Solutions to Homework 01
Question 1
What is responsible for the seasons: The tilt of the axis of Earth's daily
rotation or the change in distance between Earth and Sun? Give a reason why
the other answer would not be correct.
Answer:
The tilt of the Earth's axis is responsible for the seasons. If it were the
distance from the Sun, the northern and the southern hemispheres would have
to have have the same seasons at the same time.
Question 2
What does it mean to say that the Earth is 8 light-minutes away from the
Sun?
Answer:
It simply means that it takes light (space = vacuum) eight minutes to come
all the way from the Sun to the Earth (or from the Earth to the Sun).
Question 3
In simple words: What is the reason for the precession of the earth axis that
is responsible for the change in polar star with time?
Answer:
The reason for the precession of the Earth's axis of daily rotation lies in the
gravitational pull of the Sun and the fact that the Earth is not spherical.
(This is enough of an answer.) The asymmetry in the Earth is such that the
Sun's pull tries to change the direction of Earth's rotation axis, which
responds to this torque by precessing.
Question 4
In reality the diameters of the Sun (1.4 million km) and the Moon
(3.5 thousand km) are very different indeed.
If we measure the angle their respective diameters suspend in the sky though,
they both come out almost the same: 1/2 of a degree... Why is that?
Answer:
Because the ratio of their respective distances from Earth is about the same
as that of their respective diameters. (The marginally acceptable answer is:
Because their distances are different.)
Question 5 (the quantitative one...)
On graphs we use axes to show amounts of a physical quantity. To go up or
to the right along an axis normally(!) means to increase the quantity shown
on that particular axis. On a normal (linear) axis moving up or right by
e.g. 1 inch means a certain amount of increase in the quantity shown, and
moving by 2 inches means and increase by double that amount. If moving by 1
inch on an axis with a log scale means to increase the quantity shown on that
axis by a factor of 5, what increase will be achieved by a moving 2 inches
along that same log-scale?
Answer:
25 is the answer. Equal intervals mean equal factors, so if one inch is a
factor of 5, two inches is a factor of 5 times 5 equals 25. If one inch was
a factor of 10 (one decade), two inches were a factor of 100, three a factor
of thousand, ...
The deadline for submitting solutions to this homework has passed.
Last modified: Thu Jan 17 14:44:40 MST 2008