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Tue, 15 Aug 2017

Be prepared for the (not-so-)great eclipse


A sunset partial eclipse. This month from the UK the lower part of the Sun will be eclipsed.
Photo: Robin Scagell

Eclipse mapMany people are jetting off to the States for the total solar eclipse of Monday 21 August, the track of which crosses the USA, offering the chance of a good view for millions of people. Full details of the US eclipse are available at this NASA website. The longest duration of the eclipse is some 2m 40s, from Tennessee, but the chances of clear weather are better farther west, where the duration is a few seconds shorter.
    But we do get a small partial from this event in the UK. And while it will not be as spectacular as the main event visible only from the US, there is the chance of getting some dramatic photos from the UK as the partial occurs close to sunset. And if you want to observe it, start laying your plans well in advance. You'll need to find a location nearby where you can observe the Sun when it's low in the sky – not always easy. So get out there in the evening before the event and find a spot where you can view the Sun when it is close to the horizon. It will be in virtually the same place on the 21st; if anything, it will be a degree or two farther south.
    To the south and east of the UK, the Sun sets before the eclipse has finished. However, the more northerly you are, the less of the Sun is eclipsed. It will last less than 50 minutes from any location in the UK.
    The graphic here shows the extent and altitude of mid eclipse from various locations. The upper diagonal line shows the area below which the Sun sets before the eclipse ends, while the lower one shows where the Sun sets at mid eclipse. 
    The timings (BST) and altitudes for the various stages are:







Eclipse starts

19:37 (7.7º)

19:38 (8.1º)

19:40 (5.0º)

19:40 (5.9º)

19:40 (3.7º)

Mid eclipse time (altitude)

19:56 (5.3º)

20:01 (4.8º)

20:03 (1.5º)

20:07 (1.7º)

20:04 (0.1º)

Eclipse ends

20:14 (2.8º)

20:23 (1.7º)

not visible

not visible

not visible

Sun sets






Diameter eclipsed






    In the final row the figure shows the fraction of the Sun's diameter covered by the Moon.
Eclipse viewers
Observers using safe eclipse viewers. Photo: Robin Scagell
The low altitude of the Sun means that it will appear considerably dimmer than when higher in the sky. However, it could still be dangerously bright to observe directly, while at the same time the usual solar filters might make it dim, though in recent tests standard solar filters still gave a viewable image even when the Sun was low down. Projection methods also might not work well. So observers will need to be particularly careful when observing, and advising others.
    Even though we often look at the setting Sun, usually we don’t stare at the Sun itself. Our brain makes sure that our eyes don’t keep it on one place in our retina for very long, so the heating of the retina is minimised. However, even if it appears dim enough for a glimpse with the naked eye, observing through unfiltered binoculars or telescope gives a much larger image on the retina, and the heat could build up and cause damage. This also applies when watching through the viewfinder of a DSLR camera.  So always err on the side of caution.

Eclipse viewer warning
A press release issued on 1 August by the American Astronomical Society says:
    In response to alarming reports of potentially unsafe eclipse viewers flooding the market as the coast-to-coast solar eclipse of August 21st draws near, the American Astronomical Society (AAS) has revised some of its safety advice to the public (
    How can you tell if your “eclipse glasses” or handheld solar viewers are safe? It is no longer sufficient to look for the logo of the International Organization for Standardization (ISO) and a label indicating that the product meets the ISO 12312-2 international safety standard for filters for direct viewing of the Sun’s bright face. Why not? Because it now appears that some companies are printing the ISO logo and certification label on fake eclipse glasses and handheld solar viewers made with materials that do not block enough of the Sun’s ultraviolet, visible, and infrared radiation to make them truly safe. Some sellers are even displaying fake test results on their websites to support their bogus claim of compliance with the ISO safety standard.
    We have not heard of any eclipse viewers on sale in the UK being affected by this.


Added by: Robin Scagell