Download HW #1 (Due 9/2) Answers

HW #1 (Due 9/2) Answers
1. Explain the concept of “look-back time”. Do this in two different ways.
a. If radio astronomers receive a signal from an intelligent civilization in the Andromeda
Galaxy (M31), how long ago did the extraterrestrials send the message? Explain your
answer.
M31 is 2.2 million light years away from the Earth. Since radio waves are part of the
electromagnetic spectrum they travel at the speed of light. 2.2 million light years means it
takes light 2.2 million years to cross this distance. So the signal was sent out 2.2 million years
ago.
b. On December 8, 1941, Franklin Roosevelt’s “Day that will live in infamy” speech
was broadcast over the radio. If an extraterrestrial civilization is listening in to our
broadcasts and today they hear this famous speech coming from the Earth, how far
away are the extraterrestrials? Explain your reasoning and solve the problem.
As in part (a) radio signals travel at the speed of light. The Day of infamy speech left Earth in
1941. So it has been traveling outward from Earth for 2009 – 1941 = 68 years. Any alien
civilization that detects this signal today is at a distance of 68 light years from the Earth. (Side
note: Earth has been transmitting radio signals since about 1900. This means that aliens
detecting our broadcasts have to live within 109 light years of Earth.)
2. Explain the difference between the terms, Solar System, Galaxy and Universe. Include
in your explanation the difference in scale (size).
The Solar System includes our Sun, the eight planets, asteroids, Kuiper objects, comets and
meteors. SO this is only our Sun’s system. It is on the order tens of light hours across. The
Galaxy (or Milky Way) is the system of stars of which our solar system is a very very small
part. There are on the order of 400 billion stars in the Galaxy of which our Sun is but one.
The size of the Galaxy in on the order of 100,000 light years across. The Universe is
everything. All the galaxies, stars, gas and dust that there is. The part of the Universe that we
can see is on the order of 10 billion light years in size.
3. The Earth is 1 astronomical unit from the Sun (A.U.). Jupiter is 5.2 A.U. from the Sun.
It takes light from the Sun about 8 minutes to reach Earth. How long does it take light
from the Sun to reach Jupiter? Show your work.
5.2 AU x (8 light minutes/ 1 AU) = 41.6 light minutes. It takes 41.6 minutes for light
from the Sun to reach Jupiter.
4. Explain how the crescent phase of the Moon is different from a lunar eclipse.
Normally the Moon (and the Earth) is half lit by the Sun. The side facing the Sun is
illuminated and the side away from the Sun is in darkness (night time). When the
Moon is near the Sun in the sky it is half lit, but from our vantage point on Earth we
see mostly the night side of the Moon. The lit, or day time side is facing away from us.
So we see a little crescent of light and the rest is dark. A lunar eclipse is when the
Moon moves into the shadow of the Earth. This can only happen during full moon
when the Moon is on the opposite side of the sky from the Sun. The lunar eclipse is
actually the shadow of the Earth on the Moon. The crescent phase of the moon is not
the shadow of the Earth.
5. You have very likely heard that “the Earth’s axis is tilted”. But the Earth is just orbiting
our in the universe. Out in empty space. Explain what the Earth’s axis is and explain
what it is tilted relative to.
The rotation axis of the Earth is an imaginary line that the Earth rotates around as it spins.
The axis goes through the poles of the Earth. The Earth also orbits the Sun, once per year.
This orbit is defined by a plane that contains the Earth and the Sun and is called our orbital
plane. This plane is also called the ecliptic. The Earth’s rotation axis is tilted 23.5 degrees
from perpendicular to our orbital plane. If there were no tilt, then the axis would be
perpendicular to the orbital plane. (Side note: The result of this tilt is to make the Sun change
its elevation about the horizon between winter and summer seasons. The more direct sun light
in the Summer allows the Earth to warm up more than in the winter.)