August 18, 2017

In the shadow of the Moon – total solar eclipse visible from the USA

  • On 21 August 2017, there will be a total eclipse of the Sun visible from the USA. For the first time in almost 100 years, the conical shadow of the Moon will race from the west to the east coast, through 14 states – from Salem, Oregon to Charleston, South Carolina. The solar eclipse will mark out a 100 kilometre-wide strip and turn day into night for a maximum of two minutes and 40 seconds.
  • In the rest of the USA, all of Canada, Greenland and Central America as well as parts of South America, the Sun will only be partially covered by the disc of the Moon. This will also occur in Western Europe, where, from Oslo to London, Santiago de Compostela in northwest Spain and La Palma in the Canary Islands, a partial solar eclipse will be visible shortly before sunset – if the sky is clear.
  • The next solar eclipse will take place on 2 July 2019 over the southern Pacific Ocean and South America; the next one in Europe (Iceland and Spain) will be on 12 August 2026.

A total solar eclipse is spectacular for observers: “Around 30 seconds before the Sun disappears entirely behind the Moon, it becomes noticeably darker in the middle of the day, as if someone had quickly turned the dimmer switch for a light,” explains Manfred Gaida, an astrophysicist in the Space Administration team at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), describing the phenomenon. The Moon moves in front of the Sun until just the corona can be seen. Where the conical umbra reaches the Earth, the sky goes dark. The partial phase of the eclipse begins around 75 to 90 minutes beforehand, and lasts for the same amount of time after the eclipse, until the ‘all-around twilight’ has completely disappeared.

Solar eclipses come in three forms – partial, annular and total – and in rare cases in a combination of annular and total. The configuration of Earth, Moon and Sun when the eclipse occurs is decisive. Gaida explains: “The further the Moon is from Earth and the closer Earth is to the Sun, the more likely an annular eclipse is. However, if the Moon is relatively close to Earth and the Sun relatively far away, as in the current case, the probability of a total solar eclipse is high. Another factor is where the solar eclipse occurs on the surface of the Earth. Partial eclipses are most frequent in the polar regions."

2017 Total Solar Eclipse's Path Across the U.S.
Video 2017 Total Solar Eclipse's Path Across the U.S.


Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

The last total eclipse visible from Germany was on 11 August 1999. “Exactly 18 years, 10 days and 8 hours later, its ‘daughter eclipse’ in Saros cycle number 145 is taking place on 21 August 2017,” explains Gaida, adding: “Because there is an eight-hour time difference, Earth will have rotated approximately 120 degrees further in longitude from west to east, and the path of totality will now pass across North America. The space agency NASA will be reporting live on the 'Great American Eclipse'. This is the first total eclipse of the Sun to cross the USA from the Pacific to the Atlantic since 8 June 1918.

What is it about these cosmic spectacles that fascinate humans? “In the past, eclipses were regarded as portents of fate, because the Sun was considered to be the source of life and a deity. The ancient Babylonians were aware of mathematical rules with which solar and lunar eclipses could be predicted,” explains Gaida. They worked out that eclipses of a similar type and form occurred after periods of 6585 and one-third days and called these special cycles ‘Saros’. At the end of a Saros cycle, the Sun, Moon and Earth assume almost the same positions relative to one another, so that, after this period, solar and lunar eclipses repeat under almost the same conditions as 18 years before. Other eclipse cycles apart from the Saros need to be taken into account as well, but none is as elegant and memorable as this, according to Gaida.

Following fairly detailed geographical and mathematical analysis of solar eclipses in the 17th and 18th centuries, these events were a welcome opportunity to practise the still young science of spectroscopic research from the second half of the 19th century. Using this, the element helium – among others – was discovered during an eclipse of the Sun in 1868. At that time, astronomers were also able to use photography to clearly establish that the solar corona and arc-shaped luminous streams of material – prominences – were actually part of the Sun, not the Moon, and are not atmospheric phenomena that only occur during solar eclipses. “The solar eclipse of 29 May 1919 has long been considered the most scientifically significant, as it was used to confirm Albert Einstein’s General Theory of Relativity, published in 1915,” says Gaida, adding: “And this theory is still of vital interest to research 100 years later, as is demonstrated by the recent detection of gravitational waves.”

What happens during a total solar eclipse?

A total solar eclipse occurs in four phases. Except for the brief duration of the period of totality, special solar eclipse glasses with approved filters must be worn to protect the eyes. Anyone who looks at the Sun without such glasses risks serious eye damage or even blindness.

  1. First contact: the new Moon touches the Sun and starts the partial phase. Glasses on!
  2. Second contact: totality begins and the solar corona becomes visible. Glasses off!
  3. Third contact: totality ends and a partial eclipse returns. Glasses back on!
  4. Fourth contact: the discs of the new Moon and the Sun touch each other at a final point on their edges and then separate. Glasses still on!


Elisabeth Mittelbach

Me­dia in­quiries Ger­man Space Agen­cy
German Aerospace Center (DLR)
Com­mu­ni­ca­tions, Ger­man Space Agen­cy at DLR
Königswinterer Str. 522-524, 53227 Bonn

Manfred Gaida

German Aerospace Center (DLR)
German Space Agency at DLR
Space Science
Königswinterer Straße 522-524, 53227 Bonn