Photoperiod — and Interpreting Graphs
7th Grade Science
York Middle School
The Mystery Class graph is a line graph, which allows
students to plot and visually display the changing photoperiod of each
secret site from one week to the next, for all 11 Monday dates that
sunrise/sunset data are given for. Be sure you use a different
colored pencil (erasable ones are the way to go) for
each secret site and for your own location too (eleven colors total).
The left edge of your graph, the vertical axis, indicates day length
hours (photoperiod), from zero hours of daylight at the bottom to 24
hours at the top.
The bottom edge of your graph, the horizontal axis, denotes the dates of your
Monday photoperiod readings (eleven in total). For the first Monday, after
you've calculated the photoperiod for each secret site, mark each site's photoperiod
on the vertical line above the specific Monday date, using a different colored
pencil for each secret site, and for your own site too. Continue plotting this
photoperiod data each following week, and connect the respective site dots
from week to week with lines.
Importance of Home!
Over the years of participating in the Mystery Class activity, my students
are always confused about their role in the data collection. We collect sunrise
and sunset data for our school all year long so that the concept of what photoperiod
means and how to figure it is well ingrained before we start the Challenge.
They have been graphing our local photoperiod since school started so when
the time comes to graph, this also is something they know how to do.
confusion comes when they add the other ten Mystery Class photoperiod
graphs to their local one. The kids always think they are one of the
Mystery Classes. I have to show them that our data is graphed with the
Mystery Class information to act as a comparison tool. We live at 40
degrees North latitude so I have them compare the other lines to our
line. Usually their first reaction is that lines close to ours mean
that the locations are close to us. I sometimes let them continue with
that idea until the information comes in about longitude. This allows
them to correct their own misconceptions when they put the two ideas
More Help the Graphs Can Give
Determining Northern and Southern Hemispheres from the graphs is also
very easy, once they remember that their days are getting longer, so
graphs that are increasing like theirs must be on the same side of the
equator. Days that are getting shorter must be Southern Hemisphere locations.
Graphs that stay basically the same throughout the activity give indication
of closeness to the equator. Graph lines that begin slightly above the
12 hour line are created with data from just south of the equator and
the opposite is true with graph lines that begin slightly below the
12 hour line (of course until the Equinox!) They also come to realize
that the steeper the line, the faster daylight is changing. This usually
leads to a discussion of whether the changes associated with spring
are more drastic south of us or north of us.
Fine Tuning Our Conclusions
Once the geographical clues start coming in we spend less time with
the graphs until just before we submit our answers on where we think
the Mystery Sites are located. Then we return to our graphs for fine
tuning locations over which we are still having some debate. We use
locations we are sure about to judge approximate latitude of ones where
we can't make up our minds. After having participated in the Mystery
Class activity for several years, we can also go back to earlier graphs
and look for similarities with previously identified sites. This information
is available in the Journey North Spring archives.
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