AST101 – Planetary Laboratory
Table 1 – Azimuth Angles for Sundial Construction
Time AM
Angle
Time PM
6:00
6:00
7:00
5:00
8:00
4:00
9:00
3:00
10:00
2:00
11:00
1:00
Angle
Use the sundial for several days at home; Use a vertical gnomon to find which direction
is north, and align your sundial accordingly. Compare observations of the time it reads
with the time of day measured by a modern clock at several different times each day, and
record your results. Turn in your records, along with your comments on how accurate
your sundial is at telling time. Also, please answer the questions on the next page – show
any calculations you make for full credit. You can find many of the answers online at
numerous websites, particularly www.nasa.gov.
Date
Actual Time
Sundial Time
Difference
4-5
AST101 – Planetary Laboratory
Questions:
1. What kind of sundial was sent to Mars? Compare how it works on Earth with how it
works on Mars.
2. When does a sundial NOT work?
3. Does your sundial match your watch time? Why or why not?
4. What is the relationship between your sundial’s time, local time, and standard time?
5. Why don’t we use local solar time instead of time zones in our everyday lives?
6. Why do time zones generally run north-south instead of east-west?
7. Earth rotates once every 24 hours. How many degrees does the sun appear to move in
one hour? In four minutes? (Hint: one full rotation of the earth is 360 degrees).
8. The Sun’s diameter in the sky is about 0.5 degree. About how long does it take for
the Sun to appear to move its own diameter across the sky?
9. What would be different about a sundial at the North Pole?
10. Would your sundial read the same time as another sundial 100 miles directly
north of you? Would the shadows be the same length?
11. Describe two types of sundials other than the horizontal dial you built in this lab
exercise.
4-6
AST101 – Planetary Laboratory
Name __________________________ Date _________________ Section __________
Astronomy 101 Planetary Laboratory
Lab#4 – Sundials and Time
The primary goal of this lab exercise is to build a sundial, try it out, and see how it works.
Sundials have been used throughout human history to tell time; and even though they are
primarily used today as ornaments, some are still used and have even been sent to Mars!
The shadow of a vertical stick can show us the time of day, because on any given day its
shadow will be shortest at noon, when the Sun is on the meridian. Also, because the Sun
is highest in summer and lowest in winter, the vertical “shadow stick” (or gnomon)
sundial can also tell the seasons by the length of the noon shadow from day to day.
Furthermore, because the daily motion of the Sun is from East to West, the shadow
moves clockwise around the gnomon.
The ancient Egyptians built tall vertical gnomons called obelisks. (A famous obelisk is
called Cleopatra’s needle can be found behind the Metropolitan Museum of Art in
Manhattan. Unfortunately, acid rain is destroying the ancient artwork on the four sides of
this structure.) A gnomon’s shadow changes from day to day since the sun’s path rises
and falls during the season; so the resulting hour marks have to be changed day by day if
you set up just a vertical structure. Ancient astronomers then discovered that, if a
gnomon were not just set vertically, but tilted to be parallel to Earth’s axis, hour marks
could be made on a horizontal dial that could correctly tell the time throughout the year.
This “tilted gnomon” (or style) greatly simplified accurate sundial construction; all you
have to do is find the angles on the horizontal dial that the shadow of the style makes on
the dial for each daylight hour.
Part One: BUILDING A HORIZONTAL SUNDIAL
There are many ways to design a sundial; the principle used to build them all, though, is
the same. Construction is based on where you are on Earth – you have to angle the style
to match your latitude to keep it parallel to Earth’s rotational axis. The horizontal sundial
you will construct in this exercise has a flat horizontal base, and hypotenuse of a small
right triangle will serve as the style. The marks made on the dial will be correct at any
time of year as long the noon position is oriented to the true geographic north of the
placement position. (In modern times, that can be done by watching for the shortest
shadow of a vertical stick or using a magnetic compass – keep in mind, though, the
magnetic north pole on Earth is not exactly the same as the geographic north pole.)
Please refer to Figures 1 and 2 on the next page as you construct your sundial.
4-1
AST101 – Planetary Laboratory
Figure 1. The base of the Horizontal Dial
L
||
FIGURE 2. The style (L, the angle between the base and the hypotenuse, is your latitude)
4-2
AST101 – Planetary Laboratory
– Use a pair of sharp scissors to cut a 6” by 6” base plate from the foam board material
provided by your instructor.
– Use a ruler or protractor to find the exact center. Around that center, use a compass to
draw a circle 2.5” in radius. Also, draw a vertical line through the center.
– One inch from the bottom of the base plate, draw a large dot on the centerline. Draw
another dot on the centerline one inch from the top of the base plate. Construct a
perpendicular line through the lower dot as shown in Figure 1.
– Draw the number 12 and the two numbers 6 as shown in Figure 1.
– Now draw a right triangle on some other foam board as shown in Figure 2. (Hint: use
one of the existing corners of the board as the 90-degree angle, to create a more precise
right triangle and to conserve foam board material.) Make the base length 4 inches, and
use a protractor to make angle L the same number of degrees as your latitude on Earth.
– Cut the right triangle out; this piece will serve as the style of your sundial. Later, you
will be gluing this piece to the base. Do not glue yet, though.
Part Two: CONSTRUCTING THE HOUR LINES ON THE HORIZONTAL DIAL
Set up the celestial sphere globe used in Lab Exercise 3 for your latitude. (These
instructions are given for New York City.) The procedure is the same – have your
instructor check your setup:
– Turn the Sun Pointer Knob to put the Sun on today’s date. (Make sure the Sun does not
change position as you move the globe.) Alert your lab instructor if the Sun does not
hold its position.
