1. Earth in Space
Chap 15, Sec 1

2. Essential Questions – Chap 15, Sec 1
How does Earth move in space?
What causes the cycle of seasons on Earth?

3. Astronomy The study of the moon, stars, and other objects in space
Ancient Egyptians were some of the first people to study astronomy in about 3000 B.C. – or 5000 years ago
They understood the appearance of Sirius (a very bright star) in the morning sky indicated it was almost time for the Nile to flood and then they could begin planting

4. How Earth Moves in Space
To us, it appears that both the Sun and Moon move around the Earth each day. We see them rise in the east and set in the west.
Long ago, people believed this was exactly what happened – that EVERYTHING in space circled around Earth.
Today we know better

5. Earth moves through space in two major ways:
Rotation
Revolution

6. Rotation The spinning of Earth on its axis Causes day and night
One rotation takes approximately 24 hours – this is called a DAY
Rotation of the Earth eastward causes the Sun to appear to move across the sky to the west

7. Revolution The movement of one object around another
One complete revolution of Earth around the Sun is called a year and takes 365 ¼ days
The path of revolution is called orbit. Earth’s orbit is an ellipse – slightly oval, not a perfect circle
Why do we have leap years?
Every four years we add together the ¼ days and make one extra day. It keeps us on schedule with seasons. Otherwise, we would eventually be having summer in December.

8. Can you remember . . . What causes seasons?
Earth is tilted on its axis
How does the angle of sunlight affect heating?
Sunlight hits the equator directly for long periods of time (days are long)
Areas nearer the poles receive sunlight at a great angle and for short periods of time (days are short), which means they get less energy and less heat
What season is it if the southern hemisphere is tilted towards the Sun?
Southern hemisphere experiences summer, while we in the northern hemisphere experience winter

9. Earth in June
The north end of Earth’s axis is tilted toward the Sun – it is summer in the Northern Hemisphere
Noon sun is high in the sky and there are more hours of daylight than dark
The Southern Hemisphere is tilted away from the sun and they are having winter
The sun is low in the sky and days are shorter than nights

10. Earth in December
The Southern Hemisphere now receives the Sun’s rays more directly and days are longer than nights – it is summer there
The Northern Hemisphere is receiving sunlight at very steep angles and days are shorter than nights – it is winter there

11. Solstices
The two days each year when the sun reaches its greatest distance north or south of the equator.
Around June 21 is OUR summer solstice:
The sun is farthest north of the equator
It is the longest day of the year for us in the Northern Hemisphere
At the same time, in the Southern Hemisphere, it is their winter solstice

12. Around December 21 is OUR winter solstice:
The sun reaches farthest south of the equator
It is the shortest day of the year for us in the Northern Hemisphere
At the same time, the Southern Hemisphere is experiencing their summer solstice and longest day of the year

13. Equinoxes
These are the two days each year when neither hemisphere is tilted toward or away from the sun.
The noon sun is directly overhead at the equator
Equinox means “equal night”
Day and night are each about 12 hours long everywhere on Earth
Around March 21 is spring equinox for the Northern Hemisphere
Around September 22 is autumnal (fall) equinox for the Northern Hemisphere
Remember the seasons are reversed south of the Equator, so they have spring equinox on Sept 22 and fall equinox on March 21

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15. Essential Questions – Answered
How does Earth move in space?
Earth rotates on its axis. Earth also revolves around the Sun.
What causes the cycle of seasons on Earth?
Earth is tilted on its axis. This tilt causes the Sun’s rays to strike the Earth at different angles at different times of year.