## Saturday, February 28, 2009

### Science Saturday Challenge #5

This week’s challenge is inspired by the Journey North: Mystery Class in which our co-op is participating.

Seasons change because the earth rotates around the sun at a 23.5° angle. Because of that angle, the amount of daylight—photoperiod—changes from day to day, and on any given day it is different based on your latitude.

Here’s an interactive animation about the seasons at Teacher’s Domain (free registration.) Also check out The Reasons for the Seasons by Gail Gibbon or Sunshine Makes the Seasons by Franklyn M. Branley.

This week's challenge involves calculating and plotting photoperiods to demonstrate how they vary throughout the world.

Materials

Procedure
First create or download a photoperiod graph. The X axis is the date and the Y axis is photoperiod (from 0 to 24 hours; see here for an example. Scroll down to the Mystery Class graph.)

Figure out your own longitude and latitude by entering your U.S. town or one nearby into the USNO web site and click “Get Data” or entering your location outside the U.S. at the Getty site.

Find the longitude and latitude of 6 cities north and south of you. Spread these out from north to south as much as possible; it does not matter what longitude they are in. Find two cities east or west along the same latitude as you. You will also need to know the time zones (map here) based on Greenwich Mean Time (GMT.) Note: if you use cities in the US you can just enter the name into the USNO site and get longitude, latitude, and time zone (see below.) For cities outside the US you can use the Getty site or a map.

Use the USNO web site to find the sunrise and sunset data for your home and your chosen cities. If you are using US cities you can simply enter the city name in the top FORM A section. For cities outside the US, use the lower FORM B section by entering in the longitude, latitude, and time zone. Repeat this for several dates; I suggest doing the same day of the week for several weeks before and after the equinox, and then several weeks before and after the solstice.

Calculate the photoperiod (amount of daylight) for your nine locations and plot them on your graph, each in a different color.

Check out the slopes of the lines and consider where these locations are in relation to each other. How does moving east to west affect photoperiod? Can you tell if a location is north or south of you based on the photoperiod? How do photoperiods affect climate? How does the data change around the equinox and the solstice? There's a lot of science and math you can discuss relating to this project.