The History of the Earth: Supplemental Guide 1A | Our Home, Earth 25that a globe spins—on its axis. It takes one day, or twenty-four
hours, for Earth to make a complete rotation. 10
The map also shows the equator, an imaginary line around
the middle of the earth. 11 The equator divides the earth into two
equal halves. The area along the equator receives the most direct
sunlight and is therefore generally the warmest area on the surface
of the earth. Show image 1A-10: Earth from space
Earth is sphere-shaped, like a ball, and it is surrounded by
a thick blanket of air, called an atmosphere, where clouds fl oat
around. Most of the earth’s surface is covered with water in the
form of the fi ve oceans: 12 Pacifi c, Atlantic, Indian, Arctic, and
Southern (or Antarctic). And between these oceans there is land
in the form of seven continents: North America, South America,
Europe, Africa, Asia, Australia, and Antarctica.
People haven’t always known that the earth is round or that it
rotates on its axis as well as orbits the sun; they haven’t always
known that there are fi ve oceans and seven continents, or that
most of the surface is covered in water. It has taken hundreds of
years for scientists and explorers to develop all the knowledge
about the earth that I just described in the last few minutes. But
this barely begins to scratch the surface of what we now know
about the history of the earth. 13
Show image 1A-11: Heat, pressure, and time
There are three important words you need to keep in mind
whenever you are thinking about geology, which is the study of the
earth. Heat is the fi rst. You can feel heat from a fl ame or from the
sun on a sunny day. Heat causes many changes to the earth.12 [Point out the oceans and
continents that are visible as you
read.]
10 [Use a globe to demonstrate
rotation.] It takes one day for Earth
to go all the way around its axis. As
the earth rotates, it also orbits the
sun. [You may wish to demonstrate
both motions of Earth by turning
around and around as you walk in
a circle around a student volunteer
playing the stationary sun.]
11 [Point to the equator on the map.
Tell students that the word equator
sounds like the word equal. We can
remember that the equator cuts
the world into two equal halves.]
13 “This barely begins to scratch the
surface” is a phrase that people
say when they mean that there is
so much more left to learn about
something.