Graphing Photoperiod — and Interpreting Graphs

Gayle Kloewer
7th Grade Science
York Middle School
York, Nebraska

Graphing Basics:
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).

Vertical Axis:
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.

Horizontal Axis:
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.

The 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.

The 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 together.

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.

If you have tips you'd like to share, please write to Journey North: