The motions of the Earth impact our understanding of the passage of time and how we measure it.

Course Competencies and Learning Objectives

CC4.1 Investigate the Earth’s place in the solar system

★ LO4.1.1 Identify the Earth’s shape and size

★ LO4.1.2 Describe the different types of Earth’s motion

★ LO4.1.3 Contrast latitude and longitude

★ LO4.1.4 Perform calculations with time on Earth

LO4.1.5 Identify the phases of the Moon

LO4.1.6 Determine the effects of the Moon on the Earth

Readings

Read Chapter 16 (sections 16.1 through 16.4) in your textbook, Physical Science, 13^th edition by Bill Tillery McGraw Hill Education

Extra

Smithsonian Time and Navigation: The untold story of getting from here to thereLinks to an external site.

Media

Earth's Shape

This video from PBS Eons will explore how gravity is responsible for making planets and other large objects spherical.

Time: 10 min

ASL version of Why the Earth is Round and the Milky Way is Flat

Channel: PBS Space Time

Position on Earth

This video from the Smithsonian will show you how latitude and longitude are defined.

Time: 1 min, 16 sec

Channel: Smithsonian National Air and Space Museum

Latitude

Lines of latitude, aka parallels, circle the globe from east to west. The line of latitude that cuts the globe in half between the north and south poles is called the equator, and it is located at 0°. The North Pole is located at 90° N (degrees north of the equator), and the South Pole is located at 90° S (degrees south of the equator).

The parallel at 23.5° N is called the Tropic of Cancer, and the parallel at 23.5° S is called the Tropic of Capricorn. These two parallels define the boundaries of the tropics, which is the region of the planet that is known for its warm and humid climate.

The parallel at 66.5° N (degrees north of the equator) is called the Arctic Circle, and the parallel at 66.5° S (degrees south of the equator) is called the Antarctic Circle.

The 49th Parallel, or 49° N defines part of the boundary between The United States and Canada.

A map of the Earth that shows five major lines of latitude: equator, Tropic of Cancer, Tropic of Capricorn, The Artic Circle, and The Antartic Circle.

Longitude

Lines of longitude run north and south and are known as meridians. The Prime Meridian is at 0° and runs through the Royal Observatory in Greenwich, England. North and South America are located west of the Prime Meridian.

Angles

Lines of latitude and longitude are angles on the globe, and are recorded in degrees. These degrees can be broken down into fractional parts similar to time. There are 60 (angular) minutes in a degree, and the ' is used for shorthand: 1° = 60' There are 60 seconds in a minute, and the " is used for shorthand: 1' = 60"

The Tropic of Cancer is only approximately located at 23.5° N, and the exact latitude is 23° 26' 9.8" N.

Practice and Apply

PROMPT How does gravity make planets spherical?

Answer

A sphere is the only shape that is symmetric in all directions, this symmetry allows the force of gravity to balance out at each point on the planet. If there was a point on the planet that made it non-spherical, gravity would act to break the asymmetry and make the planet spherical.

PROMPT What causes the Earth's shape to deviate from a perfect sphere?

Answer

The Earth's spinning causes an extra force around the planet's center which make it bulge by about 20 km at the diameter relative to the pole.