The total solar eclipse of 20 March is also the largest partial eclipse to be seen over Great Britain since 11 August 1999. Sixteen years ago the path of totality crossed Cornwall, whereas this time it passes a hundred miles or so off the north-west coast of Scotland. Back in 1999 many people travelled south-west to see totality from the UK only to be thwarted by cloud. A similar fate may await the people who are travelling to see this total eclipse, since the path of totality crosses one of the cloudiest parts of our planet. As in 1999, it may be the people who stay for the partial eclipse who get the best weather.
A solar eclipse occurs when the Moon moves in front of the Sun and obscures the bright solar disc, or photosphere. The path of totality is the area on the Earth’s surface swept out by the Moon’s umbral shadow (the darkest part of the shadow). Within this region the photosphere is completely covered and a total eclipse is seen. On either side of this path observers will see a partial eclipse.
This eclipse is total along a path that starts at 9:11am GMT/UT in the North Atlantic, south of Greenland at sunrise. At this point totality lasts just over two minutes. The shadow moves north-east and crosses land for the first time at the Faroe Islands. Torshavn, which is some way south of the centreline, experiences two minutes and two seconds of totality at 9:42am. The maximum duration of totality (two minutes and 46 seconds) occurs in the Arctic Ocean and the track then moves on to the Norwegian island of Spitsbergen. Longyearbyen has two minutes and 25 seconds of totality at 10:12am. The umbral shadow races on and leaves the Earth at the North Pole, where the Sun is permanently on the horizon at this time of year.
Away from the path of totality the British Isles will get a very large partial eclipse. In London the eclipse starts at 8:34am with the Sun 20 degrees above the horizon, reaches a maximum at 9:31am and ends at 10:41am. The corresponding times for Glasgow are 8:29am, 9:34am and 10:42am. Cape Wrath, on the north-west coast, sees the largest partial eclipse visible from mainland Great Britain at 97.3 percent. If you travel over to Carloway in the Outer Hebrides the obscuration is even bigger at 98.1 percent.
Those really keen to get the largest eclipse possible from UK soil may consider travelling to Rockall. This small, uninhabited islet is part of the UK according to a 1972 Act of Parliament, although during the debate it was described as “desolate, despairing and awful.” Rockall is just outside the zone of totality and gets a 99.96 percent partial eclipse, although getting there would be a significant challenge.
Land-based options for the total eclipse are limited to Torshavn, where average cloud cover is around 80 percent, or Longyearbyen, where it is around 55 percent. Both sites have good airports but accommodation and flights on eclipse day were booked up long ago.
Escape the clouds
Since the path of totality crosses land at only two locations, an alternative is to view it from a ship. Ships can move around to take advantage of holes in the cloud but they also have been known to end up in the wrong place, particularly in difficult weather conditions. In 2012 one cruise operator managed to miss the umbra and the passengers saw a large partial eclipse rather than totality. The tour operator claimed that “close to totality” was good enough. Anyone who has seen a total solar eclipse will disagree.
A third option is to observe the eclipse from an aircraft well above the clouds. A number of flights have been organised and a great deal of skill is needed to make sure that the aircraft is in the right place at the right time with an orientation that gives the best view out of the windows. Foremost among airborne eclipse chasers is Xavier Jubier. His flight, using a Falcon F7X business jet, will fly along the path at 49,000 feet in the same direction as the shadow and will extend totality to almost four minutes. Other flights, using large commercial aircraft, will be lower but still well above the clouds.
Whether you are in the path of totality or not, the eclipse will start at first contact as the Moon begins to obscure the Sun. Whenever any part of the bright disc of the Sun is visible, you must use an appropriate filter for direct observation, or you could seriously damage your eyesight, and potentially blind yourself. Filters such as Baader solar film, available from many speciality suppliers, will allow you to observe and photograph the partial phases safely. Alternatively, solar viewers, such as those available from the British Astronomical Association are convenient and safe for naked eye observation. You can also use binoculars or a telescope to project an image as long as there is no danger of anyone looking through the instrument – remember to place a cap on the finderscope so nobody, such as young children, can look through it by accident.
