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286 CHAPTER 9 Gravitation

(a) (b)
100 90 80 100 90 80
110 70 110 70
120 60 120 60
130 50 130 50
140 40 140 40
150 Earth DEC 30 150 30
JAN
FEB perihelion
160 NOV 20 160 Saturn 20
170 MAR Venus 10 170 Jupiter 10
180 Mercury OCT Mars 0 180 0

190 APR SEP 350 190 350
200 MAY aphelion AUG 340 200 Uranus 340
210 JUN JUL 330 210 330
Neptune
220 320 220 Pluto 320
230 310 230 310
240 250 260 270 280 290 300 240 250 260 270 280 290 300

11 11 11 11 11 13 13
0 1 10 m 2 10 m 3 10 m 4 10 m5 10 m 0 0.5 10 m 1 10 m
FIGURE 9.13 (a) Orbits of Mercury,
Venus, Earth, and Mars. Elliptical orbits can
Figure 9.13a shows the orbits of the planets Mercury, Venus, Earth, and Mars on scale
appear quite circular, even when the focus is
diagrams.The orbits of Saturn, Jupiter, Uranus, and Neptune and part of the orbit of
noticeably off-center, as with Mercury and
Mars. The colored dots indicate the posi- Pluto are shown in Fig. 9.13b. Inspection of these diagrams reveals that the orbits of
tions of these planets on January 1, 2000. Mercury, Mars, and Pluto are noticeably different from circles. 1
The tick marks indicate the positions at Kepler’s three laws apply not only to planets, but also to satellites and to comets.
intervals of 10 days. (b) Orbits of Jupiter, For example, Fig. 9.14 shows the orbits of a few of the many artificial Earth satellites.
Saturn, Uranus, and Neptune, and a portion
All these orbits are ellipses. For Earth orbits, the point closest to the Earth is called
of the orbit of Pluto. The tick marks for
perigee; the point farthest from Earth is called apogee.The early artificial satellites were
Jupiter and Saturn indicate the positions at
quite small, with masses below 100 kg (see Fig. 9.15). Nowadays, satellites with masses
intervals of 1 year.

All of these
satellite orbits
are ellipses.
Sputnik I
Explorer III Explorer I

FIGURE 9.14 Orbits of the FIGURE 9.15 Sputnik I, the first
first artificial Earth satellites. Sputnik II Vanguard I artificial Earth satellite. This satellite
See Table 9.3 for more data. Sputnik III had a mass of 83 kg.

1
Pluto has recently been reclassified by the International Astronomical Union as a dwarf planet, in the
same category as Ceres and 2003 UB (X ena).
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