To compute the change in the amount of daylight from day to day.
Students need about 10 minutes each day, several days in a row, to write down the hours of sunrise and sunset, calculate the length of the daylight hours, and figure the rate of change in daylight hours for their area. If possible plan to do the activity before and after winter solstice (about December 22).
- Using the data from the weather report in the daily newspaper, write down the time of sunrise and the time of sunset, and figure the length of the daylight hours in your area (count the whole hours and then add together the number of minutes left until the next hour and the number of minutes after that hour).
- Enter this information in a table. Enter data in the table for several days in a row. Subtract the length of the previous day from the length of each new day (or vice versa) to determine how much the amount of daylight has lengthened or shortened. Enter the rate of change per day in the last column of the table. The students will find that most days are a certain number of minutes shorter or longer, and every few days the change is 1 minute less. This is because we are measuring the change in whole minutes.
- Repeat these calculations later in the school year. If your school is located north of the equator and you figure the rate of change per day before and after December 22, the students will find the amount of daylight shortening before the winter solstice and lengthening after it. The rate of change will depend on the latitude, or distance north of the equator. This rate of change also varies at any location according to the time of year. The rate is highest near the equinoxes (about September 22 and March 22) and is lowest near the solstices (about December 22 and June 22).
- Your students can also share this data with other classes. Have them complete their sunrise/sunset calculations on paper, and then upload the same figures to our web site's daylight data form. We will process this information as it comes in and will display it within 24-48 hours. In the meantime, your students can compare their sunrise/sunset figures to those of other participating classes.
- The students will soon find the rate of change per day for their area. Consider the most frequent decrease or increase the average for that latitude at that time of year.
- After you have filled out the table at different times of year, you can discuss with the students how the rate seems to change for different times of year.
- If you do the project on different sides of the winter solstice, your students will have quantitative proof of the shortening and lengthening of the days that they observe each year.
- At this point the students are bound to wonder where a newspaper gets the times for sunrise and sunset. Newspapers get the times from sunrise-sunset charts published in an ephemeris, an astronomical almanac describing movement of heavenly bodies. After many hours of observation, astronomers are able to calculate these movements based on theories that take into account the gravitational effects of all the bodies involved. The ephemeris, or tables abstracted from it, can be purchased from planetariums, observatories, boat supply stores, or the Naval Observatory in Washington, D.C. (see Resource). There are many online ephemerides. Each presents the data in a different way. Try some of these and see which one you like best.
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