APS 1030 Laboratory
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Motions of the Sun
MOTIONS OF THE SUN
SYNOPSIS: The objective of this lab is to become familiar with the apparent motions of the Sun
in the Boulder sky.
EQUIPMENT: A globe of the Earth, a light box.
Part I. Daily Motion of the Sun
The daily motion of the Sun is caused by the Earth’s rotation rather than the Sun actually moving
across the sky. (A subtle point! It took civilization thousands of years to figure this out!) The
diagram below show an overhead view of the Earth and Sun:
We will now follow a person (the stick figure) as the Earth rotates. At point ‘A’ our viewer is
rotated into view of the Sun, and the Sun appears to rise (i.e. sunrise!) Noon is defined as the
time when the viewer is located at point ‘B.’ This is the point where the Sun will be at its highest
point in the sky. At point ‘C’ our viewer is being rotated out of view of the Sun, and he or she
sees sunset. Midnight is defined as point ‘D’; the point opposite Noon. The Sun at points ‘A’,
‘B’ and ‘C’ as seen on Earth are shown below:
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Motions of the Sun
In the next part you will use the globe and light box:
I.1
Put the globe at one end of the table. At the other far end position the light box such that
it is illuminating the globe; the light box will simulate the Sun. Find Boulder on the
globe; and position the globe such that Boulder (latitude 40°, longitude 105°) is facing the
light box (at position ‘B’, i.e. Noon.) Orient the globe such that the Earth’s axis is
pointing 90° (in either direction) away from the light box. In a moment you will see that
this orientation represents the equinoxes; on these days the Northern and Southern
hemispheres each receive 12 hours of sunlight. What fraction of the Earth is illuminated?
I.2
At the time Boulder is at Noon:
• What major cities are experiencing sunrise and sunset? (pick one for each.)
• What country is at the same latitude as Boulder but on the other side of the globe?
• What time is it there?
• What are the people of Tibet doing right now?
• Are the Honda Corporation offices in Tokyo likely to be open at this moment?
I.3
Rotate the globe such that Boulder is now at sunset. How much of the Earth is
illuminated now? What time is it now for the country on the other side of the globe?
Part II. The Annual Motion of the Sun
The position of the Sun in the sky also changes as the Earth orbits around the Sun. This motion is
not to be confused with the daily motion of the Sun described above! The Earth’s axis is tilted
23.5° with respect to the perpendicular of its orbital plane (see diagram below). As a consequence
different parts of the Earth receive different amounts of sunlight depending on where the Earth is in
its orbit. Note: the orbit is circular, but it is shown here in projection:
Due to the Earth’s tilt the northern hemisphere receives more light during the summer months; and
less light during the winter. The Sun also rises higher in the sky during the summer; and this
direct sunlight is more warming. These two effects cause the summer to be warmer than the
winter. The summer is not warmer than the winter because the Earth is closer to the Sun; in fact,
the Earth is slightly closer to the Sun during the Northern hemisphere's winter! The Sun’s path
across the summer and winter sky is shown on the next page. Note that the winter Sun does not
rise as high; nor is it up for as long:
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Motions of the Sun
If you were to measure the position of the Sun every day at Noon over the span of a year, you
would notice that it slowly moves up and down with the seasons. For example, on June 21st the
Sun at Noon is at its highest, northernmost position, while on December 22nd the Sun is at its
lowest, southernmost position. These days are called the summer and winter solstices
respectively. Solstice literally means “Sun stop”; on those two days the Sun appears to stop
moving and change directions (e.g. on the Summer solstice the Sun stops moving to the North and
starts to descend to the South.)
The illustration above shows the Earth at the summer solstice. Position your globe such that
Boulder is at Noon and it is the Summer solstice. From the globe and the illustration above answer
the following questions:
II.1
What direction do the stars rotate around Polaris, the “North Star”, as seen from the
Northern hemisphere. Do they go around clockwise or counter-clockwise?
II.2
Go to the country at the same latitude as Boulder but on the other side of the globe. What
country is this? What season is this part of the world in?
II.3
Now go back to Boulder. Follow the 105° meridian south to a latitude of 40° south.
What is there? You should find Easter Island just to the north. (Did you notice South
America is much farther east than the U.S.?) What season is this part of the world in?
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Motions of the Sun
II.4
Finally, find the antipode to Boulder - the point directly opposite on the globe, through
the center of the Earth. (Hint: It is not China!)
II.5
The Tropic of Cancer marks the northernmost points where the Sun can be seen at the
zenith (i.e. directly overhead) respectively. On the summer solstice anyone standing on
the Tropic of Cancer will see the Sun overhead at Noon. Is there anywhere in the United
States where you could see the Sun at the zenith?
II.6
The Arctic Circle marks the northern region of the Earth where the Sun is sometimes,
always (or never!) visible. Find the town of Barrow, Alaska. On the summer solstice at
what times will the Sun rise and set here?
Part. III: The Southern Hemisphere
Without changing the orientation of the globe study what is happening “down under” in Australia.
III.1
On the Northern hemisphere's Summer solstice is the Sun to the North or South? What
season is Australia experiencing? Would this be a good time to ski the Snowy Mtns. in
New South Wales?
III.2
The Tropic of Capricorn marks the southernmost point at which the Sun can be seen
at zenith. Look at the town of Alice Springs in the center of Australia. Is the sun ever
directly overhead in Alice Springs? If so, when?
III.3
The Antarctic Circle is the southern equivalent of the Arctic Circle. On our summer
solstice what time does the Sun rise at the South Pole?
III.4
In Australia, do the stars rise in the east and set in the west? Which way do the stars
revolve around the south celestial pole?
Part IV: Thought Question
Everybody knows it's colder in December than in July (a tleast in the northern hemisphere!), but
why? Is it because of a change in the number of daylight hours? The height of the Sun above the
horizon? The "intensity" of the sunlight? Or are we simply closer to the Sun in the summer than in
winter?
IV.1
How might you figure out the reason for seasons? What observations might you make
over a year at the north pole, south pole, and equator?
Medieval Noon mark from Bedos de
Celles, La Gnomonique pratique
(Paris, 1790), a book on the use of
sundials. If you look carefully at
the shadows you may notice that
the artist has made a mistake.