End of Unit 4 Assignment
This assignment consists of four parts. The first is a review with
a couple of practice questions linked directly to their answers.
The second is a short quiz that you take using Blackboard. It will
be instantly scored for you by Blackboard; you only get one chance
to take it, however, so be sure you are ready! The third part is
an essay question. The question appears below;
when you are ready to answer it, log on to Blackboard and submit
your essay. Finally, for each unit, you should log on to Blackboard
and contribute a question, an answer, or a comment to one of the
posted topics. If you find the material in this unit challenging,
you might want to start with the "discussion" part of the
assignment in order to get some help with some of the ideas.
Click here for a printable
(.pdf) version of this assignment. To view .pdf files, you will need
the free software Adobe Acrobat - click
here to download Adobe Acrobat.
Brief summary of Unit Four:
This unit deals with the apparent
motion of the sun on the sky over the course of a year. This
apparent motion results from Earth's motion around the sun, and is
made complicated by the fact that Earth's equator and Earth's orbit
are not in the same plane. The North Pole of Earth points to
the same place in the sky all year, so as the earth orbits around
the sun, sometimes the sun appears north of the celestial equator
and sometimes it appears south. That is, the sun's declination
varies from -23.5 to +23.5 degrees. At the same time, the part
of the sky that is behind the sun changes, so in winter evenings
we see Orion and in summer evenings we see Sagittarius. In
other words, the sun's right ascension changes, increasing from 0
at the vernal equinox around to 24hours = 0 a year later. Right
ascension is measured in hours because it is closely related to sidereal
time. A simple way to tell sidereal time is that it equals
the right ascension of an object that is transiting the observer's
meridian. There are 23h56m of normal solar time in one
day of sidereal time, or, there are 365.2422 solar days in
one year and
366.2422 sidereal days in one year.
Practice Question One
If the arrangement of the
earth and sun are as sketched, is the declination of the sun positive
Practice Question Two
What are the right ascension and
declination of the sun on the following dates:
March 21, June 21, September 21, and December 21?
What are these dates called?
On which of these dates
will the sun rise due east?
When you are ready, go to Blackboardand
take Quiz Four.
You will get instant feedback on your score on Quiz Four
(and your instructor will also be informed of your score). If your
score is OK,
you may proceed directly to the Essay Question Four on Blackboard. Otherwise,
you might want to look at what you missed, ask your instructor about
questions you missed, or review relevant parts of the unit.
Essay Question in two parts
(a) Suppose that the tilt of the earth
were double what it actually is. Which of the following would
be affected: The dates of the
equinoxes; the declination of the sun at the equinoxes; the declination of the Sun at the solstices. Explain!
(b) What observations could an observer at the Earth's equator make to determine the declination of the Sun?
When you are ready to answer this question, go
write your answer.
Alternative to the Essay Question
You can verify the tilt of the
axis of the earth by observing the altitude of the sun
at noon over the course of several
months near one of the solstices. You can use a meridian
diagram to solve for the changing declination of the sun. Try
it and see what you get!
Don't forget to contribute to the discussion
on Blackboard on
one of the topics in this unit!