Earth, Moon, and Sun Earth in Space Gravity and Motion Phases, Eclipses, and Tides Earth’s Moon Table of Contents.

Earth, Moon, and Sun

Earth in Space

Gravity and Motion

Phases, Eclipses, and Tides

Earth’s Moon

Table of Contents


Astronomy

• Astronomy is the study of the moon, stars, and other objects in space.


How Earth Moves

- Earth in Space

Earth moves through space in two major ways: rotation and revolution.


Rotation vs. Revolution• Earth moves through space in two ways:

1. Earth rotates in a complete circle in ~24 hours (23h 56m 4.098903691s). This rotation is in an eastward direction around an imaginary axis running north and south through the center of our planet tilted at a 23.5 degree angle. The rotation of our planet causes day and night.

2. Earth also revolves around the sun in an elliptical orbit. One complete revolution takes 365.25 days (about one year). The tilt of the axis combined with revolution around the sun causes seasonal change on our planet. If the earth did not spin on its tilted axis, there would be no seasonal change!

• Calendars were designed to define the beginning, length, and division of a year.

• However, because rotation does not exactly equal 24 hours and revolution does not exactly equal 365 days, an additional day was added to the calendar at the end of February (February 29) every 4th year.


Sunlight Striking Earth’s Surface

- Earth in Space

Near the equator, sunlight strikes Earth’s surface more directly and is less spread out than near the poles.


Seasons on Earth

- Earth in Space

Earth has seasons because it is tilted as it revolves around the sun. This counterclockwise elliptical path is called an orbit.


Seasons on Earth

Equinox means “equal night.” This happens two times each year when day and night are about 12 hours long everywhere on our planet because the sun is almost directly overhead at the equator. The vernal equinox (or spring equinox) occurs around March 21 of each year and marks the beginning of spring in the Northern Hemisphere.

The autumnal equinox (or fall equinox) occurs around September 22nd of each year and marks the beginning of fall in the Northern Hemisphere.

* Please note that these same dates will result in opposite seasons in the Southern Hemisphere!


Seasons on EarthSolstices mark the greatest distances north and south of the equator the sun reaches two times each year.

The summer solstice takes place around June 21st each year. On this day, the sun is farthest north of the equator, resulting in and is the longest day of the year and this marks the start of the summer season in the Northern Hemisphere.

The winter solstice takes place around December 21st each year. On this day, the sun is farthest south of the equator, resulting in and is the shortest day of the year and this marks the start of the winter season in the Northern Hemisphere.

* Please note that these same dates will result in opposite seasons in the Southern Hemisphere!


Seasons on Earth

- Earth in Space

The height of the sun above the horizon varies with the season.


What You Know

What You Learned

Using Prior KnowledgeBefore you read, write what you know about seasons on Earth in a graphic organizer like the one below. As you read, write what you learn.

1. The sun’s rays heat Earth.2. Earth has seasons.3. In the Northern Hemisphere, fall begins in September and

spring begins in March.

1. Areas where the sun hits Earth at a more direct angle are generally warmer than areas where the sun’s rays are more spread out.

2. The tilt of Earth’s axis as it moves around the sun causes seasons.

3. Around March 21 and September 22, day and night are each 12 hours long.

- Earth in Space


End of Section:Earth in Space


Gravity

- Gravity and Motion

The strength of the force of gravity between two objects depends on two factors: the masses of the objects and the distance between them.


Gravity Versus Distance

As a rocket leaves a planet’s surface, the force of gravity between the rocket and the planet changes. Use the graph to answer the following questions.




Force of gravity on the rocket in millions of newtons and distance from the planet’s center in planetary radii

Reading Graphs:

What two variables are being graphed? In what units is each variable measured?




Four million newtons

Reading Graphs:

What is the force of gravity on the rocket at the planet’s surface?




One million newtons

Reading Graphs:

What is the force of gravity on the rocket at a distance of two units (twice the planet’s radius from its center)?




It decreases.

Making Generalizations:

In general, how does the force of gravity pulling on the rocket change as the distance between it and the planet increases?



Gravity and Inertia


Newton concluded that two factors–gravity and inertia–combine to keep the moon in orbit around Earth.


Asking QuestionsBefore you read, preview the red headings. In a graphic organizer like the one below, ask a question for each heading. As you read, write answers to your questions.

What is gravity? Gravity is the force that attracts all objects toward one another.

What is inertia? Inertia is the tendency of an object to resist a change in motion.

Question Answer



Links on Gravity

Click the SciLinks button for links on gravity.



End of Section: Gravity and

Motion


Motions of the Moon

- Phases, Eclipses, and Tides

The changing relative positions of the moon, Earth, and sun cause the phases of the moon, eclipses, and tides.


Phases of the Moon


The phase of moon you see depends on how much of the sunlit side of the moon faces Earth.


Moon Phases and Eclipses Activity

Click the Active Art button to open a browser window and access Active Art about moon phases and eclipses.



The Moon’s Orbit


The moon’s orbit is tilted about 5 degrees relative to Earth’s orbit around the sun.


Solar Eclipse


A solar eclipse occurs when the moon passes directly between Earth and the sun, blocking sunlight from parts of Earth.


Lunar Eclipse


During a lunar eclipse, Earth blocks sunlight fromreaching the moon.


Tides


Tides occur mainly due to the difference in the force of gravity between the moon and different parts of Earth.


Spring and Neap Tides


When Earth, the sun, and the moon are in a straight line, a spring tide occurs. When the moon is at a right angle to the sun, a neap tide occurs.


Previewing VisualsPreview Figure 11. Then write two questions you have about the diagram of the phases of the moon in a graphic organizer like the one below. As you read, answer your questions.

Q. Why does the moon have phases?

A. The changing relative positions of the moon, Earth, and the sun cause the phases of the moon.

Q. Do we see different sides of the moon as the phases of the moon appear?

A. No; the same side of the moon always faces Earth.

Earth’s History



End of Section: Phases,

Eclipses, and Tides


The Moon’s Surface

- Earth’s Moon

Features on the moon’s surface include maria, craters, and highlands.


The Moon’s Size

- Earth’s Moon

The moon is 3,476 km in diameter, a little less than the distance across the contiguous Untied States.


The Origin of the Moon

- Earth’s Moon

Scientists theorize that a planet-sized object collided with earth to form the moon.


Main Idea

Detail Detail Detail

Identifying Main IdeasAs you read the section “The Moon’s Surface,” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea.

The moon’s surface has a variety of features such as…

Dark, flat areas called maria, which formed from huge lava flows

Large, round pits called craters, which were caused by the impact of meteoroids

Highlands, or mountains, which cover much of the moon’s surface

- Earth’s Moon


Links on Earth’s Moon

Click the SciLinks button for links on Earth’s moon.

- Earth’s Moon


End of Section:Earth’s Moon


Graphic Organizer

Around the sun

revolves

Earth

On its axis

Years

rotates

causing causing

Night and day


End of Section: Graphic Organizer

Earth, Moon, and Sun Earth in Space Gravity and Motion Phases, Eclipses, and Tides Earth’s Moon Table of Contents.

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