter.
16Lunar Cycle – Second Week Waxing After one more week, themoon is full. Therefore,two weeks elapse betweenthe new moon and thefull moon.
17Solar and Lunar Eclipses Solar Eclipse: Lunar phase is new(compare with Lunar Cycle: First Week slide).Lunar Eclipse: Lunar phaseis full(compare with Lunar Cycle:2nd Week slide).
18Full Moon Rises at Sunset, Full Moon Sets at Sunrise
19The 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 theSince 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 otherEarthMoonThe Earth-Moon system to scaleIt isn’t easy for eclipses to occur between the Earth and Moon!
21Can see any lunar phase at some time during the day except full However, the different phases are visible only certain times during the lunar cycle. Those times are the same from one lunar cycle to the next.