One of the most convincing pieces of evidence for a spherical Earth are the pictures we have
of it from Space. Astronauts aboard the Apollo 17 shuttle took one of the most famous
photographs in history, called “The Blue Marble” (Figure24.1). This outstanding image
shows Earth as it looks from about 29,000 kilometers (18,000 miles) away in Space. The
picture shows us that Earth is spherical and looks like a giant blue and white ball. Hundreds
of years before humans ever made it into space, we knew the Earth was round. What ways
have you been able to see this for yourself?
Figure 24.1: Photograph entitled “The Blue Marble” taken by Apollo 17 Crew. ( 15 )The Sun and the other planets of our Solar System are also spheres. The Sun is found
in the center of the solar system, and the planets travel around the Sun in regular paths
calledorbits. Earth is the third planet from the Sun, and its mass is approximately 6.0 x
1024 kilograms. In contrast, the volume of planet Jupiter is about 1,000 times greater than
Earth’s volume, and the Sun’s volume is about 1,000 times greater than Jupiter’s (Figure
24.2).
While the outer planets in the Solar System are giant balls of swirling gas with very low
densities, Earth is an inner planet. The inner planets are relatively small, denser, rockier
planets than the outer planets. Three-fourths of Earth’s rocky surface is covered with water.
As far as we know, Earth is also the only planet that carries liquid water, another important
requirement for life. The entire planet is also surrounded by a thin layer of air called the