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AST 151A Lab: Why We Have Seasons
In this lab, you will use a stick figure to represent different points on the Earth and
determine why the Earth experiences different seasons at different times of the year.
[This lab uses the same NAAP software package that you previously downloaded.]
Introduction
Watch the following video, featuring Bill Nye, explaining why we have seasons through
each year … https://www.youtube.com/watch?v=KUU7IyfR34o
After you have viewed the video, open the lab report template and read through the
questions you will need to answer.
Instructions
Open the NAAP applications package. Click on the link for “2. Basic Coordinates and
Seasons.” We will skip the information on coordinates. At the bottom of the list, click on
“Orbits and Light” and read the material provided. When you are ready, go to the next
page.
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Open the Seasons and Ecliptic Simulator.
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Note that there are three main panels (left, upper right, and lower right) each of
which have two different views. Controls run along the bottom of the simulation that
affect more than one panel. Click animate and then move through the six views to
get an overview this simulator’s capabilities. We will address each of these six
views separately.
•
Experiment with the various methods to advance time in the simulator. You may
click the start animate/stop animation button, drag the yearly time slider, or drag
either the sun or the earth in the left panel to advance time.
•
Note that this animation does not illustrate the rotation of the earth. Because the
timescales of rotation and revolution are so different, it isn’t possible to effectively
show both simultaneously.
Left Panel – Orbit View
•
Practice clicking and dragging in this panel to change the
perspective. Change the perspective so that you are looking
directly down onto the plane of the Earth’s orbit.
•
Click labels. Note that you can see how the direct rays of the
sun hit at different latitudes throughout the year.
Tip: Note that if you
click and drag the
Earth,
you
will
change the date and
location rather than
the perspective.
•
Experiment with this view until you can quickly create the two views shown below.
Note that these images explain the shape of the elliptic on the celestial sphere. In
the image on the left (summer solstice) an observer on the Earth sees the sun above
the celestial equator. In the image on the right (winter solstice) an observer on the
Earth sees the sun below the celestial equator.
Left Panel – Celestial Sphere
•
This view shows the earth at the center of the celestial sphere.
The celestial equator and the ecliptic with the sun’s location are
shown. Note that you may click on the sun and drag it and read
out its coordinates.
•
Experiment with this view until you can quickly create the image
to the right – the direct rays of the sun hitting the earth on the
summer solstice.
Upper Right Panel – View from Sun
•
This view shows the earth as seen from the sun. It gives the best
view of the subsolar point – the location on the earth where the
direct rays of the sun are hitting. The noon observer’s location on
the Earth is indicated by a red parallel of latitude which can be
dragged to new latitudes (this affects the appearance of the lower
right panel). It is possible for the red parallel to be at an
inaccessible location in this view.
•
Create the image shown to the right – an observer at latitude
80°N on the summer solstice.
Upper Right Panel – View from Side
•
This view shows the earth as seen from a location in the
plane of the ecliptic along a line tangent to the Earth’s orbit.
It allows one to easily see the regions of the Earth that are
in daylight and those that are in shadow.
•
Dragging the stick figure allows one to very conveniently
change latitude. Dragging the stick figure on top of the
subsolar point effectively puts the observer at the latitude
where the direct rays of the sun are hitting.
Tip: Once the stick figure
is selected you can gain
greater precision over its
motion by moving the
mouse a distance away
from the figure.
•
Although rotation is suppressed in this simulation, keep in mind that the stick figure
is on a planet rotating on a north/south axis with a period of 24 hours. Thus, 12
hours later, it will be on the other side of the earth.
•
Set up the simulator for the image at right – the winter solstice for an observer at 80
North latitude. Since this observer’s parallel of latitude is located entirely in the
shaded region, this observer will not see the sun on this day.
Lower Right Panel – Sunbeam Spread
•
This view shows a “cylinder” of light coming from the sun. It is projected on a grid to
convey the area over which the light is spread. As this light is spread over a larger
area, its intensity decreases.
Lower Right Panel – Sunlight Angle
This view shows the angle with which rays of sunlight are striking the Earth. It lists the
noon sun’s angle with respect to the horizon (its altitude).
•
Verify that when the noon observer is at the latitude where the most direct rays of
the sun are hitting, the sun is directly overhead making an angle of 90 with the
ground.
•
Verify that when the noon observer is at the latitude where the least direct rays of
the sun are hitting, the sun is on the horizon.
Now that you have learned how to use the simulator, use it to answer the questions in
the lab report template. Be sure to save your report using the file name convention
specified in “How to Submit Lab Reports.” When you are done, post your completed lab
report to Blackboard.
Grading Rubric
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•
•
•
•
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Table 1 is complete and correct (30 pts).
Answers to questions 2 to 4 are complete and correct (30 pts).
Table 2 has complete descriptions for each scenario (20 pts).
File name format is correct (5 pts).
Name and date are included in header (5 pts).
Lab report is submitted on time (10 pts).
Name:
Date:
Lab 07 – Why We Have Seasons
Question 1: The table below contains entries for the coordinates for the sun on the
ecliptic as well as the latitude at which the most direct and least direct rays of the sun
are hitting. Use the simulation to complete the table.
Date
RA
DEC
Latitude of Most
Direct Rays
Latitude of Least
Direct Rays
2.9 h
+16.5°
16.5° N
73.5° S
February 5
March 21
May 5
June 21
August 5
September 21
November 5
December 21
Question 2: Using the data in the table above, formulate general rules relating the
declination of the sun to the latitude where the most direct and least direct rays of the
sun are hitting.
Answer:
Question 3: The region between the Tropic of Cancer and the Tropic of Capricorn is
commonly known as the tropics. Using the sunlight data table from question 1, define
the significance of this region.
Answer:
Question 4: Using the sunlight data table from question 1, define the significance of the
region north of the Arctic Circle commonly referred to simply as the Arctic.
Answer:
Question 5: Use the simulator to complete the table below. For each latitude, write a
short paragraph which describes the variations in sunlight (seasons) that are
experienced at this latitude throughout the year.
Latitude
0°
23.5° N
41° N
66.5° N
90° N
Description of Yearly Pattern of Sunlight
The noon sun’s angular height above the horizon ranges from 90° on
the vernal equinox, to 66.5° on the summer solstice, to 90° on the
autumnal equinox, and back to 66.5° on the winter solstice. Thus, the
equator always receives very direct intense sunlight throughout the
year which accounts for the very high temperatures.
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