13 September 2015 partial solar eclipse

The partial solar eclipse taking place on Sunday 13 September will be visible from southern Africa (southwards from southern Angola, Zambia, Malawi, southern Mocambique and southern Madagascar), the southern Atlantic and Indian Oceans, and Antarctica. As it will be easier to observe the eclipse the closer to the South Pole one is, the far south of Africa will be the optimal observation area. NB Never look directly at the Sun unless using special filters. The other way to observe the eclipse is indirectly, using binoculars or a small telescope to reflect the image onto white paper.

Length of the eclipse

In total, the eclipse will last approximately 4½ hours (04.41 – 09.06 UT, with the maximum eclipse taking place at 06.54UT). However, from any place the duration of the eclipse will be somewhat shorter. Maximum coverage in Cape Town and Hermanus will take place at 07.43 local time. Because of the time of year, sunrise will occur in the SW corner of the Western Cape after the eclipse has started, so it will not be possible to observe the point at which the Moon’s shadow starts to cross the Sun. However, after sunrise, the remainder of the eclipse will be visible, with maximum taking place quite low towards the east (10° altitude for Cape Town). The eclipse will end around 11 am local time. Maximum coverage of the Sun, closest to the centre of the eclipse ie closer to the South Pole will be 79%. In Africa, the south coast of South Africa will, thus, have largest eclipse coverage. For Cape Town (and Hermanus) it will be around 43% at maximum (26% in Johannesburg and only 0.07% in Harare). What is a solar eclipse? A solar eclipse occurs when the Moon passes between Earth and the Sun. A partial solar eclipse occurs in areas near Earth’s polar regions. when the centre of the Moon’s shadow misses Earth.

Mechanism of a solar eclipse

The Moon’s shadow during a solar eclipse has two parts. The umbra is the darker, narrow inner part of the shadow. Although it can rarely be as wide as a couple of hundred kilometres, it is usually only a few kilometres wide. It is only if an observer is in the path of the narrow umbra that a total eclipse can be seen. In the larger surrounding penumbra, which can be several thousand kilometres across. a partial eclipse will be visible. Partial eclipses, like the 13 September one, are also the only kind to be seen when the eclipse occurs over areas near Earth’s polar regions when the centre of the umbra misses Earth entirely. There are three types of solar eclipse: During a total eclipse, the Moon’s shadow completely covers the disc of the Sun (totality usually lasting only a brief 3-4 of minutes). In addition to this brevity, the likelihood of observing a total eclipse is very small as the Moon’s shadow falls on only a very narrow strip of Earth as it tracks across the rotating planet. Partial eclipses involve darkening of only a limited area of the Sun. They only produce a visible weakening of the strength of sunlight unless their magnitude (the fraction of the Sun’s diameter which is covered by the Moon) exceeds around 0.7 (70%). Annular eclipses are a consequence of the fact that the Moon’s orbit is slightly elliptical. When a total solar eclipse occurs when the Moon is in apogee (furthest away from Earth), the lunar diameter appears to be smaller than that of the Sun and a ring (annulus) of the solar disk remains visible at mid-eclipse. Annual eclipses can last up to 12½ minutes. As with a normal total eclipse, an annular eclipse has a narrow umbra and a wide penumbra which produces a partial eclipse.

Solar eclipse mapArea of Sun covered by the eclipse. Dark areas – up to 70% coverage. Light areas – up to 45% coverage.

 Times of solar eclipses

Solar eclipses occur within a number of cycles and/or points in cycles. For example, they always occur at new moon. Although new moons occur monthly, it is only at certain times that the positions of Earth and the Moon are lined up in a way which will cause an eclipse. The two points at which orbits like those of the Moon and the Sun intersect and an eclipse may occur is called a node. Solar eclipses occur in pairs (semester series) approximately every 177 days and four hours apart at the alternating nodes of the Moon’s orbit. The other part of the 13 September pair took place on 20 March, which was a total eclipse. All or, more likely, part of the eclipse was visible from Greenland, Iceland, Europe, north Africa, the Middle East and north-western Asia). The Moon has a 19 year cycle, the Metonic cycle. It takes this number of years to repeat the overall pattern of its motions. Because of the link between the lunar and solar orbits is fundamental to the occurrence of solar eclipses, solar eclipses also follow an almost identical 19 year cycle. There is also a relationship between the occurrence of lunar and solar eclipses. More lunar eclipses occur in one year than solar eclipses, but, whenever there is a solar eclipse, there is always a lunar eclipse close to two weeks later. Thus, there is a full lunar eclipse on 28 September 2015 (visible in the Western Cape from 02.11 – 07.22, with maximum eclipse occurring at 04.47). Just to prove how complex the cycles which produce eclipses can be, a solar eclipse can, very rarely, occur one fortnight before and after a solar eclipse. This will next happen in July and August 2018). Similarly, and equally rarely, a lunar eclipse can occur a fortnight before as well as after a solar eclipse. This will next take place in June and July 2020.

Sources: ASSA 2015 Sky Guide Africa south, Ridpath, I (Ed) 2012 Oxford dictionary of astronomy 2nd ed revised, www.en.wikipedia.org, www.earthsky.org, www.moonblink.info