A Clue About Longitude Clues
Why They Only Work on the Equinox

So far, the sunrise/sunset data has provided a lot of information about the Mystery Sites' latitude, but not about their longitude. These special longitude clues, which work only on the equinox, will now help students estimate the longitude of each Mystery Class and help narrow their search.

BEFORE Watching the Slideshow

This slideshow will help students visualize how the Earth's tilt varies from season to season. They should begin to grasp why, on the equinoxes, all locations along a longitude line have the same sunrise time. This lays the groundwork for understanding how to use the Mystery Class longitude clues to estimate each site's longitude and eventually its location.

You may want to conduct one of these activities to lay the groundwork for concepts covered in the slideshow.

Teacher Background: The Math Behind the Longitude Clues

The Longitude Clues

Students will get this information for the spring equinox:

  • The time (in Universal Time) that the sun rises at each Mystery Class site.
  • The time the sun rises at Greenwich, England (0 degrees longitude).
This map shows the earth's 24 time zones, each of which covers 15 degrees longitude.

Longitude and Time Zones
In 24 hours, the earth rotates a full 360 degrees. That means that it spins at a rate of 15 degrees longitude per hour. (360 degrees divided by 24 hours = 15.) We've established 24 time zones, each of which covers about 15 degrees of longitude. All clocks within the time zone will show the same time. Each time zone is an hour earlier than the zone east of it and an hour later than the zone west of it.

Simple Equinox Example
Sunrise in Greenwich: 05:00 UT
Sunrise at Mystery Class X location: 10:00 UT
Mystery Class Location: Must be 5 time zones (or 75 degrees longitude) west of Greenwich. (There are 5 hours difference between sunrise times. If a site's sunrise is later than sunrise in Greenwich, the site is west of there.)