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Reasons for Seasons
Sunrise, Sunset, and Seasons

Exploring Daylength (Photoperiod)

Time
1-2 periods, then ongoing is optional (see step 2)
* Try to start this activity in the fall.

Materials
chart paper, daily newspaper (optional)


Teacher Background

Standards

Overview: Students track photoperiod (daylight hours) over time and predict how daylight will change during different seasons. This helps build their understanding that ever-changing daylight is the driving force for migrations and all other seasonal changes.

Laying the Groundwork
Ask, Which season do you think has the most daylight hours? The least? If students are stumped, ask, How do you spend your time after dinner in the summer? In the winter? Why do you think the amount of sunlight changes over the year? What questions do you have?
Write students’ ideas on a chart that they can revisit and revise at the end.

Exploration
Challenge the class to discover how the amount of daylight changes throughout the school year.

  1. Ask students to hypothesize about whether the amount of daylight (called photoperiod) will increase or decrease between fall and winter, winter and spring, and spring and summer. Have them explain the thinking behind their responses.
  2. Set up a class chart, or have students use individual charts, to write down the sunrise and sunset times for your location each day, each week, or only on the equinoxes and winter solstice.

    To locate these times, students can check the local paper or use the form on this Web site: Complete Sun and Moon Data for One Day. (Because you can punch in any date on the form, you can jump ahead to future dates, such as the first day of each season. This will enable you to complete the activity in just one or two class periods.)

    Older students should make charts for 2 additional locations – one north of you and one south of you. (To make this more engaging, try to set up an exchange with other classrooms.)
    If your students are participating in Mystery Class, they will receive this information for 10 classrooms hidden around the globe!

    Adaptation: Also have students track the maximum or average temperatures for each location. (They can locate this through the National Weather Service.)

  3. Once students have gathered the data, they should calculate the photoperiod (hours of daylight) – and high or average temperatures, if they have them – for each date; see Kids Calculating Photoperiods. Finally, they can graph their data (see Graphing Ideas).

What to Expect
The number and rate of changes in daylight hours varies according to the latitude of a location. The greatest and most rapid changes occur farthest from the equator (at the poles). On the equator, the daylight hours are close to equal all the time.

Locations north of you in this hemisphere should have more hours of daylight during the summer (between the Vernal and Autumnal Equinox) and fewer hours during the winter (between the Autumnal and Vernal Equinox). Those locations south of you in this hemisphere have the opposite pattern. The tilt of the Earth’s axis as it rotates and orbits around the sun causes these changes in daylight hours through the seasons.

Making Connections

  • Have students revisit and revise their responses charted during Laying the Groundwork.
Discussion and Journaling Questions:
  • What patterns do you notice?
  • How do you think the hours of daylight will progress until the first day of summer? Add your prediction to the graph.
  • How would you explain the pattern of sunrise and sunset times during the year?
  • Which month has the greatest number of daylight hours?
  • What patterns do you notice between daylength and temperatures?

    If students compared several locations:

  • What patterns do you notice?
  • Explain how latitude is related to daylength during different seasons.
  • Which parts of the hemisphere (latitudes) have the most dramatic changes in day length? The least dramatic? How would you explain this?

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