1. Unit 1 E
The Solar System

2. 9. A Year and A Day

3. The time it takes Earth to orbit once around the sun
A year on Earth is 365 days
Vocabulary:
year
The time it takes Earth to orbit once around the sun

4. The exact amount of time is 365 ¼ days
Because of the extra ¼ day
1 day is added to the calendar every 4 years

5. That year is called leap year
February usually has 28 days
But in a leap year it has 29 days

6. So there are 366 days in a leap year
All other years have 365 days
2012 will be our next leap year

7. A year is a different length of time on each planet
This is because it takes each planet a different amount of time to orbit the sun

8. Mercury is much closer to the sun than Earth
It takes Mercury only 88 days to travel around the sun
So a year on Mercury is 88 days long

9. Pluto is very far from the sun
It takes Pluto a long time to travel once around the sun
A year on Pluto is equal to 248 year on Earth

10. Day and Night

11. This line is called Earth’s axis
An imaginary line goes through Earth from the North Pole to the South Pole
This line is called Earth’s axis

12. Vocabulary
axis
An imaginary line that goes through the Earth from the North Pole to the South Pole

13. This spinning movement is called rotation
Earth spins on its axis
This spinning movement is called rotation

14. Vocabulary
rotation
A spinning movement like when Earth rotates on its axis once every 24 hours

15. Both daytime and nighttime are part of this day
Each 24 hours is 1 day
Both daytime and nighttime are part of this day

16. Only on side of Earth faces the sun at any time.
This side is having daytime
The opposite side of Earth is in darkness.

17. When it is daytime in North America it is nighttime in India
In a 24-hour day
each part of Earth moves into the sunlight and back into darkness again

18. Because Earth spins toward the east,
The spinning motion of Earth makes the sun look like it is moving across the sky
Because Earth spins toward the east,
the sun appears to rise in the east and set in the west

19. If Earth did not rotate,
half of our planet would always have day and half would always have night