Page 80 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

44 part I The energy–Atmosphere system
The Universe is populated with at least 125 billion galaxies. One of these is our own Milky Way Galaxy, and it contains about 300 billion stars. Among these stars is an average-size yellow star, the Sun, although the dramatic satellite image in Figure 2.2 seems any- thing but average! Our Sun radi- ates energy in all directions and upon its family of orbiting plan- ets. Of special interest to us is the solar energy that falls on the third planet, our immediate home.
In this chapter: Incoming solar energy arrives at the top of Earth’s atmosphere, establishing the pat- tern of energy input that drives Earth’s physical systems and in- fluences our daily lives. This solar energy input to the atmosphere, combined with Earth’s tilt and rota- tion, produces daily, seasonal, and annual patterns of changing day- length and Sun angle. The Sun is the ultimate energy source for most life processes in our biosphere.
The Solar System, Sun, and Earth
Our Solar System is located on a
remote, trailing edge of the Milky
Way Galaxy, a flattened, disk-
shaped collection of stars in the
form of a barred-spiral—a spiral
with a slightly barred, or elongated,
core (Figure 2.1a, b). Our Solar Sys-
tem is embedded more than half-
way out from the galactic centre,
in one of the Milky Way’s spiral
arms—the Orion Spur of the Sagit-
tarius Arm. A super-massive black
hole some 2 million solar masses
in size, named Sagittarius A*
(pronounced “Sagittarius A Star”),
sits in the galactic centre. Our
Solar System of eight planets, four dwarf planets, and asteroids is some 30000 light-years from this black hole at the centre of the Galaxy, and about 15 light-years above the plane of the Milky Way.
From our Earth-bound perspective in the Milky Way, the Galaxy appears to stretch across the night sky like a narrow band of hazy light. On a clear night, the unaided eye can see only a few thousand of these billions of stars gathered about us in our “neighbourhood.”
▲Figure 2.1 Milky Way Galaxy, Solar System,
and Earth’s orbit. (a) The milky Way galaxy viewed
from above in an artist’s conception, and (b) an image
in cross­section side view. (c) All of the planets have
orbits closely aligned to the plane of the ecliptic. pluto,
considered the ninth planet for over 70 years, was reclassified as a dwarf planet, part of the kuiper asteroid belt, in 2006. (d) The four inner terrestrial planets and the structure of earth’s elliptical orbit, illustrating perihelion (closest) and aphelion (farthest) positions during the year. have you ever observed the milky Way galaxy in the night sky?
[(a) and (b) courtesy of nAsA/JpL.]
Top view
(a)
Our Solar System
is in the Orion Spur of the Sagittarius Arm
Sun and Solar System
Side view
(b) 100000 light-years
Our Solar System
Sun Asteroid belt Mercury Venus
Jupiter Mars Earth
Solar System is 11 light hours wide
Uranus
Saturn
(c)
Neptune
Mars
Earth
Venus Sun
Mercury
Closest to Sun—perihelion, January 3 147255000 km
Farthest from Sun—aphelion, July 4 152083000 km
(d)
Animation
Nebular Hypothesis
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