Longitude

• The Lines (or Meridians) of Longitude are Great Circles that pass through the North and South Poles. They are expressed in degrees East or West of the Prime Meridian at Greenwich.
• When Meridians are used in conjunction with Parallels of Latitude they can identify any geographical location in the world.

Calculation of Longitude by observation of Lunar Eclipse

• The Lunar Eclipse only occurs during a Full Moon.
• It is caused by the Earth’s shadow, which obscures the moon for up to four hours.
• A Lunar Eclipse is visible from any country on Earth experiencing darkness.
• The Eclipse times could be calculated and predicted, and written down in a star chart.
• By observing a Lunar Eclipse, Sailors could calculate their Longitude.
• It was visible at night to sailors on any Sea, when there was a Full Moon during the Eclipse Season.

Calculation of Latitude

• By measuring the angle of the North Star (Polaris), from the Horizon, Sailors could calculate their Latitude.

Roman Prime Meridian

• The Roman Prime Meridian was Mount Teide in Tenerife.