• the rotation of the Earth (15°/hour)


• a good clock or watch (Harrison provided this)


• a reference point (We use Greenwich as the Prime Meridian)

• Set your watch to match the time in Greenwich (we call this “UT” or Universal Time).
• Travel to a new location.
• Find out when the Sun is directly overhead (This is ‘Solar Noon’).
• Calculate your new longitude: # of hours difference *15°/hour=your longitude.

Remember that if solar noon occurs BEFORE your watch says noon, you are EAST of the Prime Meridian.

Here are some problems to try out:

1. If there is a 3 hour difference between San Francisco and Baltimore, how many degrees of longitude are they apart?
   • 20°
   • 45°
   • 60°
   • I thought only temperature was measured in degrees!


2. Baltimore is 5 hours BEHIND Greenwich time (UT or GMT). What is our longitude?
   • 15° * -5hours = 75° West
   • 15° * -5hours = 75° East
   • 60° West
   • 60° East


3. If your watch is set to UTC (same as GMT or Greenwich Mean Time), and your watch says 3:20pm when the Sun is directly overhead, what is your longitude?
   • 45° East
   • 45° West
   • 50° East
   • 50° West