– Rotate the globe along the meridian toward the N mark on horizon ring so that the
DEC scale on the metallic meridian reads about 49 next to the top of the horizon ring.
This action will put the NCP at +41 degrees, the same angle as the latitude of New York
City. You can verify this by counting the number of degrees from the horizon ring to the
NCP. If you’re not sure about this, ask your lab instructor for clarification.
– To set the globe for time, turn the globe so that the Sun is directly under Earth and just
above the lower metallic meridian ring. Now the Sun is at the midnight position.
– Without undoing the previous operations, turn the small inner Earth using the Earth
Rotation Knob (again, see the figure on the answer sheet) until New York is lined up
directly under the upper meridian. Gently hold the plastic globe so that it doesn’t rotate
while you do this. If you get lost, ask your lab instructor for guidance.
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AST101 – Planetary Laboratory
If all has gone well with the setup, the globe is now set at 0 hr for today’s date. Next:
– Move the Sun to the vernal equinox position (the intersection of the ecliptic and the
celestial equator that takes place in March). At this position, the Right Ascension (RA)
hour angle is 0 h.
– Place the Sun on the meridian to simulate local noon. Now imagine that Earth’s axis
(the metal rod) is receiving light from the model Sun. The axis would cast a shadow
down on our horizon (represented by the horizon ring). If you follow the 0 hour arc
through the celestial pole and then down to the horizon, it becomes the 12 hour arc for
RA. The 12 hour arc meets the horizon at the 0 degree azimuth angle for the noon
position of the Sun. Note the azimuth angles are marked on the ring. Recall the sky turns
15 degrees an hour. Turn the globe toward the west 15 degrees (it is now 1:00 PM) and
write down the azimuth angle for the 12 hour RA arc. This is the angle Earth’s axis
would project onto the horizontal plane.
Remember that the style on your sundial is a line that is parallel to Earth’s axis. That
means that the angles found by this technique are the angles to be used to construct
reference lines for other hours of the day. Use the angle you measured to construct a line
from the lower spot (on the cross line to the 6’s) to the outer dial near the 12. Label this 1
for 1:00 PM. (For example if you found the angle is 10 degrees then use your protractor
to construct a line 10 degrees from the 12 o’clock line; see Figure 1 above for reference).
ASK YOUR INSTRUCTOR IF YOU ARE UNSURE OF ANY PROCEDURES.
Proceed at 15-degree intervals to determine the azimuth angles (i.e., construction angles)
for the rest of the hours (2:00, 3:00, 4:00, 5:00, and 6:00 PM) after the 12 noon position.
Then find the angles to construct the lines for 11:00, 10:00, 9:00, 8:00, 7:00, and 6:00
AM. Draw in the lines as you find the angles. Fill in all angles found on Table 1 on page
4-5.
If you have done this correctly, you should find that the 6:00 AM and 6:00 PM positions
have an angle of 90 degrees. For reference, you have already drawn these lines in Part
One of this exercise.
Part Three: USING THE SUNDIAL
Now glue the style onto the vertical line that runs through the center of the sundial, with
the angle of latitude on the lower dot and the 4-inch base running between the lower and
upper dots. The vertical line of the right triangle should be on the upper dot.
4-4
AST101 – Planetary Laboratory
Table 1 – Azimuth Angles for Sundial Construction
Time AM
Angle
Time PM
6:00
6:00
7:00
5:00
8:00
4:00
9:00
3:00
10:00
2:00
11:00
1:00
Angle
Use the sundial for several days at home; Use a vertical gnomon to find which direction
is north, and align your sundial accordingly. Compare observations of the time it reads
with the time of day measured by a modern clock at several different times each day, and
record your results. Turn in your records, along with your comments on how accurate
your sundial is at telling time. Also, please answer the questions on the next page – show
any calculations you make for full credit. You can find many of the answers online at
numerous websites, particularly www.nasa.gov.
Date
Actual Time
Sundial Time
Difference
4-5
AST101 – Planetary Laboratory
Questions:
1. What kind of sundial was sent to Mars? Compare how it works on Earth with how it
works on Mars.
2. When does a sundial NOT work?
3. Does your sundial match your watch time? Why or why not?
4. What is the relationship between your sundial’s time, local time, and standard time?
5. Why don’t we use local solar time instead of time zones in our everyday lives?
6. Why do time zones generally run north-south instead of east-west?
7. Earth rotates once every 24 hours. How many degrees does the sun appear to move in
one hour? In four minutes? (Hint: one full rotation of the earth is 360 degrees).
8. The Sun’s diameter in the sky is about 0.5 degree. About how long does it take for
the Sun to appear to move its own diameter across the sky?
9. What would be different about a sundial at the North Pole?
10. Would your sundial read the same time as another sundial 100 miles directly
north of you? Would the shadows be the same length?
11. Describe two types of sundials other than the horizontal dial you built in this lab
exercise.
4-6
Name _________________________ Date _______________ Section ___________
Astronomy 101 Laboratory Preparation Assignment #4 – Sundials and Time
Reading Assignment:
•
•
Section 4.3 (Keeping Time) of the online OpenStax Astronomy textbook
The FAQ page of the Time and Frequency Division of the National Institute of
Standards and Technology: https://www.nist.gov/pml/time-and-frequencydivision/popular-links/web-clock-faq
Questions:
1. What is a meridian?
2. On a clock, what do the abbreviations AM and PM stand for and what do they mean?
3. What is a sidereal day, and what is a solar day?
4. What is apparent solar time, and what scientific device can be used to determine it?
5. What is mean solar time, and what scientific device can be used to determine it?
6. What is Standard Time and what is Daylight Savings Time?
7. What is Universal Time (UT)? What are some of its other names?
8. What is an Analemma?
9. What is the equation of time?
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