On Monday, August 21st, the continental United States will experience its first total eclipse of the sun for 38 years (the last total eclipse visible from the USA having occurred in 1979). Providing the weather holds good along the path of the eclipse, an estimated 220 million people will be able to see the event – providing they take the proper precautions.
An eclipse is a periodic event, occurring when the Moon passes between the Sun and Earth and either fully or partially occults (blocks) the Sun’s light. This can happen only at new moon, when the Sun and the Moon are in conjunction as seen from Earth, in an alignment referred to as syzygy. There are actually four types of eclipse:
- Partial – this occurs when the Sun and Moon are not exactly in line with the Earth, and so the Moon only partially obscures the Sun. Partial eclipses are virtually unnoticeable in terms of the sun’s brightness, as it takes well over 90% coverage to notice any darkening at all.
- Annular – occurs when the Sun and Moon are exactly in line with the Earth, but because of the variations in the Earth’s distance from the Sun, and the variations in the Moon’s distance from the Earth, the apparent size of the Moon is smaller than that of the Sun. Hence the Sun appears as a very bright ring, or annulus, surrounding the dark disk of the Moon.
- Total – occurs when the dark silhouette of the Moon completely obscures the intensely bright light of the Sun, allowing the much fainter solar corona to be visible. The complete coverage of the Sun’s disk by that of the moon – referred to as totality – occurs at its best only in a narrow track on the surface of Earth.
- Hybrid (also called annular/total eclipse) – this shifts between a total and annular eclipse. At certain points on the surface of Earth, it appears as a total eclipse, whereas at other points it appears as annular. Hybrid eclipses are comparatively rare.
The last total eclipse took place in March 2016, and was visible from South/East Asia, North/West Australia, the Pacific and Indian oceans. The 2017 event will be visible in partial forms across every continent except Antarctica and Australia. However, the path of totality will only be visible across the continental United States.
The path of totality will run from Oregon to South Carolina, as will be around 113 kilometres (70 miles) wide, offering people along it an unrivalled opportunity to view the eclipse – weather permitting -, providing the right precautions are taken.
The most important aspect of viewing an eclipse “live” is never look directly at the Sun, even during the period of totality; you should at least use a solar filter or viewer. However, if you don’t have one or the other or any specialised kit, the best way to see the eclipse in the flesh is via pinhole projection. For those who are unable to see the eclipse first-hand, there are a wide variety of ways to watch the event on television or the Internet, including:
- NASA Total Eclipse live stream is providing options to watch through NASA Edge, NASA TV, Ustream, YouTube and more. NASA’s Facebook page. These will show images of the eclipse, from 11 spacecraft, three aircraft and from more than 50 high-altitude balloons, and the astronauts on the International Space Station.
- Slooh, the on-line community observatory, will run a webcast starting at 12:oo noon EDT (1600 GMT), as a part of a 3-day celebration of the eclipse.
- The Virtual Telescope Project is hosting a free online observing session with views of the total solar eclipse beginning at 13:00 EDT (17:00 GMT).
- The Eclipse Ballooning Project will be broadcasting live views of the eclipse from the edge of space via more than 57 cameras sent up on weather balloons.
- CNN and Volvo will be providing a 360-degree view of the eclipse with 4K resolution from different locations along the eclipse path. The stream will also be viewable in virtual reality, which people can navigate by moving a phone or virtual reality headset. The live stream begins at 12:03 p.m. EDT (16:03 GMT).
- ABC will air a two-hour special on the eclipse starting at 13:00 EDT (17:00 GMT). The broadcast will also be available on Facebook Live and YouTube
There are a number of terms common to eclipses which are worth mentioning for those who wish to follow the event, but are unfamiliar with the terminology. These include:
- The umbra, within which the object in this case, the Moon) completely covers the light source (in this case, the Sun’s photosphere).
- The antumbra, extending beyond the tip of the umbra, within which the object is completely in front of the light source but too small to completely cover it.
- The penumbra, within which the object is only partially in front of the light source.
- Photosphere, the shiny layer of gas you see when you look at the sun.
- Chromosphere, a reddish gaseous layer immediately above the photosphere of the sun that will peak out during the eclipse.
- Corona, the light streams that surround the sun.
- First contact, the time when an eclipse starts.
- Second contact, the time when the total eclipse starts.
- Third contact, the time when the total eclipse ends.
- Fourth contact, the time at which the eclipse ends.
- Bailey’s beads, the shimmering of bright specks seen immediately before the moon is about to block the sun.
- Diamond ring, the last bit of sunlight you see right before totality. It looks like one bright spot (the diamond) and the corona (the ring).
A total eclipse occurs when the observer is within the umbra (they are standing in the shadow cast by the Moon); an annular eclipse when the observer is within the antumbra, and a partial eclipse when the observer is within the penumbra.
As well as the passage of the Moon between the Earth and Sun, there are a number of Earthly effects to look for if you are in the path of totality, such as a the 360-degree sunset. This may also be accompanied by an “eclipse wind” as temperatures suddenly drop. And, of course, there is the rousing of nocturnal animals, fooled by the darkness, followed by a false dawn as the Moon moves away from between the Earth and the Sun, and an accompanying dawn chorus.
The period of totality lasts only a few minutes but offers a superb opportunity for observing the Sun and its corona – hence why NASA is using a chain of three aircraft to “chase” the eclipse as the Moon’s shadows travels at an average speed of 3,683 km/h (2,288 mph) west-to-east, enabling them to carry out an extended study of the corona.
As a point of historical interest, August 21st marks the 103rd anniversary of the 1914 total eclipse, which was seen from Scandinavia through to Turkey, the middle east and India. It was the subject of a number of expeditions being sent eastwards to the Baltic and Ukraine by Britain and other European nations with the intention of studying it – only for the conflagration of the First World War to erupt.
The war foiled attempts by a British expedition which intended to use the eclipse as a means to measure relativity; however, it did give rise to another mystery: whether or not a film of the eclipse apparently made in Sweden in 1914 is the real deal or not. If it is, it might be the oldest surviving footage of an eclipse.
If you are on the path of totality, and plan to view the eclipse, do please take the proper precautions and I hope the weather cooperates with you. I’ll be following things on-line.