Download Chapter 6 - USD Home Pages

Astronomy
HOMEWORK Questions Chapter 6 - 9th edition
2. What type of chemical or molecule is most common in Earth’s atmosphere? a. carbon
dioxide; b. oxygen; c. water; d. nitrogen; e. hydrogen.
The Earth’s atmosphere is about 79% nitrogen, 20% oxygen, and 1% other gases. So, the
answer is d. nitrogen.
4. Why is Earth’s surface not riddled with craters, as is that of the Moon?
Earth has an atmosphere. This burns up small meteoroids, but does not burn up large ones,
which do strike Earth. The atmosphere also gives us weather, which gradually wears away
craters. Tectonic motion also is constantly resurfacing the Earth. Meteor Crater in Arizona is
about 50,000 years old, so this weathering/tectonics erasure doesn’t happen overnight, but it
does happen over millions of years.
6. Describe the process of plate tectonics. Give specific examples of geographic features created
by plate tectonics.
Plate Tectonics: The surface of the Earth is divided into continent-sized, and larger plates
which are constantly in motion. When one slides by the other, it produces a fault line, such as
the San Andreas. When two collide, mountains are formed, as is the case with the Alps or the
Himalayas. When two are moving apart, it produces a region where magma can ooze up, such
as the mid-Atlantic ridge.
9. How do we know about Earth’s interior, given that the deepest wells and mines extend only
a few kilometers into its crust?
By measuring its response to earthquakes (seismic waves). These waves are bent when they
pass from one material to another, much as the path of a light ray is bent when entering glass.
Furthermore, some seismic waves are stopped by a liquid.
21. What is a tidal force? How do tidal forces produce tides in Earth’s oceans?
The term “tidal force” refers to the difference between the gravitational force at some particular point (e.g., the point on Earth’s surface right underneath the Moon) and the average
gravitational force over the entire object, such as the entire Earth.
This force leads to tides because: a) the average gravitational force on Earth keeps it almost
exactly in orbit with the Moon; and b) the force is STRONGER THAN AVERAGE at the
point on Earth closest to the Moon, and WEAKER THAN AVERAGE at the point on Earth
farthest from the Moon. The net effect is to try to make the Earth slightly football-shaped,
with the points pointing away from, and towards, the Moon.
22. What is the difference between spring tides and neap tides? During which phases do they
occur?
Spring tides refer to the times that tides are pronounced: high highs, low lows. These occur
when the Sun and Moon work in cahoots to help each other, which happens not only at new
Moon, but also at full Moon. Neap tides are when the tides are least pronounced, highs and
lows show less variation. These occur when the Sun and Moon work against each other, at first
and last quarter Moon.
41. What if: The Moon orbited Earth in the opposite direction from Earth’s rotation, rather
than in the same direction? What would be different about Earth and life on it? Assume that
today the counter-revolving Moon is at the same distance as our Moon.
At first, is seems that differences would be fairly minor. The Moon would go through the
same phases, but on the opposite side of the Sun. Waxing crescent would be prominent in
the pre-dawn sky, folllowed by first quarter rising at midnight, etc. The old waning crescent
Moon would be prominent in the western sky after sunset. The synodic month would be a few
days shorter than the sidereal month, so werewolves would appear every 25 days or so instead
on every 29.5 days. But one has to look at the past. The counter-revolving Moon would be
spiraling inward toward Earth as Earth slows down. This means that it was farther away in
the past; hence Earth didn’t get the benefit of extreme tides and stresses early in its life. Less
salt would be in the oceans, and the weathering away of land would not have taken place. This
would probably have delayed evolution of life. In the future, the Moon might spiral inward to
the point that Earth’s tidal stresses would tear it up, leaving a very nice ring system.
45. What IF? Earth were now in synchronous rotation with the Moon? That is, that the Earth
rotated at the same rate as the Moon orbits it. What would be different about Earth and life
on it?
In the first place, this means that the solar cycle would be 29.5 days long, since the Earth’s spin
is slowed to match the Moon’s orbital period. The Moon seems to hang over one particular point
on Earth. Let’s put it at 90 degrees west longitude, so that it’s over the Western Hemisphere.
So the Moon would always be in the sky for inhabitants of the Western hemisphere, and nobody
in the Eastern hemisphere would ever see it. As far as phases, we would have a new Moon at
noon, a first-quarter Moon at sunset, and a full Moon at midnight. We would also have only
weak tidal variations, due to the Sun.
Incidentally, this mutual synchronous rotation is the situation with Pluto and its large satellite
Charon.