The Partial Solar Eclipse of Friday 20th March 2015
The largest partial solar eclipse visible from Northern Ireland since 29 June 1927, when
97% of the Sun was obscured, occurs between approximately 08:26 and 10:38 on the
morning of Friday 20th March 2015. Even the famous solar eclipse of 11 August 1999,
which was total over a small part of Cornwall and led to a distinct darkening of the day in
Northern Ireland around the time of eclipse maximum (11:14 BST), only obscured 86.8%
of the Sun as seen from Belfast.
The 20 March 2015 eclipse is the first partial solar eclipse visible from Northern Ireland
since 1 August 2008, when less than 20% of the Sun was obscured. Eclipse maximum,
which is the deepest part of the eclipse, occurs at 09:30 as seen from Armagh. At this
time, only a thin crescent or ‘smiley’ of the Sun will be left uncovered and more than 93%
of the Sun’s disc will be obscured by the Moon, which is passing slowly across the face of
the Sun from West to East during its nearly one-month orbital period of revolution about
the Earth.
Figure 1: Left: The path of totality for the solar eclipse of 20th March 2015 crosses part
of the North Atlantic Ocean roughly halfway between Britain and Iceland and then into the
Arctic Ocean. The central part of the Moon’s shadow (the umbra) only touches land on the
Faroe Islands, to the North-West of Ireland, and the Svalbard Archipelago, Norway, in the
Arctic Ocean. Right: Locations of cities for which the eclipse parameters are listed in Table 1.
Image from the NASA Eclipse Website: http://eclipse.gsfc.nasa.gov/SEgoogle/SEgoogle2001/
SE2015Mar20Tgoogle.html.
This eclipse will be seen as total along a path that passes over part of the North
Atlantic Ocean and the Arctic Ocean, tracking roughly halfway between Ireland and
Greenland/Iceland, as shown in Figure 1, and then across Svalbard, north of Norway, in
the Arctic Ocean. The remote Faroe islands and the Svalbard Islands, roughly halfway
between Norway and the North Pole, are the only places on Earth where the total eclipse
may be seen from land, weather permitting. Others hoping to see the amazing spectacle
of a total solar eclipse will have to book a passage on a ship or an aircraft.
Because the path of totality lies to the North-West of Ireland, locations in Northern
Ireland farther west and/or farther north will have a slightly better view of this exceptionally large partial solar eclipse. For example, whereas 93.15% of the Sun is obscured from
Armagh, in Belfast it is 93.03% but in Derry/Londonderry it is 94.38%. In Dublin it is
91.52%, and in Cardiff and London only 86.66% and 84.41% respectively. The parameters
of the eclipse from these locations are shown in Table 1, while the development of the
eclipse as seen from Northern Ireland is shown in Figure 2.
1
Other Solar Eclipses Visible from Northern Ireland
Eclipse Frequency
Partial solar eclipses are not particularly rare, occurring with an average frequency (as
seen from Northern Ireland) of approximately once every 3 years during the period 1900–
2099. However, the last such eclipse was on 1 August 2008, when less than 20% of the
Sun’s disc was obscured; and the next will not occur until 21 August 2017, when only 3%
of the disc will be covered. These figures illustrate the fact that the majority of partial
solar eclipses visible from any particular location on Earth usually produce the appearance
of only a small ‘bite’ taken out of the Sun. This is because in order to see a total solar
eclipse or a large partial eclipse, the position of the Moon, which for a solar eclipse lies
between the Sun and the Earth, must lie nearly exactly on a straight line drawn from the
observer’s location towards the Sun.
Deep partial solar eclipses, as seen from any particular location on Earth, are therefore
very rare, occurring on average only once or twice in a human lifetime. Total solar eclipses,
when the Sun, Moon and Earth happen to be perfectly lined up as seen from the observer’s
location, are even rarer. At the latitudes of Northern Ireland, according to the astronomer
Jean Meeus a total eclipse will occur on average with a mean interval of approximately
305 years, slightly more often than the mean interval (375 years) between total eclipses
for a random place on Earth.
Calculations show that the next total eclipse visible from any part of the island of
Ireland will not occur until 23 September 2090, when the path of totality will cross the
extreme SW of Ireland. After this event, which will produce a very large partial eclipse as
seen from Northern Ireland, the following total eclipses over different parts of the island
of Ireland are those of 14 June 2151 (crossing NE Ulster), 8 November 2189 (crossing SW
Ireland) and 14 April 2200, which passes straight across Ulster and will be visible from
Armagh. In addition, an annular eclipse will be visible from Ulster on 23 July 2093. This
rare type of eclipse occurs when the Sun, Moon and Earth are perfectly lined up, but
when the Moon is too far from Earth and/or the Earth is too close to the Sun for the
Moon’s disc to completely cover the Sun.
