Lunar and Planetary Institute

 Children investigate and graph the changing day length through the year and across latitudes. Preparation Using the U.S. Naval Observatories Astronomical Observations Department, Data Services, Table of Sunrise/Sunset, Moonrise/Moonset, or Twilight Times for an Entire Year), download sunrise and sunset data tables for several locations at corresponding north and south latitudes (for example Moscow is approximately 55 degrees north; Cape Horn is approximately 55 degrees south latitude). Ask the children if the day length changes through the year. In what ways? Days tend to be longer in the summer and shorter in the winter. Which day is the longest? The shortest? Select several cities, one per group, that represent a wide variety of latitudes. For each city, highlight the last day of each month. Divide the children into groups, with a different city data table for each group. Ask each group to calculate the number of hours between sunrise and sunset for their city for the last day of each month and to make a table showing this information. What trends do they observe for their city? Invite the children to graph the number of hours of daylight for the months of the year. What kind of graph will they use? Because they will compare their results, the graph axes also should be comparable. Help the children decide on a standard graph, axes, and key and then have each group plot the 12 points (one for each month) for their city on their graph. As each group finishes with their graph, have them look on the globe to find the approximate latitude of their city. Have the groups post their graphs sequentially on a wall, with northern hemisphere cities above southern hemisphere cities of corresponding latitude. Invite the children to examine the plots. What do they observe? Do locations with similar latitudes, but in different hemispheres, have the same number of daylight hours for the same time of year? (patterns are opposite) What patterns do the children observe for latitude? (the higher the latitude, the greater the difference in day length from summer to winter) Which location(s) had the largest extremes? (cities closer to polar regions) The smallest? (cities closer to the equator) Invite the children to find these locations on the globe. Where were the days the longest? (cities closer to polar regions) Shortest? (cities closer to polar regions) Is there a time of year when the day length is close to the same for all the locations? (at the spring and fall equinoxes) What do the children observe about the largest and smallest number of daylight hours of cities in different hemispheres? (the largest and smallest are opposite for corresponding latitudes. For example, during the time of year (winter) when days are shorter at 45° north latitude, the days are longer, and it is summer, at 45° south latitude) What do the children think is the cause of these changes in daylight hours? (the tilt of Earth's axis, which controls which hemisphere is tilted toward the Sun) Invite them to use their materials to model the interaction of our Sun and Earth to explain the changes. Lead a discussion of the causes of changing day length. An animated cartoon illustrating the movement of Earth around the Sun and the resulting change in day length can be found at this site. Day and Night Cycles offers a hands-on enhancement of this discussion. More Activities Last updated January 4, 2007 Who? Ages 13–15 How Long? 120 minutes (can be divided into segments) What's Needed? For each group of 4–5 • Highlighter • Graph paper • Paper • Pencils• Colored markers or pencils (optional) internet access or printout of USNO data tables • Materials to demonstrate day and night cycles (optional) Connections to the National Science Standard(s) Standards A& D(grades 5–8): Understand and communicate the scientific explanation of how objects in the solar system have regular and predictable motion, such as the apparent daily movement of the Sun in the day/night cycle and annually across seasons. Standard A (grades 9–12): Use tables and graphs to communicate results of investigation of Earth's day/night cycle.