Rare total eclipse of the sun with easy US viewing coming Aug 21, 2017

This will be the 1st time in 40 years that a total solar eclipse will be visible from the continental United States.

By Bruce Betts, PhD
Dir of Science
The Planetary Society

Map of the solar eclipse on Aug 21, 2017. The area marked from Oregon to South Carolina is the path of totality. Yellow lines parallel to the path indicate the fraction of the Sun's diameter that will be covered by the Moon along those lines. Green lines roughly perpendicular to the path indicate times of maximum eclipse. Brown lines within the path indicate the duration of totality.

On Aug 21, 2017, the Moon will totally eclipse the Sun for the 1st time as seen from the continental US in more than 40 years. A partial solar eclipse will be visible throughout North America and in northern South America and Western Europe.

Solar eclipses are spectacular events, but they require some knowledge and planning to safely observe them. Pilots may be particularly well-positioned to get to the path of a total eclipse. In this article I provide data about eclipses in general, and specific information about this upcoming spectacular eclipse.

What are eclipses?

The basic concept of eclipses is straightforward, but the details and variations from one eclipse to another get somewhat complicated due to the nature of the geometries involved.

Eclipses occur when a celestial body passes in front of a 2nd celestial body as seen from a 3rd celestial body. In other words, it is when the 3 bodies line up (which is given the fun name syzygy).

A lunar eclipse, as seen from the Earth, occurs when the Moon passes into the shadow of the Earth, that is when the Earth blocks some or all of the Sun as seen from the Moon. Lunar eclipses, which occur during full Moon, can be seen from the entire night side of the Earth and they last for several hours.

Also as seen from the Earth, a solar eclipse occurs when the Moon passes in front of the Sun. In a total solar eclipse, the Moon completely blocks out the Sun, so those who experience a total solar eclipse are completely in the Moon's shadow. Unlike a lunar eclipse, a total solar eclipse is only visible from a limited area: the line traced by the Moon's shadow as it moves across the Earth. Typically this line has a width of a few tens of kilometers, thus one must either be very lucky or must travel in order to see a total solar eclipse. A partial solar eclipse, which occurs over a much broader area, occurs when the Sun is only partially blocked by the Moon.

Why don't we have a solar eclipse every time the Moon is full, so between the Sun and the Earth, or a lunar eclipse every time the Moon is new, so the other side of the Earth from the Sun? The reason is because the Moon's orbit is tilted 5° compared to the plane of the Earth's orbit, thus the Moon only can cause these eclipses when those 2 planes approximately intersect. This occurs twice a year, and there are at least 2 solar eclipses of one kind or another each year. However, because the area affected is limited, particularly for total eclipses, people often travel great distances to witness them.

Even though there are at least 2 solar eclipses per year, there are not necessarily 2 total solar eclipses per year. That is because both the Moon's orbit and the Earth's orbit are somewhat elliptical, or non-circular. Thus, the distances between the Earth, the Moon and the Sun vary from eclipse to eclipse, causing the angular diameters of the Moon and the Sun to vary from eclipse to eclipse.

So, in addition to having a total eclipse when the Moon's angular diameter is at least as large as the Sun's angular diameter, there are also eclipses where even though the Moon is centered relative to the Sun, it does not appear large enough to cover the entire Sun. This is referred to as an annular eclipse because of the annulus or circle of Sun that surrounds the Moon as seen from Earth.

Because the Earth is round, there is a 3rd permutation called a hybrid eclipse where from some locations on Earth the eclipse appears total, and in other locations that are slightly farther from the Moon, it appears as annular. The result of all of this geometry is that a total solar eclipse for any given location is very rare.

Eclipses in history

In history, eclipses have been taken as everything from portents of doom to portents of peace. Some ancient empires believed them to be signs of bad fortune for their rulers. Others interpreted them differently. The ancient Greek historian Herodotus wrote that an eclipse that occurred during a battle between the Medes and the Lydians caused both sides to put down their weapons and declare peace.

Eclipses have also been used by modern historians to help with chronologies of ancient civilizations. For example, a solar eclipse recorded in an Assyrian text was correlated with a solar eclipse that would have occurred in 763 BC. Chinese records of eclipses begin at a similar time, and indicate their ability to predict eclipses.

Eclipses in science

Solar eclipses have enabled scientific studies and discoveries since at least the mid-19th century. During a total solar eclipse, the Sun's upper atmosphere, known as the corona, becomes visible. Eclipses enabled the 1st study of the corona, which is normally not visible because it is overshadowed by the brightness of the photosphere, the upper "surface" of the Sun from which we see light emanating.

Amazingly, initial evidence of the element helium came from observations made during an 1868 total solar eclipse. Spectra created by the breaking up of the Sun's light into its wavelength components indicated an unknown element, which was eventually determined to be helium. Because of the method of discovery, it is named after the Greek word for Sun which is helios.

A 1919 solar eclipse was used to test Einstein's theory of general relativity. By taking pictures of stars during the eclipse, scientists were able to show that their light was indeed bent by the gravity of the Sun as predicted by Einstein.


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