Recent Solar Eclipses Visible from Northern Ireland
This year’s partial solar eclipse, on 20 March 2015, is the largest partial eclipse visible
from Northern Ireland since 29 June 1927. The latter was the last of three comparably
deep solar eclipses visible from Northern Ireland during the 1920s, but the next very
large partial eclipse visible from these shores will not occur until 12 August 2026 between
approximately 18:11 BST and 20:03 BST, with maximum eclipse as seen from Belfast
occurring at 19:08 BST. At eclipse maximum on that date, nearly 93.1% of the Sun will
be covered.
After this year’s 20 March 2015 partial eclipse, the next larger partial solar eclipse
visible from Northern Ireland will not occur until 23 September 2090. On that occasion,
some 96.7% of the Sun will be obscured, with eclipse maximum as seen from Belfast
occurring at 18:24 BST, although as noted there is another comparably deep partial solar
eclipse on 12 August 2026.
The year 2090, perhaps even 2026, is a long time to have to wait for another large
partial solar eclipse, emphasizing the fact that this March’s partial eclipse is a rare and
unusual event. Every effort should be made to observe it safely and to record its appearance and any associated environmental effects that might be seen. As with all time-critical
2
Location
Eclipse
Magnitude
Obscuration
(Percent)
Start (UT)
Partial Eclipse
Maximum (UT)
End (UT)
Armagh
Belfast
Derry/Londonderry
0.937
0.936
0.947
93.15
93.03
94.38
08:26
08:26
08:26
09:30
09:31
09:30
10:38
10:39
10:38
Dublin
Galway
0.924
0.937
91.52
93.12
08:24
08:22
09:28
09:26
10:37
10:34
Cardiff
Edinburgh
London
0.886
0.937
0.868
86.66
93.10
84.41
08:23
08:30
08:25
09:28
09:35
09:31
10:37
10:43
10:41
Table 1: Circumstances of the Partial Solar Eclipse from various locations in the UK and Ireland
shown in Figure 1. Eclipse parameters courtesy of UK Nautical Almanac Office, Hydrographic
Office (UKHO); http://astro.ukho.gov.uk/eclipse/0112015/.
Figure 2: Development of the 20 March 2015 partial solar eclipse as seen from Northern Ireland
from first contact at 08:26 to eclipse maximum at around 09:30. The upper panel shows the
eclipse increasing from First Contact at 08:26 to eclipse maximum; the lower panel shows the
Moon moving away from the Sun towards the end of the eclipse at Fourth Contact, which occurs
at 10:39 (as seen from Belfast). These images have been extracted from the excellent UKHO
website http://astro.ukho.gov.uk/eclipse/0112015/ (also see http://astro.ukho.gov.uk/eclipse/
and links therein).
astronomical events, we can only hope that the weather will cooperate and produce clear
skies.
Weather Prospects
The weather in March can be highly variable, as for example with the adage ‘If March
comes in like a lion, it will go out like a lamb’, and vice-versa. Armagh Observatory
has been recording the weather in Armagh since around 1795, and maintains the longest
daily climate series from a single site in the UK and Ireland, and one of the longest in
the world. The Observatory’s weather records are available for inspection on the website:
http://climate.arm.ac.uk/.
3
From these data, and considering only the last 20-year period (1995–2014), we find
an average of only 3 hours strong sunshine per day during the last twenty March 20th’s,
and on only four of those days could the Sun reasonably have been seen around 9.30 am.
This suggests that there is a probability of about 20–30% for the clouds in Armagh to
part sufficiently to enable observations of the full duration of the eclipse. However, even
if the partial eclipse cannot be seen in whole or in part, there will still be interesting
temperature, sky-brightness and other environmental effects that will be worth recording.
Eye Safety
The following information has drawn heavily, in some cases verbatim or nearly so, from
material available in articles on the Internet regarding eye safety during solar eclipses or
during transits of Mercury or Venus across the face of the Sun. In particular, the author
wishes respectfully to acknowledge the following sources: (1) the frequently quoted article
on eye safety by Professor B. Ralph Chou, which may be found for example through a
link from the Royal Astronomical Society of Canada’s website (https://www.rasc.ca/tov/
safety) and from the NASA website http://eclipse.gsfc.nasa.gov/SEhelp/safety2.html);
(2) the NASA eclipse website guidance on how to view the Sun safely during a partial
or total solar eclipses (see http://eclipse.gsfc.nasa.gov/SEhelp/safety.html); and (3) the
Royal Astronomical Society’s advice on eye safety during eclipses (https://www.ras.org.
uk/search/article-archive/545-pn99-19) and links therein.
