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OVERVIEW

Earth is a common object in the solar system, one of eight planets that goes around the Sun once a year in an almost
circular orbit. Earth is the third planet out from the Sun, it is fifth in mass and diameter, and it has the greatest
density of all the planets (Figure 16.1). Earth is unique because of its combination of an abundant supply of liquid
water, a strong magnetic field, and a particular atmospheric composition. In addition to these physical properties,
Earth has a unique set of natural motions that humans have used for thousands of years as a frame of reference to
mark time and to identify the events of their lives. These references to Earth’s motions are called the day, the month,
and the year.
Eventually, about 300 years ago, people began to understand that their references for time came from an Earth
that spins as a top as it circles the Sun. It was still difficult, however, for them to understand Earth’s place in the
universe. The problem was not unlike that of a person trying to comprehend the motion of a distant object while riding
a moving merry-go-round being pulled by a cart. Actually, the combined motions of Earth are much more complex
than a simple moving merry-go-round being pulled by a cart. Imagine trying to comprehend the motion of a distant
object while undergoing a combination of Earth’s more conspicuous motions, which are as follows:

1. A daily rotation of 1,670 km/h (about 1,040 mi/h) at the equator and less at higher latitudes.
2. A monthly revolution of Earth around the Earth-Moon center of gravity at about 50 km/h (about 30 mi/h).
3. A yearly revolution around the Sun at about an average 106,000 km/h (about 66,000 mi/h).
4. A motion of the solar system around the core of the Milky Way at about 370,000 km/h (about 230,000 mi/h).
5. A motion of the local star group that contains the Sun as compared to other star clusters of about
1,000,000 km/h (about 700,000 mi/h).
6. Movement of the entire Milky Way galaxy relative to other, remote galaxies at about 580,000 km/h (about
360,000 mi/h).
7. Minor motions such as cycles of change in the size and shape of Earth’s orbit and the tilt of Earth’s axis. In
addition to these slow changes, there is a gradual slowing of the rate of Earth’s daily rotation.
Basically, Earth is moving through space at fantastic speeds, following the Sun in a spiral path of a giant helix as
it spins as a top (Figure 16.2). This ceaseless and complex motion in space is relative to various frames of reference,
however, and the limited perspective from Earth’s surface can result in some very different ideas about Earth and
its motions. This chapter is about the more basic, or fundamental, motions of Earth and its moon. In addition
to conceptual understandings and evidence for the motions, some practical human uses of the motions will be
discussed.

this band, which is called the ecliptic. As viewed from Earth, the
16.1 SHAPE AND SIZE OF EARTH
Sun appears to move across the background of stars, completely
The most widely accepted theory about how the solar system around the ecliptic each year.
formed pictures the planets forming in a disk-shaped nebula Today, almost everyone has seen pictures of Earth from
with a turning, swirling motion. The planets formed from sepa- space, and it is difficult to deny that it has a rounded shape
rate accumulations of materials within this disk-shaped, turning (Figure 16.3). During the fifth and sixth centuries b.c., the
nebula, so the orbit of each planet was established along with its ancient Greeks decided that Earth must be round because
rate of rotation as it formed. Thus, all the planets move around (1) philosophically, they considered the sphere to be the per-
the Sun in the same direction in elliptical orbits that are nearly fect shape and they considered Earth to be perfect, so therefore
circular. The flatness of the solar system results in the observ- Earth must be a sphere; (2) Earth was observed to cast a circular
able planets moving in, or near, the plane of Earth’s orbit, which shadow on the Moon during a lunar eclipse; and (3) ships were
is called the plane of the ecliptic. observed to slowly disappear below the horizon as they sailed
When viewed from Earth, the planets appear to move only off into the distance. More abstract evidence of a round Earth
within a narrow band across the sky as they move in the plane was found in the observation that the altitude of the North Star
of the ecliptic. The Sun also appears to move in the center of above the horizon appeared to increase as a person traveled

406 CHAPTER 16 Earth in Space 16-2