Kids Calculating Photoperiods

Here are three ways in which students can calculate the photoperiod of a location from sunrise and sunset data.

1. Use noon as a pivot point.
If the sunrise is at 6:14 a.m. and the sunset is at 9: 36 p.m., you would divide the photoperiod into two parts: the amount of time before noon and the amount of time after noon. Most students would work it through like this: from sunrise (6:14) to 6:30 is 16 minutes, from 6:30 to noon is 5 hours and 30 minutes, so from sunrise to noon is 5 hours and 46 minutes. From noon to sunset is 9 hours and 36 minutes so the total would be 14 hours and 82 minutes. Since they know that there are only 60 minutes in an hour, it becomes 15 hours and 22 minutes. It may help to state it visually:

sunrise   noon   sunset
6:14 morning photoperiod 12:00 afternoon photoperiod 9:36

2. Convert sunrise and sunset numbers to a 24-hour clock and subtract sunrise from sunset.

sunset 9:36

convert to 24 hour clock



sunrise 6:14


- 6:14





15 hours and 22 minutes

This can be challenging when one of the hours has to be converted to 60 minutes to enable proper subtraction. For example:

sunset 9:08

converted 21:08



- sunrise 5:52







15 hours and 16 minutes

3. Mystery Class veteran teacher, Jo Leland offered this helpful Teacher Tip for calculating photoperiod too:

Students may also come up with their own methods that come out correctly. Let them use whichever method they feel comfortable with as long as it works consistently.

National Science Education Standards

Science as Inquiry
Use math in all aspects of scientific inquiry. (5-8)

Earth and Space Science
Most objects in the solar system are in regular and predictable motion. Those motions explain phenomena such as the day, the year, phases of the moon, and eclipses. (5-8)

National Math Standards

Numbers & Operations
Understand numbers, ways of representing numbers, relationships among numbers, and number systems.

Compute fluently and make reasonable estimates.

Problem Solving
Build new mathematical knowledge through problem solving.

Recognize and apply mathematics in contexts outside of mathematics.