On the morning of 20 March, at around 8:30 GMT, the Moon will begin to slide in front of the Sun from our Earth-based viewpoint. Anyone lucky enough to be on the Faroe Islands, the Svalbard Archipelago or a ship in that area will see the Moon eventually cover the entire Sun, at which point a total eclipse will occur and the Sun’s tenuous atmosphere will be momentarily revealed. For anyone living in the UK, the Moon will not entirely obscure the Sun and a partial eclipse will occur. But it will still be well worth going out to have a look.
How much of the Sun is covered depends on exactly where you are; in Glasgow almost 95% of the Sun will be hidden whereas in London the figure is closer to 85%. The timings vary slightly too, but roughly the eclipse will begin around 8:30 in the morning, will reach its maximum around 9:30 and will be over by 10:40. Information on the precise timings for your location can be found here: http://www.timeanddate.com/eclipse/solar/2015-march-20
Solar eclipses occur when the moon blocks out the sun’s light and a shadow is cast on the earth. By a lucky coincidence the Sun is 400 times bigger than the Moon but is also 400 times further away from us, so they are roughly the same size in the sky. At least twice a year, the Moon moves in between the Earth and sun is able to cover the sun either partially or totally. The shadow sweeps across the Earth because of the Moon’s motion and the shadow is small – only covering about 1% of the Earth’s surface. The safest way to observe the event is to use specialist eclipse viewers or make a pinhole camera. The Society for Popular Astronomy has a guide here: http://www.popastro.com/news/newsdetail.php?id_nw=329
The Royal Society has played an important role in publishing the results from scientific investigations of solar eclipses with one project in particular standing out to me. In 1715 an eclipse was observable from the UK, and it was the first to be seen in the country for 500 years. Edmond Halley, the British mathematician and astronomer and Fellow of the Royal Society who is famous for his work on the orbits of comets, wanted to know how fast the eclipse shadow would sweep over the Earth. To answer this question, Halley had to find some helpers who could make observations from various points along the path of totality. He realized that the eclipse would generate excitement and that he could capitalise on this to get the help he needed. He devised what was probably the first citizen science project when he requested that the “curious” of the country that were along the path of totality observe “what they could” and make a note of the time and duration of totality from their location. When the observations came in, Halley was able to calculate that the shadow swept over the Earth at a staggering 2,800 km per hour. Find out more about Halley’s helpers and the citizen science project in this Science Stories film.