In exactly a week, a total eclipse will be visible, spanning along a narrow path through the United States. In readiness for the event on 21 August 2017, we asked physicist and eclipse-chaser professor Frank Close eight questions about eclipses, and how to watch them.
What is a solar eclipse?
The moon is 400 times smaller than the sun, but it’s also 400 times closer to earth, which means that remarkably, the two bodies appear to us as exactly the same size. For 14 days a month, the orbiting moon is on the ‘sunny’ side of the spinning earth, and the sunlight casts a shadow.
Almost all the time, that shadow is projected way off into space; but on very particular occasions, the shadow falls onto the earth – the moon is obscuring our view of the sun.
From the human observer’s point of view, your attention is of course on the sun, as you watch the moon slowly move in front of it.
Relative to the earth, the moon and sun are moving at slightly different speeds, which means the shadow sweeps across the earth’s surface; it passes from West to East at about 2,000 miles an hour.
Are all eclipses the same size?
No. Because the moon’s orbit around the earth is elliptical, sometimes it’s closer, sometimes it’s further away. So for each eclipse, the size of the shadow will be different.
On an occasion when the moon is very far from the earth, it will appear fractionally smaller than the sun. Rather than total, the eclipse is ‘annular’ – with a thin ring of sun visible around the moon.
How wide is the path of the eclipse?
Typically, the whole shadow will be several thousand miles across; the central area which gives ‘totality’ will usually be between ten and 100 miles across.
How long does totality last?
That depends on the size of the shadow. The longest totality has been around seven minutes; most are around three, and some are shorter than that.
It also depends very much on where you are standing. The closer you are to the centre-line of the shadow’s path, the longer you will experience totality. If you are say 30 miles off-centre, you will only get a few seconds.
How often does totality occur?
Twice a year (or, in fact, twice in 355 days) some sort of solar eclipse happens somewhere on the earth’s surface. But the spectacular total eclipses happen about six times a decade – and most often in locations not easily accessible to most of us.
How long will I have to wait to see the next eclipse?
It depends how far you want to travel! Since 1999 I have seen six: Cornwall in 1999, Zambia in 2001, Sahara Desert in 2006, two in the South Pacific, and one off the Cape Verde Islands. The August 2017 in Wyoming will be my seventh.
Which cities will experience totality in 2017?
The shadow makes landfall south of Portland, Oregon, and it leaves the continent in South Carolina. Cities on the path include: Corvallis, Albany; Lebanon, Oregon; Idaho Falls, Idaho; Casper, Wyoming; Lincoln, Nebraska; St Joseph, Missouri; Kansas City, Kansas; St Louis, Missouri; Nashville, Tennessee; Greenville, South Carolina.
What safety tips would you recommend?
First, don’t walk about. It’s surprisingly easy to trip over in the strange darkness, especially when there are lots of other people around you.
Second, be absolutely certain to look away as totality finishes. During totality, you will be looking at the blackened disk that is (or is not) the sun. The sunlight returns literally in a flash, and your pupils will be wide open.
Third, watch out for wild animals! When totality falls, birds and animals behave as if it’s night-time. Birds roost, crickets chirp, night-time predators set about their business. In 2001, I was watching from the banks of the Zambezi river; fortunately, our guards were ready for the hippopotami.
Fourth, save up. Experiencing a total eclipse is the most extraordinary and wonderful sensation. You are sure to want to finance the journey to another one, another year.
Featured image credit: annular solar eclipse by Takeshi Kuboki. CC BY 2.0 via Flickr.