Saturday, 12 September 2015

Partial Solar Eclipse to be visible from Southern Africa, Madagascar and Antarctica on 13 September 2015.

A partial Solar Eclipse will occur on Sunday 13 September 2015, visible from all of Namibia, Botswana, South Africa, Lesotho, Swaziland, Zimbabwe and Reunion Island and parts of Angola, Zambia, Mozambique, Madagascar and Antarctica. The eclipse will occur between 4.41 am and 9.06 am GMT.

The area over which the 13 September 2015 partial Solar Eclipse will be visible. Areas in darker grey will be able to observe the entire eclipse, in the lighter grey areas the eclipse will either begin before sunrise or end after sunset, so only part of the event will be visible. HMNautical Almanac Office.

Eclipses are a product of the way the Earth, Moon and Sun move about one-another. The Moon orbits the Earth every 28 days, while the Earth orbits the Sun every 365 days, and because the two Sun and Moon appear roughly the same size when seen from Earth, it is quite possible for the Moon to block out the light of the Sun. At first sight this would seem likely to happen every month at the New Moon, when the Moon is on the same side of the Earth as the Sun, and therefore invisible (the Moon produced no light of its own, when we see the Moon we are seeing reflected sunlight, but this can only happen when we can see parts of the Moon illuminated by the Sun). 

The relative positions of the Sun, Moon and Earth during a Solar eclipse. Starry Night.

However the Moon does not orbit in quite the same plane as the Earth orbits the Sun, so the Eclipses only occur when the two orbital planes cross one-another; this typically happens two or three times a year, and always at the New Moon. During Total Eclipses the Moon entirely blocks the light of the Sun, however most Eclipses are Partial, the Moon only partially blocks the light of the Sun.

How the differing inclinations of the Earth and Moon's orbits prevent us having an eclipse every 28 days. Starry Skies.

Although the light of the Sun is reduced during an Eclipse, it is still extremely dangerous to look directly at the Sun.

Animation showing the shadow of the Moon at five minute intervals on Friday 20 March 2015. Andrew Sinclair/HM Nautical Almanac.

See also...

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The June (or Northern) Solstice falls on Sunday 21 June in 2015, the day on which the Sun rises highest in the sky and the longest day of the year in the Northern Hemisphere (where it is the Summer Solstice) and...

A total Lunar Eclipse will occur on a April 2015, starting at about 9.00 am GMT. It will be visible across much of the Pacific, as well as most of Alaska, the Russian Far East, eastern Japan and eastern Australia. Part of...

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