SUNDIAL
ACTIVITY
Sundials are great scientific instruments that can help us tell
time! When we read the time on the sundial it is giving us the
SOLAR TIME which tells us when the sun is crossing our line
of longitude. If we are in Florida, are we on the same longitude
line as students who are in Spain? No, we are on a different
longitude line!
We are near the longitude line where the blue arrow is
pointing.
Students in Spain are closer to the longitude line where the
red arrow is pointing.
As junior scientists we need to do some math to find out the WATCH
TIME which is the time we see when we look at a clock or our watches!
To do this we need to use the graph plaque on the “Man of the Cosmos”
sundial.
Across the top of the graph ( x-axis) , the months of the year
are shown January through December
Running down the graph on the right side (y-axis) we have
numbers.
Notice that zero has a thick line extending from it. That line
tells you whether or not you have to add minutes or subtract
minutes.
Steps to calculating the watch time:
1. Read the hour and minutes on the sundial
2. Determine the correction of minutes using the graph
 Ask yourself, what month are we in?
 Put your finger on that month on the graph
 Move your finger straight now until your finger
runs into the squiggly line
 Look to the left and see what number you stopped
at
 Ask yourself, is my finger above or below the thick
line?
o If above you are going to add that number to
your minutes
o If below you are going to subtract that
number from your minutes
3. Determine the correct hour by adding 1 to the number
you saw on the sundial
LET’S PRACTICE!
Step 1: Locate the shadow on this picture
Review questions:
1. How many minutes in an
hour?......................................................60 minutes
2.
Using the picture, how many minutes does each little
mark go up by?..................5 minutes
Step 2: Read the hour an minutes on this close- up of the
previous picture to find the solar time
The sundial says that it is 3:45 in the afternoon.
Step 3: This picture was taken in April so use the graph to
correct the minutes so that they are in watch time.
 place your finger on April
 Follow it down….
 and stop, your finger should hit the graph at the ten
mark. You will also notice that your finger is
BELOW the thick line so we are going to
SUBTRACT 10 minutes from 3:45 p.m. This will give
us a time of 3:35 p.m.
Step 4: Now we have our minutes corrected now we have to
correct our hours. Go back and read the last step in our “Steps
to reading a sundial” section. It says we have to add one hour
to our time to get the watch time.
3:35 p.m. + 1 hour = 4:35 p.m.
Step 5: We are done and we have officially calculated the
WATCH TIME using out sundial and of course the sun!
Let’s practice calculating a few more watch times.
1. Dana went to see the “Man of the Cosmos” sundial in
February. She looked at the sundial and determined that
it was 5:35 p.m. solar time. What is the watch time?
2. Patrick lives in Chicago and he decided that he wanted to
get up really early so he could go and see the “Man of the
Cosmos” sundial. It was December and really cold, but he
made it there. When he got there, he saw that the sundial
said that it was 6:30 a.m. What is the watch time?
3. Molly visited the sundial with her family in August after
they spent a whole day at the Adler Planetarium and
Astronomy Museum. When they got there Molly
discovered that the sundial said that it was 7:00 p.m.
What is the watch time?
ANSWERS:
1. 6:40 p.m.
2. 7:20 a.m.
3. 7:55 p.m.
Picture source:
http://www.wsanford.com/~wsanford/exo/sundials/aphmbes.ht
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