Eye Safety Warning
There is very little tendency under normal circumstances to look at the Sun in a way that
might damage the eye, especially as the Sun is so bright that it is difficult to stare at it
directly. However, during a large partial solar eclipse the Sun is the centre of attention,
and with so much of the Sun covered it is tempting and slightly easier to stare at it.
NEVER DO SO.
It is never safe to look at the Sun without proper eye protection. Even when 99% of
the Sun’s surface is obscured during a very deep partial eclipse or either side of the short
period of totality of a total eclipse, the intensity of the remaining parts of the solar disc
that are still visible is still strong enough to cause a retinal burn even though the ambient
sky brightness may be comparable to twilight. The retina has no pain receptors, and so
severe damage can be done without you becoming immediately aware of the danger.
Failure to use proper observing methods can result in permanent eye damage and
severe retinal loss. Viewing the Sun through binoculars, a telescope, or other optical
devices without the proper protective filters can result in immediate and permanent retinal
injury owing to the irradiance of the magnified image.
How Eye Damage Occurs
The radiation from the Sun that reaches the surface of the Earth ranges in wavelength
from the near ultraviolet (UV), with wavelengths longer than around 290 nm, through the
visible spectrum (approximately 370–700 nm) and near infrared (wavelengths greater than
about 700 nm), right through to radio waves with wavelengths of the order of a metre or
so. The tissues of the normal human eye focus a substantial part of the ultraviolet, visible
and near-infrared radiation with wavelengths spanning approximately 380 nm (near-UV)
to 1400 nm (near-infrared) on to the light-sensitive retina at the back of the eye. In turn,
4
the retina can be permanently damaged by the intensity of the focused image of the Sun
at the back of the eye.
In particular, while environmental exposure to UV radiation is known to contribute
to accelerated ageing of the outer layers of the eye and the development of cataracts, the
primary concern with improper viewing of the Sun during an eclipse is the development
of solar retinopathy, or retinal burns.
Any exposure of the retina to intense visible light causes damage to its light-sensitive
rod and cone cells. The light triggers a series of complex chemical reactions within cells
that damage their ability to respond to a visual stimulus and in extreme cases can destroy
them. The result is a loss of visual function, which may be temporary or permanent
depending on the severity of the damage.
When a person looks repeatedly, or for a long time, at the Sun without proper eye
protection, this photochemical damage may be accompanied by a thermal injury caused
by the high level of visible and near-infrared radiation. The radiation first burns, and
then literally cooks the exposed tissue. The thermal injury, known as photocoagulation,
destroys the rods and cones, and creates a permanent blind area. There is a significant
danger to vision because there are no pain receptors in the retina and so photic retinal
injuries can occur without any feeling of pain. Any visual effects are sometimes not
noticed until several hours after the irreparable damage is done.
Take Care
Despite the well-known danger of looking at the Sun without suitable eye protection,
there are always some people who either through ignorance or foolhardiness choose to
ignore this advice. Research over the last fifty years has shown that the principal causes
of eclipse-related retinal burns are (1) viewing the partially eclipsed Sun without suitable
eye protection; (2) looking directly at the Sun through the pinhole of an indirect projection
viewer; and (3) viewing the Sun through ordinary sunglasses, photographic neutral-density
filters, or a wide range of similarly inappropriate devices.
In summary, it is never safe to look at the Sun without proper eye protection. No
filter should be used unless it has been specially designed for the purpose, and no filter
should be used with any optical device (e.g. telescope, binoculars or camera) unless that
filter has been specially designed for that purpose and is firmly mounted at the front end
of the instrument. Filters should never be placed in any position where they might burn
or crack under the Sun’s heat.
How to Observe the Partial Eclipse Safely
Projection Method
The safest and most inexpensive method to observe the Sun is the method known as
‘projection’. A small pinhole or a small opening in a piece of card is used to project
an image of the Sun on to a shaded screen placed about a metre behind the opening.
Alternatively, a small mirror can be used to reflect the image of the Sun onto a distant,
shaded wall. Even the holes in a colander, or the leaves of a tree, can be used as ‘pinholes’
to project multiple images of the Sun on to a nearby surface, opening up the possibility
to produce a variety of interesting ‘artistic’ photographs of the eclipse.
