Page 418 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch15_377-404.indd Page 395 9/3/10 6:17 PM user-f465
tiL12214_ch15_377-404.indd Page 395 9/3/10 6:17 PM user-f465 /Users/user-f465/Desktop

TABLE 15.6
Divide your group into three subgroups, with each subgroup
Distances from the Sun to planets known in the 1790s selecting one of the three categories above. After preparing for a
few minutes, have each group present reasons why we should
Distance Predicted Actual Distance
Planet n by (n ∙ 4)/10 (AU) (AU) understand UFOs to be natural phenomena, aliens from other
planets, or psychological misperceptions. Then have the entire
Mercury 0 0.4 0.39 group discuss the three categories and try to come to a consensus.
Venus 3 0.7 0.72
Earth 6 1.0 1.0
Mars 12 1.6 1.5
Such patterns of apparent regularity in the spacing of
(Asteroid belt) 24 2.8 —
planetary orbits were of great interest because if a true pattern
Jupiter 48 5.2 5.2
existed, it could hold meaning about the mechanism that de-
Saturn 96 10.0 9.5 termined the location of planets at various distances from the
Uranus 192 19.6 19.2 Sun. Many attempts have been made to explain the mechanism
of planetary spacing and why a belt of asteroids exists where
the Bode numbers predict there should be a planet. The most
successful explanations concern Jupiter and the influence of its
gigantic gravitational field on the formation of clumps of mat-
of all the planets known at that time, but the numbers also pre-
dicted a planet between Mars and Jupiter where there was none. ter at certain distances from the Sun. In other words, a planet
Later, a belt of asteroids was found where the Bode numbers does not exist today between Mars and Jupiter because there
predicted there should be a planet. This suggested to some peo- never was a planet there. The gravitational influence of Jupiter
ple that a planet had existed between Mars and Jupiter in the prevented the clumps of matter from joining together to form a
past, and somehow this planet was broken into pieces, perhaps planet, and a belt of asteroids formed instead.
by a collision with another large body (Table 15.6). There is no theoretical evidence that explains the Bode
numbers, and they are believed to be simply a mathematical
coincidence.

15.4 IDEAS ABOUT THE SOLAR SYSTEM
Myths, Mistakes, & Misunderstandings
If you observe the daily motion of the Sun and Moon and the
UFOs and You nightly motion of the Moon and stars over a period of time, you can
easily convince yourself that all the heavenly bodies revolve around
UFOs (unidentified flying objects) are observed around the a fixed, motionless Earth. The Sun, the Moon, and the planets do
world. UFOs are generally sighted near small towns and out in
the country, often near a military installation. Statistically, most appear to move from east to west across the sky, and the stars seem
sightings occur during the month of July at about 9 p.m., then at to be fixed on a turning sphere, maintaining the same positions
3 a.m. UFOs can be grouped into three categories: relative to one another as they move as units on the turning sphere.
To consider that Earth moves rather than the Sun, the Moon, and
1. Natural Phenomena. Most sightings of UFOs can be
explained as natural phenomena. For example, a bright the planets seems contrary to all these observations.
light that flashes red and green on the horizon could be There is a problem with the observed motion of the planets,
a star viewed through atmospheric refraction. The vast however, one troublesome observation that spoils this model of
majority of all UFO sightings are natural phenomena such a motionless Earth with everything moving around it. When
as atmospheric refraction, ball lighting, swamp gas, or the planets are observed over a period of a year or more, they
something simple such as a drifting weather balloon or do not hold the same relative positions as would be expected of
military flares on parachutes, high in the atmosphere and this model. The stars and the planets both rise in the east and
drifting across the sky. This category of UFOs should also set in the west, but the planets do not maintain the same posi-
include exaggeration and fraud, such as balloons released tions relative to the background of stars. Over time, the plan-
with burning candles as a student prank. ets appear to move across the background of stars, sometimes
2. Aliens. The idea that UFOs are alien spacecraft from other slowing, then reversing their direction in a loop before resum-
planets is very popular. No authentic, unambiguous evidence
exists, however, that would prove the existence of aliens. In ing their normal motion. This retrograde, or reverse, motion for
fact, most unidentified flying objects are not objects at all. the planet Mars is shown in Figure 15.20.
They are lights that can eventually be identified.
3. Psychological Factors. This group of sightings includes THE GEOCENTRIC MODEL
misperceptions of natural phenomena resulting from
unusual conditions that influence a person’s perception and Early Greek astronomers and philosophers had attempted to ex-
psychological health. This includes people who claim to plain the observed motions of the Sun, Moon, and stars with a
receive information from aliens by “channeling” messages. geometric model, a model of perfect geometrical spheres with
attached celestial bodies rotating around a fixed Earth in perfect

15-19 CHAPTER 15 The Solar System 395