ASSA Solar Section

Extracted from the ASSA Sky Guide 2004 -
used with permission from the editor, Auke Slotegraaf.

The Sun

At over 1.4 million kilometers wide, the Sun contains 99.86 percent of the mass of the entire solar system: well over a million Earths could fit inside its bulk. The total energy radiated by the Sun averages 383 billion trillion kilowatts, the equivalent of the energy generated by 100 billion tons of TNT exploding each second.
But the energy released by the Sun is not always constant. Close inspection of the Sun's surface reveals a turbulent tangle of magnetic fields and boiling arc-shaped clouds of hot plasma dappled by dark, roving sunspots
Observing the Sun

WARNING: Never look at the Sun without proper protection - permanent blindness can result from the shortest look through binoculars or a telescope

An easy and safe way of observing the Sun is by projecting its image through a pinhole onto a darkened screen. Take a cardboard box, remove one side, and tape a white sheet of paper on the inside of one end. Poke a small hole in the opposite end, cover it with aluminium foil and make a pin-hole in the foil. Stand with your back to the Sun and look at the sheet of paper, while pointing the box at the Sun. A large image of the solar disk will be projected inside the box. If the image is too faint, enlarge the hole. If the hole is too large, the image will be unclear.
Careful examination of the solar disk should show sunspots, usually grouped together. Sunspots come and go in a roughly 11-year cycle. Maximum was reached in 2000 and the next minimum will occur around 2007. Each spot lasts a few days or weeks. Solar observers keep a careful count of the daily number of sunspots, and make drawings showing the distribution of sunspots on the solar disk. This method of solar observing also works well for viewing the progress of an eclipse, or the passage of Mercury or Venus during a transit (see pp42-43 for details of the transit of Venus on June 08).
A brighter, sharper image can be produced if a telescope (or half a binocular) is used to project the image onto a white sheet of paper. Keep in mind that the concentrated heat can damage eyepieces, so allow the optics to cool down periodically. It's also a good idea to reduce the diameter of the lens by a piece of paper or cardboard with a round hole in it, so that the eyepiece is not damaged by the intense light passing through it.
To view sunspots, faculae and granulation safely through a telescope, you will need a solar filter that will dim its light by a factor of about one million. Only filters designed for solar viewing should be used - other filters, no matter how dark, generally transmit too much infrared which can damage the retina . Filters are usually made of glass or plastic coated with metal. Mylar, an aluminised plastic, can often be bought very cheaply, and if properly mounted, works well. The filter must be located in front of the telescope.
To view solar flares, prominences and elaborate surface detail, you will need a special hydrogen-alpha (Ha) filter.
In addition to visual observing, electronic devices can be used to monitor solar activity. A simple VLF radio receiver can be used to monitor the strength of signals received from distant transmitters. When a solar flare erupts, signal strength increases abruptly in an event known as a sudden ionospheric disturbance (SID). Such flares sometimes disrupt the Earth's magnetic field causing a magnetic storm that can be recorded with a magnetograph.


Two eclipses of the Sun occur in 2004. The partial eclipse on April 19 is visible in Southern Africa, as well as Antarctica and Madagascar. Local details are given in Table 1 below.
The partial eclipse on October 14 is not visible from Southern Africa (maximum eclipse at 04:59), but can be seen from north-east Asia, Japan, the Hawaiian Islands, and the western part of Alaska.
A solar eclipse occurs when the Moon blocks sunlight that normally falls on Earth and can only occur at the time of New Moon. Because the plane of the Moon's orbit is tilted slightly to the ecliptic, in most months the Sun-Moon-Earth alignment is not exact and the Moon's shadow misses the Earth. When the alignment is correct, the Sun is eclipsed. Each year, between two and five solar eclipses can occur.
Directly under the Moon, the eclipse is either total or annular, depending on the distance to the Moon. When the Moon is at its farthest from Earth, it does not completely cover the solar disk, leaving a ring of sunlight visible. This is an annular eclipse (Latin annulus, 'ring'). For thousands of kilometres to either side, the eclipse is partial.
From any given location, there will be a partial eclipse every couple of years, an annular eclipse once every 224 years, and a total solar eclipse once in 375 years.

Solar telescopes in South Africa

At the SAAO's observing site in Sutherland, the University of Birmingham (UK) operates an automated telescope that monitors low-degree solar oscillations. This is one of six automatic stations distributed globally, giving 24-hour coverage of the Sun. A better understanding of these solar oscillations will help explain the origin of the solar cycle, which may influence climate changes on Earth.
The 4-inch heliographic telescope at SAAO in Cape Town, erected in 1876, is used to demonstrate sunspots.
At Boyden Observatory near Bloemfontein, there is a 20-cm coelostat with instrumentation for making narrow-band Ha observations.

Go to the ASSA Bloemfontein Solar Activities webpage