The Great American Eclipse

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In preparation for the wonderful event in August, here is some useful information about eclipses, their history, and how to observe them.

THE AUGUST 21, 2017 TOTAL SOLAR ECLIPSE

Great American Eclipse Poster Image
11"x17" poster (14.5MB PDF)
eclipse map image
August 21, 2017 great American total solar eclipse path of totality, Credit to: Michael Zeiler, GreatAmericanEclipse.com
Eclipse students wearing protective glasses imageViewing the sun with pinhole projectors image
IMPORTANT! Top: Special eclipse glasses are needed to safely view an eclipse. You can permanently damage your eyes by viewing the eclipse before or after totality. Even when 99% of the Sun is covered by the Moon you can still damage your eyes. Only for the ~2 minutes during totality it is safe to look at the corona with no protection. Bottom: The Sun can also be safely observed through pinhole projectors, 1963.

The first total solar eclipse in the continental US since 1979 will occur on August 21, 2017. Day will turn to night for two minutes during this time and the Sun’s enigmatic faint extended atmosphere, or “corona,” will shine in the dark sky. HAO director, Scott McIntosh, has been spearheading an effort called The Eclipse Megamovie Project, to unite people from around the world to share their images and experiences around total solar eclipses and other astronomical events. The hope is to create ultra-high time resolution movies of this spectacular natural event using collected images from professionals and citizens from across the coast-to-coast path of totality.

See Director Scott McIntosh’s Explorer Series presentation titled—The Sun, the Moon, and Us: A Guide to the Great American Eclipse of 2017:
Slides: LocalGuideSlides.pdf and
Movie: explorer_series_scott_mcintosh2.mp4

Websites for the August 21, 2017 Total Solar Eclipse:

Websites for general information on solar eclipses:

WHAT IS A SOLAR ECLIPSE?

Total eclipse (India, 1980) image
Photography by High Altitude Observatory/NCAR (India, 1980)

A solar eclipse occurs when the Moon passes between the Sun and Earth, causing the Moon to block the Sun’s light and casting a shadow on Earth. Typically the Moon only partially blocks the Sun’s light during an eclipse, however occasionally the Moon completely blocks the Sun, causing what is called a total solar eclipse. For a solar eclipse to take place, the Sun, Moon, and Earth must be aligned in a perfect or near perfect straight line, an alignment that astronomers call syzygy. This can only happen during the new Moon phase.

The following list outlines the 4 types of solar eclipses that are determined by what part of the Moon's shadow falls on the Earth:

Total: A total solar eclipse takes place when the Moon completely covers the Sun and casts its shadow on Earth. A total eclipse of the Sun can only take place when the Moon is at perigee or when its orbit is closest to the earth.
Partial: Partial solar eclipses happen when the Moon does not completely cover the Sun's disc and casts only partial shadow on Earth.
Annular: Annular solar eclipses occur when the Moon's disc covers the center of the Sun's disc, leaving the Sun's outer edges uncovered. An annular eclipse of the Sun can only take place when the Moon is at apogee or when its orbit is farthest from the earth.
Hybrid: Hybrid eclipses are rare. They happen when an annular eclipse turns into a total solar eclipse.

HISTORY

Visible since before recorded history, the ancients regarded eclipses with mystery, wonder, and fear. If you could predict the occurrence of eclipses you could wield great power!

Babalonian Tablet image
Figure 1: Babalonian tablet is a record of an ancient eclipse

The ancient Babylonians were the first to discover the “Saros Cycle” more than 22 centuries ago and could predict the occurrence of cycles very accurately. The saros cycle relates to the lunar cycle and is about 6,585.3 days long. The earliest known Babylonian record is of the eclipse that took place on May 3, 1375 BC, recorded on a stone table (Figure 1).

