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

- Earth in Space Sunlight Striking Earth’s Surface • Near the equator, sunlight strikes Earth’s surface more directly and is less spread out than near the poles.

- Earth in Space Seasons on Earth • 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 Earth • Solstices 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!

- Earth in Space Seasons on Earth • The height of the sun above the horizon varies with the season.

- Earth in Space Using Prior Knowledge • Before 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. What You Know The sun’s rays heat Earth. Earth has seasons. In the Northern Hemisphere, fall begins in September and spring begins in March. What You Learned 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. The tilt of Earth’s axis as it moves around the sun causes seasons. Around March 21 and September 22, day and night are each 12 hours long.

End of Section:Earth in Space

- Gravity and Motion Gravity • 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 and Motion 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? - Gravity and Motion Gravity Versus Distance

Four million newtons Reading Graphs: What is the force of gravity on the rocket at the planet’s surface? - Gravity and Motion Gravity Versus Distance

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)? - Gravity and Motion Gravity Versus Distance

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 Motion Gravity Versus Distance

- Gravity and Motion Gravity and Inertia • Newton concluded that two factors–gravity and inertia–combine to keep the moon in orbit around Earth.

- Gravity and Motion Asking Questions • Before 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. Question Answer 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.

- Gravity and Motion Links on Gravity • Click the SciLinks button for links on gravity.

End of Section: Gravity and Motion

- Phases, Eclipses, and Tides Motions of the Moon • The changing relative positions of the moon, Earth, and sun cause the phases of the moon, eclipses, and tides.

- Phases, 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.

- Phases, Eclipses, and Tides Moon Phases and Eclipses Activity • Click the Active Art button to open a browser window and access Active Art about moon phases and eclipses.

- Phases, Eclipses, and Tides The Moon’s Orbit • The moon’s orbit is tilted about 5 degrees relative to Earth’s orbit around the sun.

- Phases, Eclipses, and Tides Solar Eclipse • A solar eclipse occurs when the moon passes directly between Earth and the sun, blocking sunlight from parts of Earth.

- Phases, Eclipses, and Tides Lunar Eclipse • During a lunar eclipse, Earth blocks sunlight fromreaching the moon.

- Phases, Eclipses, and Tides Tides • Tides occur mainly due to the difference in the force of gravity between the moon and different parts of Earth.

- Phases, Eclipses, and Tides 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.

- Phases, Eclipses, and Tides Previewing Visuals • Preview 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. Earth’s History 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.

End of Section: Phases, Eclipses, and Tides

- Earth’s Moon The Moon’s Surface • Features on the moon’s surface include maria, craters, and highlands.

- Earth’s Moon The Moon’s Size • The moon is 3,476 km in diameter, a little less than the distance across the contiguous Untied States.

- Earth’s Moon The Origin of the Moon • Scientists theorize that a planet-sized object collided with earth to form the moon.

- Earth’s Moon Identifying Main Ideas • As 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. Main Idea The moon’s surface has a variety of features such as… Detail Detail Detail 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.

End of Section:Earth’s Moon

Graphic Organizer Earth rotates revolves Around the sun On its axis causing causing Night and day Years

End of Section: Graphic Organizer

Table of Contents