1. Section 19.1
Earth in Space

2. Astronomy: The study of the moon, stars, and all other objects in space.
Has been studied for thousands of years.
Ancient Egyptians used the location of a star (Sirius) to predict approaching flood seasons.

3. Earth moves through space in two major ways: rotation & revolution.
Axis: The imaginary line that passes through Earth’s center from the North to South pole.
Rotation: The spinning of the Earth on its axis.

4. The rotation of the Earth causes our days & nights.
As the Earth spins toward the East, the sun appears to move West across the sky.
It is day on the side of the Earth facing the sun, and night on the opposite side.
It takes about 24 hours for the Earth to rotate once (a.k.a. – a day! ).

5. As Earth rotates on its axis, it also continually revolves around the sun.
Revolution: The movement of one object around another.
Earth follows a path, or orbit, as it revolves around the sun. Our orbit is an ellipse, or slightly elongated circle.

6. Leap Years… A simple fix?
It takes Earth approximately days to orbit the sun.
We round the number to , so every four years we add an extra day to the calendar.
If we did not, important dates would eventually fall in different seasons as we lose days.
There are exceptions! Century years (2000, 1900, etc.) are only leap years if they are divisible by 400. Why?

7. Why do most places on Earth (excluding Phoenix) have 4 distinct seasons?
Due to the tilt of Earth (23.5° from vertical), places near the equator receive the most direct sunlight & remain warmer throughout the year & everyone else has varying seasons.
Without the tilt, temperatures worldwide would remain fairly constant & there would be no seasons.
As the Earth moves around the sun, the Northern & Southern hemispheres alternate being angled toward the sun.
The hemisphere that is angled toward the sun experiences summer, while the other experiences winter.

8. Breaking Down the Year…
June: The northern hemisphere is tilted toward the sun & we have more daylight hours than darkness. In the southern hemisphere, there is less direct sunlight & fewer hours of daylight.
December: The southern hemisphere is tilted toward the sun & they have more daylight hours than darkness. In the northern hemisphere, there is less direct sunlight & fewer hours of daylight.

10. Solstices
The sun reaches its farthest point north or south of the equator.
Summer Solstice (Northern Hemisphere) – June 21. The sun is at the farthest point north of the equator. The longest day of the year!
Winter Solstice (Northern Hemisphere) – December 21. The sun is at the farthest point south of the equator. The shortest day of the year!

12. Equinoxes
“Equal night.” Neither hemisphere is tilted toward or away from the sun, so the day & night are each about 12 hours long.
Noon sun is directly overhead at the equator.
Vernal (Spring) Equinox – March 21, Northern Hemisphere. Marks the beginning of Spring.
Autumnal Equinox – September 22, Northern Hemisphere. Marks the beginning of fall.

14. Section 19.2
Gravity and Motion

15. What is gravity?
An attractive force that pulls objects toward each other. Measured in Newtons (N).
Law of Universal Gravitation: States that every object in the universe attracts every other object.
The strength of gravity depends upon the masses of the objects & the distance between them.
If distance increases, gravity decreases.
If mass increases, gravity increases.

16. How do massive objects like the Earth & moon remain in orbit?
Inertia (the tendency of an object to resist a change in motion) & gravity combine to keep them in orbit.
The greater the mass of an object, the greater the inertia.
If not for gravity, the moon would continue out into space in a straight line, but Earth continuously pulls it back down.
The same can be said for the Earth around the sun.