Lunar Eclipse

  • In the Ancient World, a city calculated its Longitude by the times of a Lunar Eclipse.
  • The Lunar Eclipse can only be observed during a Full Moon when the Sun, Earth and Moon are in alignment, known as a Syzygy.

Syzygy

  • There are between two to five Lunar Eclipses every year..
  • This requires the Sun, Earth and Moon to be in a straight line, known to the Greeks as a Syzygy
  • A Lunar Eclipse can only occur at the Full Moon, when the Sun, Earth and Moon are in alignment, and the Earth obscures the Moon.
  • A Solar Eclipse can only take place at a New Moon, when the Moon’s alignment obscures the Sun.

Why a Lunar Eclipse does not occur every month

  • The Moon’s orbit crosses the Ecliptic (the Earth’s Orbit around the Sun), twice every month.
  • If the Ecliptic and the Moon’s Orbit were aligned, then the Earth would obscure the Moon once a month, and the Moon would obscure the Sun once a month (Solar Eclipse).
  • As they are not aligned the Eclipses occur only when the Moon’s Orbit passes through the Ecliptic.

Observation

  • 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 Earth in any country at the same time but only in one hemisphere. The Ancient Greeks and Romans could not observe Eclipses in the Southern Hemisphere.
  • It was visible at night to sailors on any Sea, when there was a Full Moon during the Eclipse Season.

The Saros Cycle

  • The Eclipse times could be calculated and predicted by the Babylonians and the Ancient Egyptians. They knew that groups of Eclipses reoccur in a cycle that repeats itself just over every 18 years, known as the Saros Cycle.
  • Once the time of a previous eclipse is logged, it becomes possible to calculate the next group of eclipses.

Calculation of Longitude

  • By observing a Lunar Eclipse, and knowing the Time, Cities could calculate their Longitude.

 

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