Section 19.1 Earth in Space

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 365.2442 days to orbit the sun. • We round the number to 365.25, 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.