R2
= (1.25 × 104 kilometers)2
= ( × 1.25 × 1.25) × 104+4 kilometers2
= 4.90 × 108 kilometers2Is the "dark side of the Moon" always in the dark, and why?
The Moon is tidally locked to the Earth, which means that
the same side of the Moon always faces the Earth. The "dark side of the Moon" is thus the
side of the Moon that always points away from the Earth (called dark because
it is always hidden to us on Earth), not the side of the Moon which is
shadowed from the Sun. As the Moon goes around the Earth each month, the dark
side points towards the Sun during the new moon phase and then away from the
Sun during the full moon. Thus the dark side of the Moon can be fully
illuminated, and it is not always dark.
What are 2 observations, possible with the naked eye, which contradicted the Ptolmeic model?
[a]
the observed phases for the planet Venus, which could not be
explained if Venus orbited the Earth
[b]
the moons of Jupiter, which orbit Jupiter rather than the Earth
The Terrestrial planets in our solar system that have substantial atmospheres are:
[a] the Moon and Mercury
[b] Mercury, Venus, and the Earth
[c] Mars, Venus, and the Earth
[d] Mars, Jupiter, and the Earth
The more __________ an object is, the further back in time we look when we
observe it.
[a] bright
[b] faint
[c] nearby
[d] distant
Recall that the average distance between the Sun and Neptune is 30 AU.
How much sunlight per unit area falls on Neptune, relative to Earth?
Neptune is 30 times further away than the Earth is from
the Sun. The strength of light falls off as
the square of distance, so the amount of sunlight per unit area that falls
on Neptune is 30 × 30 = 900 times less than on Earth.
Recall that the radius of the Earth is RE = 1.25 ×
104 kilometers, and the radius of Neptune is RN = 2.50
× 104 kilometers.
How large is the area of the silhoutte of Earth (the area of the disk on the sky),
in square kilometers?
The Earth is a sphere (round). If you look at it from a
distance, it appears as a circle on the sky. We need only to calculate the
area of this circle.
R2
= (1.25 × 104 kilometers)2
= ( × 1.25 × 1.25) × 104+4 kilometers2
= 4.90 × 108 kilometers2How large is the area of the silhoutte of Neptune, in square kilometers? How much
larger is the silhoutte of Neptune than the silhoutte of Earth?
We need only to calculate the area of the equivalent
circle on the sky formed by Neptune.
R2
= (2.50 × 104 kilometers)2
= ( × 2.50 × 2.50) × 104+4 kilometers2
= 1.96 × 109 kilometers2We then compare the size of the two circles. The silhoutte of Neptune on the sky is 4 times larger than that of Earth.
How much total sunlight falls on Neptune, relative to Earth?
Neptune is 4 times larger than Earth on the sky, but
receives 900 times less light per unit area.
Where would you place Neptune in the solar system, in units of A.U., in
order for it to receive the same amount of total sunlight that the
Earth receives?
Neptune is 4 times larger than Earth on the sky, so where
would it receive 4 times less light per unit area? Light falls off as the
square of distance, so at a distance of 2 A.U. Neptune woule receive 4 times
less light per unit area than the Earth.
What planet lies closest to that position?
Mars lies at a radius
of 1.5 A.U., and is the planet which is closest to 2 A.U. from the Sun.
| Useful Numbers | ||
|---|---|---|
| 1 centimeter | = | 0.01 meters |
| 1 kilometer | = | 1,000 meters |
| 1 A.U. | = | 1.50 × 1011 meters |
| 1 year | = | 365 days |
| 1 year | = | 3.15 × 10 7 seconds |