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

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?