Lesson 2

Scaling the Sun-Earth-Moon System

Why the Sun and Moon Appear To Be the Same Size:

It may seem that the Sun is close to Earth, but it is actually about 150 million kilometers away (or nearly 12,00 Earths away). The Sun’s diameter is 1,392,000 kilometers, while the Earth’s diameter is 12,756 kilometers. That means that 109 Earth’s could fit across the Sun’s diameter. One million Earth’s could fit inside the Sun.

The Moon is 3500 km in diameter or 400 times smaller than the Sun. The Moon is also 400 times closer to Earth than the Sun is. Because an object that is close appears larger, from Earth the Moon appears to have the same diameter as the Sun.

Using Earth as a Standard Unit:

It is common in astronomy to use Earth’s distance from the Sun –150 million km - as a standard unit when stating how far a planet is from the Sun. Earth’s distance from the Sun is referred to as 1 astronomical unit (AU). For example, Jupiter is 778,400,000 km from the Sun. Using astronomical units, we can say that Jupiter is 5.2 AU (or 5.2 “Earth distances”) from the Sun.

ASTRONOMICAL UNIT
An astronomical unit, or AU, is equal to the mean (average) distance from the Earth to the Sun, about 93 million miles (150 million km). It takes a beam of light about 8.3 minutes to travel 1 AU.

Why We Study Relative Size, Position and Distance:

Relative size, position and distance provide the foundation for studying other phenomena.

Seasons – determined by the relative positions of the Sun and Earth, including the tilt of the Earth’s rotation axis along its orbital plane.

Phases of the Moon – reflects the Sun’s light. The Moon’s phases occur because we see only portions of the Moon’s illuminated side, depending upon the position of the Moon relative to Earth.

Solar and lunar eclipses – The Moon is closer to Earth than the Sun, and therefore has an apparent diameter similar to that of the Sun, even though the Moon is smaller than the Sun. The Moon can block sunlight from reaching parts of Earth during a solar eclipse. During a lunar eclipse, the larger Earth can block sunlight from reaching the smaller Moon.

Solar energy – The size of the Sun helps to heat and illuminate planets, moons, and asteroids throughout the solar system. A planet’s distance from the Sun and the planet’s position on its axis affect the amount of solar energy that the plane receives overall.

Planetary orbital motion- The massive size of the Sun is responsible for the great gravitational pull that holds the planetary bodies in orbit around it.

A. Earth’s Moon:

1. Moon’s age is between _____ and ______ billion years.

2. Theories about the Moon’s formation are:

i. ___________ collided with Earth.

ii. ___________ from the collision was thrown into ________.

iii. Newly formed Moon traveled at correct speed to remain within control of Earth’s ___________ __________.

3. Moon is Earth’s natural _______________.

i. Moon ________ around, or orbits, Earth.

ii. Moon is the _______ object to be seen by humans.

4. Moon used to study the history and origin of the _______ ___________.

5. The Moon is approximately ________ size of Earth.

B. Natural Calendar:

1. Earth rotates, or orbits, on its _______.

2. Earth revolves, or orbits, around the _________.

3. The Moon rotates on its _______.

C. Day and Night:

1. Day and night are caused by the Earth’s ________ on its ________.

2. Earth takes approximately _________ hours to rotate ____ time through day and night.

D. Month:

1. Moon takes approximately one __ , or ___ days to orbit Earth.

2. The ____________ cycle of the Moon’s ______ change in shape led to measure each _______.

E. Year:

1. Earth orbits the Sun one time in ____________ days.

2. Every four years we have _______ ______ to make up for the ¼ day to complete the orbit.

F. Our Sun the Star:

1. Our Sun is the only _________ in our solar system.

2. Our galaxy is called the ______ _________.