Estimated Time to Complete Lesson
The activity should take one-and-a-half class periods. It will take one period
to make the astrolabe, which students can then take home at night to determine
their latitude. The next day students can compare their results and calculate
an average latitude based on the class results.
Teaching StrategyBackground information
An astrolabe is an ancient scientific instrument that the Greeks used in the
second century B.C. to determine the latitude at which they were located. The
astrolabe, which allows the user to measure vertical angles, was very important
to sailors in ancient times. Determining the angle between your position on
Earth and Polaris determines your latitude. A person standing at the North Pole
will find Polaris straight overhead (90°), which is her latitude. A person
standing at the equator will find Polaris at the horizon (0°), also her
latitude. Polaris is not visible below the Equator. A local night star map would be helpful for this activity. Polaris is
not a very bright star and students may need to identify the Big Dipper, which
will then point them to Polaris.
Have students cut out the astrolabe silhouette from the activity sheet
(heavy black lines).
Have students tape this silhouette onto their manila folder sheet and cut
out this figure.
Have students make a tiny hole at the index point (marked by an "X," which is
the midpoint of the horizontal axis), push the end of
a 7-inch piece of string through the hole and tape that end to the backside of the
cardboard. Have students attach a paper clip to serve as a weight at the other
end of the string that falls over the degree markings on the front of their
Have students fold the cardboard along the dashed line as indicated on the
diagram so that the sides with the triangular notch form a right angle with the
face of the astrolabe.
Have students take their astrolabes home to use at night. First they need to
locate the Big Dipper, which will point them to Polaris. Refer students to a
local star map. Then have students face Polaris and raise their astrolabe until
they can sight Polaris through both notches. At this point they should press
the string against the cardboard and record the angle at which the string
crosses the scale. This angle is the altitude of Polaris, which is also their
Next day in class have students pool their latitude data and determine the
class average for the latitude value. How does this average compare to the
actual latitude value found on a map? Ask students to identify reasons for
error in the determinations they made.
To conclude the lesson, discuss other kinds of navigation. One form of
navigation that Ernest Shackleton's crew used was a sextant. To help students
better understand how a sextant works, have them do the online activity,
Escape from Antarctica.
To acquaint students with more modern navigation
techniques, have them check out NOVA Online's GPS: The New
Find your city's actual latitude (and longitude) by typing your town name and
zip code in at this site.
Students may be assessed through:
their participation in the activity.
the accuracy of the astrolabe they constructed.
the accuracy of their latitude determination.
Calculate the percent error in their latitude determination. Students
can use the following formula to do this:
% error = difference between the students value and the true value (from map) x 100
true value (from map)
Calculate the percent error of the class average. Do the same as
described in step 1 but for the entire class. How does this average compare to
their own individual value? Have students discuss the merits of an average
value versus one person's result.
Search for other constellations. They can look for other
constellations such as Orion, Perseus, and the Little Dipper in their night sky
and diagram those they found.
Research other methods past sailors used to determine their location.
Students can start with this NOVA Online site, Longitude, which tells
the story of how one man figured out how knowing what time it was in your home
port could tell you where you were at sea.