If there are any sunspots on the solar disc, it will be interesting to watch them disappear behind the advancing lunar limb. If you own a hydrogen-alpha solar telescope, you could draw or photograph any prominences that you see and then compare them with photographs taken during totality.
Imaging the eclipse
Taking photographs or video during the partial phase is relatively easy. You will need a solar filter over the lens and it is best to set manual focus and manual exposure if that is possible. A good project would be to take pictures using the same settings at regular intervals and then put the photos together as a time-lapse or composite image. You can do this with a long lens to record the Moon moving across the Sun or use a wide-angle lens on a tripod to record the changing lighting conditions. It is even possible to get really good images of the partial phase simply by holding your camera or phone to the eyepiece of a suitably filtered telescope.
During the partial phase have a look for pinhole projections of the eclipsed Sun on walls or on the ground. You can use natural pinholes in foliage or make your own using some foil and a pin. At around 95 percent obscuration, shadows take on an unusual character since the sunlight is coming from a narrow slit rather than an extended disc. If the sky is clear, you should also be able to see Venus around 35 degrees to the left of the Sun. Unless you are in the path of totality this is as good as it gets, but if you are lucky enough to be in the right place at the right time the partial phase is just a warm-up. The main feature is about to start.
Just before totality the crescent will break up into multiple bright points called Baily’s beads. These rapidly reduce in size until, at the last moment, only one or two tiny parts of the photosphere are visible. These form the famous ‘diamond ring’ at second contact (when the Moon is fully on the Sun). Once the ring is over, totality has begun and it is safe to observe without filters since you will not see any of the phenomena of totality otherwise. This is particularly important to remember if you have filters over your camera lenses!
Take in the view
Immediately after the diamond ring, binoculars or a telescope will show a red band along the Moon’s edge near the second contact point. This is the chromosphere, a region of low-density ionised gas around 2,000 kilometres thick that sits above the photosphere. With optical aid and some luck it should also be possible to see some deep red prominences sprouting from behind the lunar limb.
Even if you plan to photograph or video totality, I would strongly recommend that you set aside at least a minute to look around with binoculars or the naked eye. There are majestic atmospheric effects that no image or video can ever convey. Look for the bright planets – Venus should be easy, but Mercury to the right of the Sun should also be visible.
Without doubt, the most spectacular feature of a total eclipse is the solar corona. This is the Sun’s extended atmosphere and, with the photosphere hidden from view, it shines out from the dark lunar disc. The shape of the corona varies depending on the state of the solar cycle but at this eclipse we would expect a reasonably active, symmetrical structure. High resolution images of the corona require long exposures and accurate tracking, but you can get nice atmospheric wide-angle pictures using a hand-held camera. Just remember to disable the flash!
As the end of totality approaches, the chromosphere will reappear from behind the Moon’s trailing limb. This is a warning that the third-contact (when the Moon starts to move off the Sun) diamond ring is imminent. If you are using an optical aid prepare to look away before the first glint of photosphere appears. Now is the time to resume using filters to safely watch the partial phase as the Moon gradually uncovers the Sun.
I am sure that there will be plenty of spectacular images of this total eclipse but, even if you are not lucky enough to be under the umbra, the large partial eclipse visible across the British Isles will still be spectacular. The next big partial eclipse in this part of the world will not occur until 2026, so make the most of this one.
Nick James is a council member of the British Astronomical Association, Assistant Director of its Comet Section, Assistant Editor of The Astronomer and an avid eclipse chaser. This article appeared in the March 2015 issue of Astronomy Now.
Inside the magazine
In the April 2015 issue of Astronomy Now, we tell you everything you need to know about solar eclipses. Get your copy in the shops or order online.
Never miss an issue by subscribing to the UK’s biggest astronomy magazine. Also available for iPad/iPhone and Android devices.