3 How Earth MovesAstronomy –the study of the moon, stars and other objects in spaceEarth moves through space in two major ways: rotation and revolutionRotation- the spinning of Earth on its axisAxis – imaginary line through Earth’s center and North and South polesRotation causes day and nightTakes 24 hours for one rotationRevolution- movement of one object around anotherOne complete revolution of Earth around the sun is a yearEarth follows a slightly elliptical orbit around sun
5 CheckupHow many rotations of Earth take place in one revolution around the sun?
6 Calendars A Calendar is system of organizing the time that defines the beginning, lengthand divisions of a yearPeople of many cultures have added to our idea of a calendarEgyptians determined that there are 365 days in a yearMany people followed moon cycles – there are about 29 ½ days in a moon cycle or monthProblem = 12 moon cycles of 30 days only adds up to 354 days.
7 Calendar Romans borrowed Egyptian calendar of 365 days Problem: It actually takes the Earth 365 ¼ days to complete one revolution. The Romans added 1 day every 4 years. The 4th year is known as a leap yearProblem: Roman calendar still off by 11 min. Over the centuries the minutes added up and by 1500’s the beginning of spring was coming 10 days earlyPope Gregory XIII dropped 10 days from the Calendar to correct. He also made other minor changes.This is the calendar we use today
8 Check up What is a leap year? Explain why it was difficult for ancient peoples to develop workable calendars
9 The Seasons on Earth Temperate places on Earth have 4 seasons: Winter, spring, summer, fallHow sunlight hits the Earth affects how warm an area is.Sunlight hits more directly and a smaller area at the equator so it is warmerSunlight hits at an angle and over a greater area at the poles so it is colder
10 Earth’s Tilted Axis Earth has seasons because its axis is tilted as it revolves around thesun. (23.5° from vertical)As Earth revolved around the sun, the north end is tilted towards the sun for half of the year and tilted away the other halfSummer and winter are caused by the Earth’s tilt not by changes in Earth’s distance from the sunEarth is actually farthest from the sun during the northern hemisphere’s summer
11 Earth in June North end of Earth’s axis is tilted toward the sun Sun is higher in the sky and there are more hours of sunlightSun’s rays are more directIt is summer in Northern HemisphereSouth end of Earth’s axis is tilted awaySun is low in the sky, days are shorter than nightsSun’s rays are more slantedIt is winter in southern hemisphere
12 Earth in DecemberSouthern Hemisphere receives most direct sunlight – summerNorthern Hemisphere receives slanted rays - winter
13 SolsticesSolstice – when sun is farthest north or south of the equatorDay when the sun is farthest north of the equator = about June 21 = Summer Solstice in the Northern Hemisphere and Winter Solstice in Southern HemisphereDay when the sun is farthest south of the equator = about December 21= Winter Solstice in Northern Hemisphere and Summer Solstice in Southern Hemisphere
14 EquinoxesHalfway between the solstices, neither hemisphere is tilted toward or away from sunNoon is directly overhead at the equatorDay and night are equal lengths - about 12 hours eachOccurs twice a year around March 21(Vernal or spring equinox) and September 21 (autumnal equinox)