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# Section 1; Earth in Space

## How does Earth move?

Earth spins on its axis.  When it does that, it is called rotation.  Earth's rotation causes night and day.  For Earth to rotate one time, it takes 24 hours.  And each time it rotates one of those 24 hour rotation cycles of night and day, it is called a day.

Not only does Earth rotate on its axis, but it also travels in a path around the sun.  This is called revolution, because it's the movement of one object around another.  A year, that is caused by one complete revolution of Earth around the sun.  The path that Earth follows around the sun, is also called the orbit.  The orbit is slightly elongated circle, or ellipse, not quite circular.

## What causes the cycle of seasons on Earth?

There are four distinct seasons...winter, spring, summer, and fall.  But in different places, there are great differences in temperature.

It is warmer in places more near to the equator.  This is because of the way that sunlight hits Earth.  Sunlight hits Earth's surface most directly near the equator.  Sunlight arrives at a steep angle near the poles.  The result of this is that it is spread out over a greater area.  That is why it is warmer near the equator than the poles.

Because of Earth being tilted on its axis, that makes it so that the temperatures are not fairly consistent all year round.  It makes it so that there are big differences in one place throughout the year in temperature.  It makes it so that there are seasons, because if Earth was not tilted on its axis, there would be no seasons.  Therefore, Earth has seasons because its axis is tilted as it revolves around the sun.  The North end of the Earth's axis is tilted away from the sun and toward the sun each for part of the year.  This is because of the way the Earth revolves on its axis.  And this also says that summer and winter are caused by Earth's tilt as it revolves around the sun.  Earth's distance from the sun is not what causes change in seasons, because in fact, when it is summer in the Northern Hemisphere, Earth is farthest away form the sun.

Twice each year, the sun reaches its greatest distance north or south of the equator.  This is called a solstice...when the sun is farthest north, or farthest south away from the equator.  A summer solstice is when the sun is farthest north of the equator in the Northern Hemisphere, but this also means that it is a winter solstice in the Southern Hemisphere.  This solstice occurs around June 21st each year.  In the Northern Hemisphere, it is the longest day of the year, and in the Southern Hemisphere, it is the shortest day of the year.

And pretty much the same thing happens around December 21st of each year.  The sun is farthest south of the equator...and this is when the winter solstice is in the Northern Hemisphere, and the summer solstice is in the Southern Hemisphere.

Halfway between those two solstices, not either of the hemispheres are tilted toward, or away from the sun.  This, just like the solstices, also occurs twice a year.  When the noon sun is directly overhead at the equator.  Each of these two days is called an equinox.  It means "equal night".  During these equinoxes, both day and night are about 12 hours long everywhere on Earth.

The vernal, or the spring, equinox occurs around March 21st every year and this equinox marks the beginning of the season of spring, in the Northern Hemisphere, that is.  And there's another equinox, and this marks the beginning of fall every year in the Northern Hemisphere.  It's called the autumnal equinox.

# Section 2; Gravity and Motion

## What determines the strength of the force of gravity between two objects?

Isaac Newton, an English scientist was the first person to answer the questions like, what keeps the moon and Earth in orbit?  Like why don't they just fly off into space??  Isaac Newton told a story about how he saw an apple fall from a tree, and it made him wonder about the moon's orbit.  He realized that there must be a force between the moon and Earth keeping the moon in orbit...right?

Well, a force is a push or a pull.  Most everyday, forces require objects to be in contact.  But, the moon and Earth are not in contact.  So, Newton also realized that the force that holds the moon in orbit has to be different than the everyday "force".

Just like the force that pulls the apple to the ground, Newton thought that the same type of force must pull the moon toward Earth.  This force, is called "gravity".  Gravity attracts all objects toward each other.  And although Newton did not discover gravity, he was the first person to realize that gravity occurred everywhere.

Newton made the "law of universal gravitation", stating that every object in the universe attracts every other object.

All objects around you are pulling on you, just as you are pulling on them.  When you're reading a book, let's say, you don't notice a pull between you and the book because the strength of gravity depends in part on the masses of each objects.  Mass is the amount of matter in an object.

## What two factors combine to keep the moon and earth in orbit?

Inertia is a factor that makes it so such collisions like the Earth falling into the sun don't happen.  And another factor, is Orbital Motion.  These two factors combine to keep Earth in orbit around the sun and the moon in orbit around Earth.

## Section 3; Phases, Eclipses, and Tides

### What causes the phases of the moon?

The changing relative positions of the moon, Earth, and sun cause the phases of the moon, eclipsed, and tides.  Just as Earth, the moon also rotates on its axis.  It rotates in the same amount of time as it revolves around Earth.  This is what causes a day.  And also it causes a year.  The different shapes of the moon, are called phases.  And as I said before, what causes the phases is the changing relative positions of the moon, Earth, and sun.

### What are solar and lunar eclipses?

An eclipse is when an object in space comes between the sun and a third object, it cats a shadow on that object, causing an eclipse to take place.  And there two types of eclipses, solar eclipses, and lunar eclipses.  A solar eclipse occurs when there's a new moon, and it blocks your view of the sun.  A lunar eclipse occurs at a full moon when Earth is directly between the moon and the sun.

## What causes tides?

There are different types of tides.  You can see them in the button at the bottom labeled "Chapter 1 Vocabulary".  But what are tides in general?  Well, they are the rise and fall or ocean water, and they occur every 12.5 hours or so.  They are caused by mainly differences in how much the moon's gravity pulls on different parts of Earth.

# Section 4; Earth's Moon

## What features are found on the moon's surface?

Features of the moon's surface include; maria, craters, and highlands.  Maria is dark, flat areas on the moon's surface.  Craters are large round pits.  Highlands are like mountains and they're light colored features.

## What are some characteristics of the moon?

It's size and density, the moon is 3,476 kilometers in diameter.  This is a little less than the distance across the United States.  It's also about one-fourth of Earth's diameter.  The moon's mass is only one-eightieth of Earth's mass.

Temperature and Atmosphere...temperatures range from 130°C to -180°C.  And the reason why the temperatures on the moon vary so much is because the moon has no atmosphere.

There is no liquid water on the moon.  But, there is some evidence that there may be some large patches or ice near the moon's poles.

## How did the moon form?

Scientists have man possible theories for how the moon formed.

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Bringing 15 people

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maybe but prob not

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