Download Chapter 1: THE SCALE OF THE COSMOS HOMEWORK 1, RQ 2,6,7

Chapter 1:
THE SCALE OF THE COSMOS
HOMEWORK 1, RQ 2,6,7; P 4,5,9
Review Question 2:
In figure 1-4, the division between daylight and darkness is at the right on the globe of
Earth. How do we know this is the sunset line and not the sunrise line?
Answer:
Earth rotates toward the east, from left to right in Figure 1-4 (note that you can identify
the Baja peninsula and southern California). As Earth turns from left to right in the
image, the land will be carried from daylight into darkness. Thus the boundary is the
sunset line that separates afternoon from evening and night.
Review Question 6:
Why are light years more convenient than miles, kilometers, or astronomical units for
measuring certain distances?
Answer:
It is always convenient to use a unit of measure that is approximately the same size as the
object or distance being measured. Hence, we use centimeters or inches when measuring
a book or countertop and kilometers or miles when measuring distances between towns.
Most of us would never consider measuring the distance to even the nearest town in
inches because the number would be very large and cumbersome to work with. Also, we
probably won't know the distance to the nearest town to within one centimeter or one
inch. Consequently, when we measure the distances to stars, the light-year is convenient
because stars are separated by a few light-years and we probably won't be able to
determine the distance to the stars more accurately than a few tenths of a light-year.
Review Question 7:
Why is it difficult to detect planets orbiting other stars?
Answer:
Seeing planets around other star systems is very difficult for three reasons. First, they
will appear as point sources. Planets are smaller in radius than the stars they orbit, but at
the same distance. Hence their angular diameter would be even smaller. Second, the
planets will also be much fainter than the stars they orbit. The light received from a
planet is light from the star that reflects off the planet's surface. The planet's surface
probably isn't perfectly reflecting and is smaller in size than the star; therefore, the light
from the planet will be much fainter than that from the star. Finally, we are trying to look
for this very faint light from the planet right next to the much brighter light from the star,
making the planet even more difficult to notice.
Problem 4: Venus orbits 0.7 AU from the sun. What is that distance in kilometers?
Answer:
We know that,
1 AU = 1.496 × 1011 m = 1.496 × 10 8 km (page 456, table A-5)
Hence,
0.7 AU = 0.7 × 1.496 × 10 8 km ⇒
⇒ 0.7 AU = 1.047 × 10 8 km
Problem 5: Light from the sun takes 8 minutes to reach Earth. How long does it take to
reach Mars?
Answer:
We know the average distance between Mars and Sun d is:
d = 1.5237 AU = 2.279 × 10 8 km
(page 369, Data File Six)
Hence the light needs,
t=
d
c
sec
to travel from Sun to Mars, or:
2.279 × 10 8 km
t=
⇒ t = 757.67 sec
3 × 10 5 km sec
Problem 9: The nearest large galaxy to our own is 2 million light years away. How many
meters is that?
Answer:
If d is the distance between our galaxy and the nearest one (Andromeda galaxy),
d = 2 million l. y. = 2 × 10 6 l. y.
But,
1 l. y. = 9.461 × 1015 m
(page 456, table A-5)
Hence,
d = 2 × 10 6 × 9.461 × 1015 m ⇒
⇒ d = 1.89 × 10 22 m