Curt Nason: Total solar eclipse redux

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While giving solar eclipse presentations in the months leading up to the amazing event on April 8, I was asked how astronomers can predict when and where they occur so accurately. Others were concerned about children and pets being outside during the eclipse, and hence this month’s Night Sky column.

Whereas eclipses of the sun and moon were considered to be omens, good or bad, records were kept of when they occurred. Such records allowed Babylonian astronomers, 2,500 years ago, to discover patterns of eclipses that repeated over about 18 years. However, predicting solar eclipses accurately would require the knowledge and mathematical skill of Isaac Newton three centuries ago, with further refinements by Friedrich Bessel a century later.

Three aspects of the moon’s orbit must be considered: a 29.5 day period from one new moon phase to the next, an 18.6 year period in which it wobbles like a top and changes where it crosses the ecliptic, and an 8.8 year period of its changing perigee or closest point to earth. For a total solar eclipse to happen, a new moon must occur within six days of it crossing the ecliptic and the moon must be close enough to cover the sun completely.

In addition, earth’s changing distance from the sun must be considered, and an extremely precise standard of time is paramount. All of these aspects are known to much greater precision than given here, and refinements must be made frequently to account for subtle variations. With the moon moving at a kilometre per second, a half-second error can result in a prediction of the moon’s shadow being off by a few hundred metres.

Because we hear warnings about looking at the sun only for an eclipse, many people have the impression that it makes the sun brighter and more dangerous. The encroaching moon reduces the amount of sunlight, but it offers a spectacle that we want to look at. An extended direct view or several short glimpses without the proper protection can result in eye damage, usually temporary, which worsens with longer exposure. People were surprised at how bright the sunlight was even when the sun was 99 per cent covered by the moon. The sun is a million times brighter than a full moon, so even that narrow sliver is 10,000 times brighter. For those who are worried about their pets, animals do not look at the sun; being outside during an eclipse is no less safe than on any other sunny day.

Thousands of New Brunswickers purchased or were given eclipse glasses over the past several months, so what should we do with them? With solar activity nearing the peak of its 11-year cycle, there could be sunspots large enough to be visible through the glasses. Also, we have four partial solar eclipses occurring over the next five years. Store your eclipse glasses somewhere where they will not be damaged, and check them for punctures before reusing.

After a long spell without having a total solar eclipse in New Brunswick – 52 to nearly 1,100 years, depending on the area – we were very fortunate to have been blessed with clear skies on April 8. Thousands of eclipse chasers cancelled plans to observe from normally sunny areas in the south for better prospects in the north, including our province. My family enjoyed our first experience of totality in Nackawic, following plans made a year ago.

The sudden arrival of the moon’s shadow gave a new perspective to the word nightfall, and the sight of the sun’s beautiful corona has instilled the desire to become an eclipse chaser in many a New Brunswicker.

For information about the Saint John Astronomy Club contact Curt Nason (nasonc@nbnet.nb.ca) or visit the website http://www.sjastronomy.ca/.