Key ConceptsHow does Earth move in space?What causes the cycle of seasons on Earth?

Key Terms

Astronomy Axis Rotation Revolution

Orbit Calendar Solstice Equinox

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

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 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

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

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

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

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 Earth’s axis were not tilted, what would happen to the length of days over the course of a year?