3rd May: Solar Eclipses

On this date in 1375 the earliest recorded eclipse of the sun was observed in Usarit, Syria. Also on this date in 1715 Edmund Halley observed the total eclipse phenomenon "Baily's Beads", nine years after the first known telescopic observation of a total solar eclipse was made in France in 1706.

1. Total solar eclipses happen because the Sun, which is 400 times bigger than the Moon is also 400 times further away, making them look the same size to us on Earth. Earth is the only planet in the Solar System where the phenomenon is possible. Other planets have different eclipses - Jupiter, for example, can have a triple eclipse, in which three moons cast shadows on the planet simultaneously. In about 600 million years we won't get total eclipses on Earth anymore because the moon is slowly drifting away from us and by then it will be too small to cover the sun completely.
2. They can only happen at certain times, called eclipse seasons. An eclipse season lasts from 31 to 37 days, about every 6 months. There is guaranteed to be at least one solar eclipse per season and one lunar one. Sometimes there are two solar eclipses. A solar eclipse can only occur during a new moon when the Sun and the Moon are in conjunction as seen from Earth in an alignment called a syzygy. There's also something called the Saros cycle, which means 18 years and 11 days after an eclipse, there will be an identical one, lasting the same amount of time.
3. If the Moon were in a perfectly circular orbit, a little closer to the Earth, and in the same orbital plane, there would be total solar eclipses every month - but it isn't, so total solar eclipses are rare events. In any given place they will only happen once every 360 to 410 years, but if you're willing to travel, they occur somewhere on Earth about every 18 months.
4. Solar eclipses are predictable. We know exactly when and where they are going to happen for the next few thousand years. It works in retrospect, too. We know when they happened throughout history and before records even began (which, FYI was in China around 720 BC). If ancient writings mention an eclipse occurring at the same time as a significant event or disaster, historians can work out exactly when it happened by looking at eclipse cycles. Some have speculated that the darkness on the first Good Friday was caused by a total eclipse and therefore it should be possible to work out the exact date of the crucifixion. Except - it happened at Passover which coincides with a full moon - and eclipses can only happen when the moon is new. Also, the darkness lasted three hours which is far too long, which brings us neatly to...
5. How long solar eclipses last. It does vary slightly but a total eclipse can never last more than 7 minutes 32 seconds. The longest eclipse that has been calculated thus far will happen on July 16, 2186, when totality will last 7 minutes and 29 seconds. They are getting faster. By the 8th millennium, the longest possible total eclipse will be less than 7 minutes and two seconds. The longest total eclipse of the 20th century was on June 20, 1955 and lasted 7 minutes 8 seconds. There will be no total solar eclipses over 7 minutes in duration in the 21st century. During an eclipse, the moon’s shadow, or umbra, travels across Earth’s surface at up to 5,000 miles per hour. If you have access to a supersonic jet you can observe totality for much longer by chasing the path of the umbra. On June 30, 1973 passengers aboard on Concorde were able to stretch totality for this eclipse to about 74 minutes.
6. Just before totality, “Baily’s beads” appear. They are caused by sunlight shining through valleys on the moon. The last bead creates the impression of a diamond ring in the sky.
7. Strange things can happen during totality. Nocturnal birds like owls get up, while daytime birds go to bed. Bees and Squirrels work even harder to find food, while cicadas quieten down to save energy. Hippos hide under water until it's all over. Gravity sometimes does funny things as well. Maurice Allais, a French polymath who went on to win the Nobel Prize in Economics was first to notice that pendulums may behave differently during eclipses. As Electricity is increasingly generated by solar panels and wind farms, an effect we may see more of in future is a loss of power. Not only from the solar panels. Winds tend to drop during totality so the wind farms are affected, too.
8. Scientists love eclipses because of the observations they can make. French astronomer Pierre Janssen observed an eclipse from India in 1868 and, using a spectroscope, spotted light from a new element — Helium — decades before it was found on earth. Observations during a 1919 total solar eclipse showed a star appear to shift in the sky due to the sun’s gravitational influence, so confirming Einstein's Theory of Relativity.
9. If you're a time traveller, or dealing with primitive cultures who haven't worked out that solar eclipses are a natural phenomenon, knowing when an eclipse is due can come in very handy and possibly even save your life. While stranded in Jamaica, Christopher Columbus warned that the moon would disappear if they did not start supplying his men with food. He knew from his almanac that there would be an eclipse on February 29, 1504, which he had read about in his almanac. He took a gamble that the eclipse would be visible from Africa as well as Europe, and it paid off.
10. Eclipses are significant in Astrology. Ancient astrologers saw them as omens of large scale disasters. The type of disaster depended on which sign of the zodiac the eclipse occurred in. An eclipse taking place in a fire sign (Aries, Leo or Sagittarius) meant wars; in Earth signs (Taurus, Virgo, Capricorn) famines, droughts and Earthquakes; in Air signs (Gemini, Libra, Aquarius), wind, storm and revolutions; in Water signs (Cancer, Scorpio, Pisces), not surprisingly, heavy rain and floods. Modern astrologers view them as predicting change and upheaval in whatever fields are ruled by the sign, planet or house the eclipse is associated with, or at least highlights where reform is needed. This extends to personal charts, where an eclipse is said to predict change and upheaval in a particular area of a person's life.