Models of the Solar System: SS-P1: Solar Eclipses

Solar eclipse is when the moon passes between the sun and the earth partially or fully blocking it resulting in a shadow falling on earth. There are 2 shadows.

1. Penumbra

The Moon's faint outer shadow.
Partial solar eclipses are visible from within the penumbral shadow.

2. Umbra

The Moon's dark inner shadow.
Total solar eclipses are visible from within the umbral shadow.

For this to occur the moon must be in the “new moon” phase, which happens every 29 and a half days. However we do not get an eclipse once a month. It happens twice a year when the Moon's orbit around Earth is tilted 5 degrees to Earth's orbit around the Sun. As a result, the Moon's shadow usually misses Earth as it passes above or below our planet at New Moon. At least twice a year, the geometry lines up just right so that some part of the Moon's shadow falls on Earth's surface and an eclipse of the Sun is seen from that region.

A solar eclipse can only be seen at specific parts of the world. For example the last eclipse on the 11th of July 2010 was seen over the south of the Pacific Ocean, making it one of the most remote solar eclipses in recorded history. It also covered easter island, and this famous quote was made by Fred Espenak, a NASA astrophysicis:

"One of the most unique things about this particular eclipse is that it crosses a unique and interesting archaeological site: Easter Island. On Easter Island there are these great statues... There's a lot of mystery about these statues, but in any case, this is the first total eclipse to hit the island in about 1,400 years."

On Easter Island the eclipse was witnessed for 4 minutes 41 seconds.

The Saros Cycle determines when and how frequent an eclipse will occur. A Saros Cycle is a period of 6585 days. It is applicable to both lunar and solar eclipses. Only eclipses separated by the Saros cycle have the same geometry and period as the other. With each of these eclipses the path of the eclipse is shifted 120 degrees westward of the last eclipse. Therefore the eclipse returns to the same geographical position every 3 Saros (54 years and 34 days)

The Saros cycle was formed from the synodic months which is a period of 29.53 days, which is also the period it takes for a new moon to form. The saros cycle consists of 223 synodic months which is equal to 242 draconic months. Draconic months are 27.21 days long and are the time it takes for the earth to pass two successive nodes (which are points where the orbit of moon ecliptic, which is the path sun traces out in the sky.

Historically eclipses were often explained as acts of god. People didn’t know about the different types of eclipses but they were thought to be a sign from god. Chinese astronomy was the first to record the frequency of eclipses, way ahead of any other civilisations. This often required extreme amounts of patience and complex calculations. Other Chinese ideas included The Moon was represented by a mirror. During an eclipse, it was believed that a dragon swallowed the Moon during an eclipse and beating the mirrors would cause the dragon to cough it up and return it to the sky.

Babylonians, Sumerians, & Assyrians often known as the Chaldeans were known for their passion for astronomy and astrology from about 1200 BC onwards. They first proposed the system of 60 seconds. (60 seconds in a minute) and also understood the occurrence of lunar eclipses! This is quite astonishing since their civilisation was around at about 1200 BC. They did this by careful and precise observations.

However not all ancient civilisations were as wise. A greek astronomer called Anaximander (c 610 – 547 BC) believed that the earth was a cylinder shape and that Eclipses also came from holes in the Sun and the moon filled with fire when it came to an eclipse.

The Pomo tribe from native America, named solar eclipses "sun got bit bear" which told a story where a” bear” goes for a walk in the solar system and meets with the sun. The pair then start to argue about who would move out of the others way. The argument turned into a fight, which was represented by an eclipse of the Sun. the bear continued on its way and soon met up with the suns sister and the same fight ensued. This cycle was repeated to explain the eclipses.