In all these cases, there will be a trade-off between the size of the pinhole or the
usable size of the mirror and the brightness of the resulting image. Smaller pinholes are
5
better from the point of view of sharpness of the image, but larger pinholes produce a
brighter image. The mirror method was further developed by Dr Hugh Hunt (University
of Cambridge) and used to good effect at Trinity College, Cambridge, to display the 2004
Transit of Venus. Full details can be found at https://in-the-sky.org/work/transit.php
and http://www2.eng.cam.ac.uk/∼hemh/transit.htm.
A better approach is to use binoculars or a small telescope or spotting scope, to
project a magnified image of the Sun on to a sheet of white paper or card as shown
in Figure 3. If using a pair of binoculars, make sure that the dust cap is firmly attached to the objective glass of one side of the binoculars, to avoid inadvertent burns
and to ensure that only one image is projected from the device, and never look directly through the instrument at the Sun. Further information on projection methods can be found at http://www.exploratorium.edu/eclipse/how.html and http://www.
skyatnightmagazine.com/search/node/solar%20projection.
All these methods can be used to project a safe view of the Sun to a group of observers,
but great care must be taken to ensure that no-one ever looks at the Sun through the
optical device. Phenomena that might usefully be measured are the times of first and last
contacts; the times of passage of the Moon’s apparently black limb across any (reddishbrown looking) sunspots, and so on.
Figure 3: Projecting the Sun with a small telescope and two pieces of card: one to display the
Sun’s image and the other to provide a sunshade to enable the image to be more easily seen.
This arrangement can readily be improved to provide an easy and robust method to monitor the
appearance and growth of sunspots from day to day, as well as the rotation of the Sun roughly
once every 27 days at the mid-latitudes where most sunspots are seen. In the image at right,
notice the phenomenon of limb darkening. This is caused by the observer’s line of sight closer
to the edge of the Sun only being able to penetrate to higher, and therefore cooler, levels of
the Sun’s gaseous ‘surface’ than at the centre of the disc. The limb darkening is a significant
additional cause of the general darkening of the sky observed during the final ‘crescent’ phase
of a deep partial eclipse.
6
Solar Scope Projection Viewer
The instrument known as a solar scope was invented as a relatively cheap device to enable
an image of the Sun to be projected safely to a small group of people. It can be obtained
from the manufacturer (http://www.solarscope.com/en/index.en.html), or for example
from retailers such as Green Witch Telescopes in the UK (see http://www.green-witch.
com/lightec-solar-projection-telescope-1529-p.asp). This is a very simple device, and very
safe for use under supervision.
Direct Observation Method
The Sun can only be viewed directly when specially designed solar filters are used to
block out the intense and potentially damaging UV, visible and IR radiation. Eclipse
shades or eclipse cards are readily available from a wide range of manufacturers and
retailers, for example from retailers such as those linked to http://amazon.co.uk/ (and
found by searching for ‘solar eclipse glasses’), or from companies that organize specialinterest eclipse tours (e.g. http://www.astroadventures.net/eyesafety.html).
If you intend to look directly at the Sun, you must avoid all risks and use the correct
solar filters designed for the purpose.
Environmental Effects
During a solar eclipse it is interesting to monitor radio reception (e.g. http://www.ukssdc.
ac.uk/ionosondes/eclipse/detailexp.html), or record any changes in ambient light levels,
temperature (both air temperature and ground temperature), and other meteorological
and associated phenological phenomena, for example a tendency for bird song to die
down, perhaps even birds roosting, or flower petals beginning to close as if preparing for
‘night’, and other effects. If the sky is partly cloudy you might notice cumulus clouds
beginning to dissipate around the time of eclipse maximum, in the same way that the sky
sometimes begins to clear around sunset as fair-weather clouds that may have built up in
the afternoon disappear.
The essence of science is observation, and there are many phenomena of the natural
world around us that may be observed — and recorded — during a large partial solar
eclipse. Enjoy this partial eclipse as seen from Ireland, as the next comparably deep event
visible from these shores will not occur until 12 August 2026.
Acknowledgements
I thank colleagues, especially Apostolos Christou, Gerry Doyle, Maria Madjarska and
Terry Moseley for helpful comments, corrections and suggestions, which have greatly
improved this leaflet. This work was supported by the Northern Ireland Department of
Culture, Arts and Leisure.
Mark E. Bailey
Armagh Observatory
2015 February 18
7