Eyes down for the partial solar eclipse

WE WILL be able to see a partial solar eclipse in Guernsey tomorrow, when the moon appears to move across part of the sun’s disk.

The eclipse will appear as if the sun has a growing, and then diminishing, ‘bite’ out of it. As we will be looking at the shadowed side of the moon, the moon itself will not be visible except in silhouette where it overlaps the sun.

Do NOT look at the sun directly or through any optical equipment – in this article we explain various ways to safely view the partial eclipse.

Partial eclipse timings in Guernsey

From Guernsey, a maximum of about 30% of the sun’s diameter will be eclipsed by the moon.

The diagram shows how the Moon’s orbit intersects with the Earth’s orbit around the Sun, causing a regular cycle of possible eclipses. (29628315)

The partial eclipse will start in Guernsey about 10.03am, with maximum 29.5% eclipse at 11.05am, and finish about 12.12pm. As it is a morning eclipse, the sun at the start of the eclipse will be in the east, rising high to the south.

Depending on what equipment you may be able to use, you can observe the shape of the partially-eclipsed sun, and maybe sunspots.

Annular eclipse on a northerly path

Although in Guernsey we will be nearer the edge of the eclipse, a few northerly parts of the world will lie in the full path of this eclipse. As the moon is at a further point in its orbit, it will not fully cover the sun and through maximum will be seen as an annular eclipse, leaving a ‘ring of fire’ of sun visible at maximum eclipse. The annular eclipse will be visible from parts of northern Canada, Greenland, and north-eastern Russia.

Along the full eclipse path, the first location to see the full annular eclipse will be at 10.50am and the last at 12.33pm. There will no doubt be live internet feeds from some of these locations if you wish to follow progress and see images.

Eclipses – when they happen and why

Solar eclipses can only be viewed from the day side of Earth when the new moon passes in front of the sun, making a narrow full eclipse shadow which moves along a track over parts of the Earth. Lunar eclipses can only be viewed from the night side of Earth when the full moon passes through Earth’s shadow.

A partial eclipse occurs when the moon only partly falls into line with Earth and the sun.

Also, a partial solar eclipse is seen from parts of the Earth not in the full solar eclipse track.

Typically eclipses occur in groupings of two or three within around a month, when the nodes of the moon’s orbit – where the moon’s orbit intersects with Earth’s orbit – roughly coincide with the full and new moon.

The rest of the time, the moon does not line up very well with the Earth and the sun, as its orbit is inclined about five degrees from our own, so no eclipses occur.

Eclipses can occur approximately every six months, as the sun crosses a node about every 173 days.

Diagram from www.timeanddate.com. (29628344)

Many parts of the world saw a lunar eclipse on 26 May, though this occurred during daytime in Guernsey so we could not see it. In May/June 2021, the lunar eclipse is followed by an annular solar eclipse.

As the May lunar eclipse also occurred at the moon’s perigee when it appeared particularly large, the solar eclipse about two weeks later occurs when the moon is approximately at its apogee, or furthest away from Earth. Its apparently smaller size is not enough to completely cover the sun in the solar eclipse, giving the annular eclipse.

Why does the moon look the same size as the sun?

The moon is approximately 400 times smaller, but also on average about 400 times closer to Earth than the sun. This means that the apparent size of both the moon and sun in the sky are about the same at the present epoch, at about half a degree.

Therefore when the moon is in a position to move across the sun, it can cover it almost exactly – sometimes appearing slightly bigger, sometimes slightly smaller.

When is the next eclipse?

Before dawn on 19 November 2021 we will see the beginning of a partial (almost total) lunar eclipse, though we won’t get to see a very good view of this in Guernsey, with mostly penumbral (partial) shadow before the moon sets at dawn. The best parts of the eclipse will be seen in time zones to the west.

This is followed by a total solar eclipse on 4 December, though this will only be visible in deep southern latitudes.

Ways to view the partial solar eclipse safely

It is not safe to look at the sun directly, whether with the naked eye or optical aid.

There are various ways to view the sun and the partial solar eclipse safely, and many of those are free or inexpensive or use materials or items you may have already, for example:

1. Eclipse glasses. If you have a pair of solar eclipse glasses, in good condition, you can use these to view the sun’s outline safely for a limited time. Just don’t try walking around while wearing them, as you can’t see anything else.

2. Simple pin hole projection. Use two pieces of card: a dark card with a neat pin hole, held in view of the sun, and in its shadow hold a white card to display the tiny image. The further apart the cards are, the bigger but dimmer the image.

3. Pin hole projection in a box or tube, for better contrast. You can find guides for this online. This will project a small image of the sun through a pin hole in foil in the sunny end of the box, onto the inside of the far end of the box, which you can view through a side cut-out in the box. Placing a white card inside at the display end produces a brighter image. The longer the box or tube, the larger the image, though it may become less crisp and bright and will be more difficult to find and keep centred.

How to project an image of the Sun for safe viewing - simple pinhole projection with two pieces of card. This can be enclosed in a box for greater contrast, and the pinhole can be made in foil for a crisper image. (29628355)

4. Mirror reflector. Cover a small flat mirror with black paper or card, leaving just a tiny cut-out no more than 5mm across. You could use a small hole punch but any shape hole will do – this will act as a pinhole that you can use to reflect an image of the sun onto a shaded white wall – experiment using different distances between mirror and wall and see how big a reflected projection you can make without losing too much detail.

5. Use a tree or colander or interlaced fingers as a multi-pinhole projector. For example stand in the shade of a tree that lets small glimpses of the sun shine onto the ground. During the partial eclipse you will see instead of glimmering circles, crescents, though these will be stretched as the ground lies at an angle away from the sun. You can put down a white sheet or white card to show up the shapes better, and perhaps angle this full on to the sun to see more representative shapes.

A kitchen colander (not a sieve) can also be used to split the light into many images – the further the colander is away from the ground, the bigger but dimmer the shapes will be, and they may overlap.

You can also try this method with interlaced fingers, leaving tiny gaps, or by punching random small holes into a piece of card.

6. Binocular or telescope projection. If you have a small telescope or binoculars and a tripod, you may be able to project a larger image of the sun onto white card – but as this may damage your equipment, please research this properly first, it is at your risk.

Make sure no one can look through directly as this is very dangerous.

A long pinhole projector made from cardboard tubes and set on a tripod with tracking to keep the image in view – this set-up produces a 2cm diameter image of the Sun, enough to show the shape of a partial eclipse. (29628396)

You can roughly aim by standing with your back to the sun and trying to cast the smallest shadow with the binoculars or telescope, then trial and error to find the sun image.

Use the normal focusing method of your equipment to obtain a crisp image on the card. With this method, larger sunspots may be visible.

Adjust to keep the sun showing fully on the card when in use, and cover up the equipment when not in use, to protect your equipment from excessive heat.

7. Use a solar filter. Solar filters are available to safely view the sun through binoculars or a telescope on a tripod, or to cover a lens on a camera. Or for a less expensive option, construct your own solar filter with dedicated solar filter film such as Baader AstroSolar Safety Film.

Make sure any sun filter is fitted securely and preferably taped on.

8. Come to our open morning. La Societe Guernesiaise Astronomy Section is holding an open morning at the Observatory in Rue du Lorier, St Peter’s from 10am tomorrow for people to observe the partial eclipse. They will have solar filters on telescopes as well as telescopic projection, pin hole projection, and eclipse glasses to try and members will be on hand to advise on safety and answer questions.

9. Look for live feeds or news of the eclipse. Amateur and professional observers of the annular eclipse will also post images online in the days afterwards, so do take a look.

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