How Earth Moves Astronomy –the study of the moon, stars and other objects in space Earth moves through space in two major ways: rotation and revolution Rotation- the spinning of Earth on its axis Axis – imaginary line through Earths center and North and South poles Rotation causes day and night Takes 24 hours for one rotation Revolution- movement of one object around another One complete revolution of Earth around the sun is a year Earth follows a slightly elliptical orbit around sun
Rotation vs Revolution
Checkup How many rotations of Earth take place in one revolution around the sun?
Calendars A Calendar is system of organizing the time that defines the beginning, length and divisions of a year People of many cultures have added to our idea of a calendar Egyptians determined that there are 365 days in a year Many people followed moon cycles – there are about 29 ½ days in a moon cycle or month Problem = 12 moon cycles of 30 days only adds up to 354 days.
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 4 th year is known as a leap year Problem: Roman calendar still off by 11 min. Over the centuries the minutes added up and by 1500s the beginning of spring was coming 10 days early Pope Gregory XIII dropped 10 days from the Calendar to correct. He also made other minor changes. This is the calendar we use today
Check up What is a leap year? Explain why it was difficult for ancient peoples to develop workable calendars
The Seasons on Earth Temperate places on Earth have 4 seasons: Winter, spring, summer, fall How sunlight hits the Earth affects how warm an area is. Sunlight hits more directly and a smaller area at the equator so it is warmer Sunlight hits at an angle and over a greater area at the poles so it is colder
Earths Tilted Axis Earth has seasons because its axis is tilted as it revolves around the sun. (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 half Summer and winter are caused by the Earths tilt not by changes in Earths distance from the sun Earth is actually farthest from the sun during the northern hemispheres summer
Earth in June North end of Earths axis is tilted toward the sun Sun is higher in the sky and there are more hours of sunlight Suns rays are more direct It is summer in Northern Hemisphere South end of Earths axis is tilted away Sun is low in the sky, days are shorter than nights Suns rays are more slanted It is winter in southern hemisphere
Earth in December Southern Hemisphere receives most direct sunlight – summer Northern Hemisphere receives slanted rays - winter
Solstices Solstice – when sun is farthest north or south of the equator Day when the sun is farthest north of the equator = about June 21 = Summer Solstice in the Northern Hemisphere and Winter Solstice in Southern Hemisphere Day when the sun is farthest south of the equator = about December 21= Winter Solstice in Northern Hemisphere and Summer Solstice in Southern Hemisphere
Equinoxes Halfway between the solstices, neither hemisphere is tilted toward or away from sun Noon is directly overhead at the equator Day and night are equal lengths - about 12 hours each Occurs twice a year around March 21(Vernal or spring equinox) and September 21 (autumnal equinox)
Solstices and Equinoxes
Check up If Earths axis were not tilted, what would happen to the length of days over the course of a year?