1. The Solar System
Objectives: Analyze the theories of the formation of the universe and solar system Analyze planetary motion and the physical laws that explain that motion:
Rotation.
Revolution.
Apparent diurnal motions of the stars, sun and moon.
Effects of the tilt of the earth's axis.

2. Why Astronomy? Astronomy the scientific study of the universe
In the process of observing the universe, astronomers have made exciting discoveries, such as new planets, stars, black holes, and nebulas.
By studying these objects, astronomers have been able to learn more about the origin of Earth and the processes involved in the formation of our solar system.

3. Organization of the Universe
The solar system includes the sun, Earth, other planets, and many smaller objects such as asteroids and comets.
The solar system is part of a galaxy.
The galaxy in which the solar system resides is called the Milky Way galaxy.
The nearest part of the universe to Earth is our solar system.

4. How do we Measure Distances in the Universe?
Astronomical unit - the average distance between the Earth and the sun; approximately 150 million kilometers (symbol, AU)
Astronomers also use the speed of light to measure distance.

5. Telescopes
Are instruments that collect electromagnetic radiation from the sky and concentrate it for better observation.
Telescopes that collect only visible light are called optical telescopes.
Two types of optical telescopes:
refracting telescope - uses a set of lenses to gather and focus light from distant objects
reflecting telescopes - uses a curved mirror to gather and focus light from distant objects

6. How did the solar system form?
Scientists have long debated the origins of the solar system, which is the sun and all of the planets and other bodies that travel around it
In the 1600s and 1700s, many scientists thought that the sun formed first and threw off the materials that later formed the planets. This was incorrect.

7. Solar System formation, ctn’d
In 1796, French mathematician Pierre Simon, advanced a hypothesis now known as the nebular hypothesis.
Modern scientific calculations support this theory and help explain how the sun and planets formed from an original nebula of gas and dust.
solar nebular - rotating cloud of gas and dust from which the sun and planets formed; also any nebular from which stars and planets may form
The sun is composed of about 99% of all of the matter that was contained in the solar nebula.

8. Formation of Planets
While the sun was forming in the center of the solar nebula, planets were forming in the outer regions.
Some planetesimals (small bodies from which a planet originated in the early stages of development of the solar system) joined together through collisions and through the force of gravity to form larger bodies called protoplanets.
Protoplanets’ gravity attracted other planetesimals, collided, and added their masses to the protoplanets.
Eventually, they became very large and condensed to form planets and moons.

9. Earth’s Movement Rotation - the spin of a body on its axis
Earth’s rotation takes about one day.
Earth rotates from west to east.
The hemisphere of Earth facing the sun experiences daylight, while that facing away from the sun experiences nighttime.

10. Earth’s Movement, cnt’d
Revolution - motion of a body that travels around another body in space; one complete trip along an orbit
Each complete revolution of Earth around the sun takes 365 1/4 days, or about one year.
Earth’s orbit around the sun is an ellipse, so Earth is not always the same distance from the sun.

11. Earth’s Movement, cnt’d
Perihelion - the point in the orbit of a planet at which the planet is closet to the sun
Aphelion - the point in the orbit of a planet at which the planet is farthest from the sun

12. Why do we have seasons?
Earth’s axis is tilted at 23.5˚, which causes changes in the angle at which the sun’s rays strike Earth’s surface
When the North Pole tilts away from the sun, the angle of the sun’s rays falling on the Northern Hemisphere is low.
This means the Northern Hemisphere experiences fewer daylight hours, less energy, and lower temperatures.
The opposite occurs in the Southern Hemisphere during this time: more sun rays and warmer temperatures.

13. Equinox = Equal amounts of light and dark
Equinox - the sun’s rays strike Earth at a 90° angle along the equator. The hours of daylight and darkness are approximately equal everywhere on Earth on that day.
The autumnal equinox occurs on September 22 or 23 of each year and marks the beginning of fall in the Northern Hemisphere.
The vernal equinox occurs on March 21 or 22 of each year and marks the beginning of spring in the Northern Hemisphere.

14. Solstice = Sun far from equator
Solstice - the point at which the sun is as far north or as far south of the equator as possible
The summer solstice occurs on June 21 or 22; marks the beginning of summer in the Northern Hemisphere.
The winter solstice occurs on December 21 or 22; marks the beginning of winter in the Northern Hemisphere.

15. Earth-Moon System
Earth and the moon revolve around each other. Together they form a single system that orbits the sun.
The balance point of the Earth-moon system is located within the Earth’s interior, because Earth’s mass is greater than the moon’s mass.
The orbit of the moon around Earth forms an ellipse
When the moon is farthest from Earth, the moon is at apogee.
When the moon is closest to Earth, the moon is at perigee.

16. Moon’s Rotation and Revolution
The moon rises and sets 50 minutes later each night. This happens because of both Earth’s rotation and the moon’s revolution.
The moon moves in its orbit around Earth. It takes 1/29 of Earth’s rotation, or about 50 minutes, for the horizon to catch up to the moon.
The moon revolves around Earth in 27.3 days.
The moon’s revolution around Earth and its rotation on its axis take the same amount of time. Therefore, observers on Earth always see the same side of the moon
The part of the moon illuminated by sunlight changes as the moon orbits Earth.

17. Moon Phases
As the moon revolves around Earth, different amounts of the moon are lighted.
Therefore, the apparent shape of the visible part of the moon varies. The varying shapes are called phases.
When the size of the lighted part of the moon is increasing, the moon is said to be waxing.
At full moon, the entire near side of the moon is illuminated by the light of the sun.
When the lighted part of the near side of the moon appears to decrease in size, the moon is waning.
The period from one new moon to the next one is 29.5 days.

18. Eclipses
An eclipse occurs when one body passes through the shadow of another.
During a total solar eclipse, the moon passes between Earth and the sun. The sun’s light is completely blocked by the moon. The shadow of the moon falls on Earth.
A lunar eclipse occurs when Earth is positioned between the moon and the sun and when Earth’s shadow crosses the lighted half of the moon.

19. Bibliography
Allison, DeGaetano, Pasachoff, Holt Earth Science. 2008, Holt, Rinehart and Winston, New York