Earth in Space

1. Key Concepts How does Earth move in space? What causes the cycle of seasons on Earth? Earth in Space

2. Key Terms • Astronomy • Axis • Rotation • Revolution • Orbit • Calendar • Solstice • Equinox

3. 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 Earth’s 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

4. Rotation vs Revolution

5. Checkup • How 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, 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.

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 year • Problem: 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 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

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, 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

10. Earth’s 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 Earth’s tilt not by changes in Earth’s distance from the sun • Earth 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 sunlight • Sun’s rays are more direct • It is summer in Northern Hemisphere • South end of Earth’s axis is tilted away • Sun is low in the sky, days are shorter than nights • Sun’s rays are more slanted • It is winter in southern hemisphere

12. Earth in December • Southern Hemisphere receives most direct sunlight – summer • Northern Hemisphere receives slanted rays - winter

13. 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

14. 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)

15. Solstices and Equinoxes

16. Check up • If Earth’s axis were not tilted, what would happen to the length of days over the course of a year?