Understanding the Relationship between Time and the Sun
sun dagger at Chaco Canyon is thought by many to be a sort of ancient
timekeeping device. By creating a place where the movement of the
sun could be tracked day after day, Chacoans could mark the passage
of time and gain some idea of when seasons were changing. They used
a location they knew—nearby Fajada Butte—to
figure out what they didn’t know—the time.
Time: 45 minutes to 1 hour
Download this activity:
Word | PDF
See our Teacher Tips
for standards, discussion ideas, and more.
This works because there is a cyclical relationship between the
passage of time and where the earth is in its daily and yearly rotations.
In fact, it is these very rotations—the spin of the earth
on its axis and the length of its tour around the sun—that
we use to measure days and years.
For centuries, people have relied on the position of the sun in
the sky as a way to tell the time. That position is, of course,
dependent on where you are on the globe. The sun might be high overhead
in California at the same moment when it is setting on the horizon
activity, Modeling the Seasons shows
you more about why our experience of the sun changes with time and
If we can use our location and the sun to tell us about time, can
we use time and the sun to tell us about our location? In this easy
experiment, you’ll see how the place of the sun in the sky
is related to where we are on the earth.
North with a Watch
All watches tell time. But a watch with hands can show you directions,
• a dial watch (the old-fashioned kind with hands that move)
• a sunny day
• a compass (optional: for the activity in the “Going
About daylight saving time:
In most of the United States (and in many other countries), people
reset their clocks and watches to daylight saving time for the summer
If you try this activity using a watch that is set to daylight saving
time, you’ll be one hour too early. Before you start an activity,
set your watch to standard time. If you’re on daylight saving
time, reset your watch to one hour earlier—from 3:00 p.m. to
2:00 p.m., for example. Then use the time on your watch when you’re
doing the activity.
In the United States, daylight saving time (DST) begins on the first
Sunday in April and ends on the last Sunday in October. In Europe,
it begins on the last Sunday in March and ends on the last Sunday
Here’s a link where you can find out whether you’re on
Scroll down to the bottom of the page to find start and end dates
for DST around the world.
To Do and Notice
This way of finding north works anywhere in the Northern Hemisphere.
outside between 6:00 a.m. and 6:00 p.m. and find a place where you
cast a shadow on level ground. Make sure your watch is set to the
proper time (remember, if you are on daylight saving time, reset your
watch to one hour earlier).
Turn your watch so that the hour hand (the little hand) points in
the same direction as your shadow.
Stand so that one foot lines up with both your shadow and the hour
hand. Line up the other foot with the 12 on the watch face. (If it’s
before noon, you’ll line up your right foot with the 12. If
it’s after noon, you’ll line up your left foot with the
12.) Now your body is facing north.
You can also find north by imagining a line that passes from the center
of your watch halfway between the hour hand and the 12—this
line will point due north. (There are two possible ways to get from
the hour hand to 12. Divide the smaller of them with the imaginary
What’s Going On?
Shadows change as the sun moves. The shadow of a stick standing straight
up, for instance, pivots around the base of the stick as the day wears
on. (You can find teacher activities related to this idea at the NSTA
with a Stick.)
At exactly noon, the stick’s shadow (or your shadow) will point
due north because the sun is due south. So even with a digital watch,
you can find north at noon.
At any other time of day, a shadow is at some angle away from north.
The size of the angle changes with the motion of the sun. In the morning,
the angle between the shadow and north is large. It gets smaller until
noon, when the shadow and north line up. As noon passes, the shadow
crosses north, and the angle grows larger again for the rest of the
The earth rotates at a steady rate. Because the earth is spinning,
the sun seems to move across the sky at a steady rate, and the angle
of a shadow changes at a steady rate. The earth makes one complete
turn, or spin, every 24 hours. A complete spin is 360 degrees. Since
it spins through 360 degrees of angle in 24 hours, the earth’s
rate of rotation is 360 degrees ÷ 24 hours, which equals 15
degrees per hour. A shadow’s angle away from north changes at
that same steady rate.
While you’re outside, make a mark or lay an object on the ground
in the direction you’ve found to be north. Then, take out a
compass and compare that reading for north with the one you’ve
found using the watch. Do the two readings agree with each other?
If not, does that mean that one of them is wrong?
Unless you’re in one of a small number of places on the globe,
you’ll find that the two readings differ slightly. North as
indicated by the watch activity is known as “polar north,”
“geographical north,” or “true north.” This
is the direction toward the North Pole, the northernmost point on
the planet. If you were to stand on the North Pole while the earth
rotated on its axis, you would remain in the same place while the
earth south of you spun in a circle.
Your compass, on the other hand, is showing you “magnetic north,”
which is the direction of the earth’s magnetic pole. This actually
lies 700 miles from the North Pole, in the Queen Elizabeth Islands
in Canada. The difference between true north and magnetic north is
called “magnetic declination,” and it depends on where
you are on the globe. For example, in San Francisco, the declination
is 15 degrees east, meaning that your compass will point 15 degrees
east of true north. If you only have a compass and you want to adjust
the reading for true north, you can find declination for your location
at this Web site: http://ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp.
Which of the values for north do you think the Chacoans used when
constructing their buildings? Why?
What other instances in history can you think of where people used
the sun to show them true north?
Is it more useful for modern ship navigators to use true north or
Make Your Own Petroglyphs
| Modeling the Seasons | Seasons