Irish carved stone image
Figure 2: Loughcrew carved stone, Irish, 3340 BC

The first recorded eclipse, in 3340 BC, was carved into giant stones by the ancient Irish at Loughcrew, outside Oldcastle, in County Meath (Figure 2). The landscape of rolling hills is littered with numerous Neolithic monuments. The Irish Neolithic astronomer priests recorded the events on three stones relating to the eclipse, as seen from that location. Many believe that the ancient Celts created the “festival of light” to welcome the eclipse, which they predicted.

Image by David DePasquale of mythical beast eating the sun
Figure 3: Image by David DePasquale of mythical beast eating the Sun

In ancient China, it was believed that solar eclipses provided information on the health and success of the Emperor. Failing to predict one could endanger the Emperor's health. Legend has it that two astrologers were executed for failing to predict the solar eclipse of 2134 BC; that would make it one of the oldest solar eclipses ever recorded. The image shows a tiangou, a mythical beast eating the Sun (Figure 3).

The Greeks built the first ancient analogue computer and orrery called the Antikythera Mechanism (Figure 4); the device is a complex clockwork mechanism composed of at least 30 meshing bronze gears. The Antikythera Mechanism was used to predict astronomical positions and eclipses for calendrical and astrological purposes, as well as the Olympiads, the cycles of the ancient Olympic Games.

Greek Antikythera Mechanism image
Figure 4: Greek Antikythera Mechanism
First photograph image
Figure 5: The first successful photograph taken of a solar eclipse, 1851, by Johann Berkowski
Drawing by E. Holden image
Figure 6: Drawing by E. Holden of the 1889 eclipse

The first successful photograph taken of a total eclipse was a daguerreotype captured in July of 1851 (Figure 5) by Johann Berkowski of the Royal Observatory in Konigsberg, Prussia. Due to the poor quality of early photos, most eclipse observations during this time were recorded by drawing, like this one by E. Holden of the 1889 eclipse (Figure 6).

Digital photography image
Figure 7: Digital photography produces amazing detailed images like this one of the 2009 eclipse

With the advent of digital photography and image processing techniques, amazingly detailed images of the corona can now be produced, like this one (Figure 7) of the 2009 eclipse in the Marshall Islands (courtesy of Miloslav Druckmuller.)

Prediction requires knowledge of the Synodic Month (lunar cycle), Draconic Year (sun’s revolution), and the Anomalistic Month (calendar month.) Those Three cycles combine to create all of the eclipses seen on Earth in an approximately repeating pattern every 18 months.

On August 16, 1868, a total solar eclipse provided an opportunity for the discovery of Helium, the second lightest and second most abundant element known to us. It was discovered independently by French astronomer Jules Janssen and British scientist Sir Norman Lockyer. Janssen used a spectroscope to observe a line of yellow light, thought to be sodium, but the wavelength didn’t match up. His passion to discover the composition of the new element led to his invention of the “spectrohelioscope.” During the same time period, Joseph Lockyer, working to identify the same line of yellow light, succeeded in viewing the solar prominences in regular daylight. Both men are credited with the first sighting of helium.

The total solar Eclipse of May 29, 1919, had a totality of 6 minutes 51 seconds; it was one of the longest solar eclipses of the 20th century. Sir Arthur Eddington photographed this eclipse during an expedition to the island of Principe (off the west coast of Africa.) Eddington used the positions of star images within the field near the Sun to test Albert Einstein's prediction of the bending of light around the Sun from his general theory of relativity.

This photo mosaic, (Figure 8), illustrates the March 9, 2016, total solar eclipse from Palu, Indonesia. It was photographed by Justin Ng of Singapore, who wrote: The...“Sun’s corona extends millions of kilometers into space and it is visible to the unaided eye during a total solar eclipse. During totality, temperature at my location dropped by 10 to 15 degrees Celsius and I was able to see planet Venus and Mercury when the area became dark. I took 12 different exposures during totality to cover as much dynamic range as possible and stacked them in Photoshop to produce this image.”

Solar eclipse of March 9, 2016 image
Figure 8: A total eclipse collage from early on March 9, 2016, Palu, Indonesia.–Credit Justin Ng