Annular Eclipse of the Sun - May 31, 2003
(note: all times in GMT - add one hour for BST)

There will be an eclipse of the Sun commencing in the early hours of 31st May. The circumstances of the eclipse with the Moon close to apogee (greatest distance from Earth) means that at mid-eclipse the Moon will cover only a portion of the Sun.

Such eclipses are referred to as "annular" (from "ring-shape") since the visible portion of the Sun at mid-eclipse appears as a bright ring or circle of intense light. From Orkney at the present eclipse the form of this ring will be asymmetric, the visible portion of the Sun appearing as a crescent (dia 1).

At the time of sunrise in Orkney (0311 GMT) the eclipse will have been underway 17 minutes (dia 2). Mid-eclipse occurs at 03 46.5 GMT, when the altitude above the horizon of the Sun and Moon will be low at 3 degrees. The eclipse ends at 04 44, by which time the Sun will be nearly 9 degrees above the eastern horizon.

Annular eclipses are not as important nor are they as interesting as a total eclipse, in which the entire surface of the Sun is hidden for a short time.

On May 31 only 88.77% of the Sun will be obscured by the Moon at mid-eclipse; the residual 11.23% will produce considerably more light than many, many full Moons*. Nevertheless, more of the Sun will be hidden from view than we experienced here in Orkney during the great total eclipse which brushed southern Britain in August 1999.

*The light from the Sun is close on half a million times that of the full Moon. However, since the Sun's light falls off close to the limb, the proportion by which the overall brightness is diminished at an annular eclipse is not simply the ratio of the visible area against the entire area of the full, visible disc. Although only a relatively small portion of the Sun will be visible, observers should not attempt to view the event direct. The projection method as outlined in previous articles should be adopted. For safety reasons this will be emphasized and repeated whenever observations involving the Sun are mentioned.

However, because at the time of rising the Sun will be in eclipse, there is the possibility that mist or cloud will make observing with the unaided eye quite safe at this stage. Use discretion in such circumstances but always err on the side of safety.

Dia. 1 Mid-eclipse 03h 46.5m GMT

Dia. 2 Phase of the eclipse at sunrise, 03h 11m GMT

Techniques for observing an eclipse.

Observing an eclipse of the Sun should be approached with caution. To look directly at the Sun, even when most of the disc is obscured, can be very damaging to the eyes and should not be attempted.

The use of smoked, plain glass through which to observe the Sun direct cannot be recommended. It was in fact the method by which I was introduced to the subject as a child by those who knew no better. The soot can so easily be removed by abrasion that the risks involved are simply not worth it.

The use of filters made for the purpose should also be checked by an expert. In no circumstances apply such filters to a binocular or telescope.

A marine sextant, equipped as it is with proper filters, provides a safe and useful way for observing the Sun in the hands of an experienced observer. The projection method is recommended since it is not only safer when properly set up but allows several observers to watch the event collectively.

Dia 3. Primary azimuthal points of compass: Azimuth of sunrise (0311 GMT) on 31 May is 41.5 Note: Magnetic north is approximately 5.5 degrees west of geographical north in 2003. When using Ordnance Survey maps also allow for grid displacement from the meridian by referring to the notes accompanying the map - Landranger 6 Orkney Mainland is recommended.

Pinhole method.

Since the Sun will be at a low elevation throughout the eclipse, it should be possible to us the "pinhole" method from an eastfacing room. Not only is this a cheap way round the problem but it also entails the least amount of manoeuvring of apparatus.

If you have a white wall opposite the window so much the better. The object is to screen the window off (thick curtains will help) allowing the Sun's light to pass through a hole of about 1 cm diameter.

Use a large sheet of white card or paper at a distance of about 2 metres from the hole. This should give an image of around 5 cm (2 inches) diameter. If a wall at a greater distance can be used then the image will be larger but fainter.

Another method (below) will give an even larger image of the Sun.

Observing the eclipse safely using a mirror
and piece of card

This is a simple but effective way to observe the sun using very basic equipment. It again works on the principle of the pinhole camera. All you need is a good, small plain mirror (say about 3 inches x 4 inches), and piece of card thick enough to prevent sunlight from passing through it.

Cut a small, central hole of about ½ inch in diameter. To give a sharp image the hole should not be any larger than this; much smaller and your image will be too faint. You can Sellotape the card over the face of the mirror. Of course, all the reflecting surface must be covered except in the region of the hole.

Fig 1. Observing with "pinhole" mirror - not to scale

The diagram (Fig. 1) shows the arrangement. With the distance between the mirror and the inside wall at 50 feet, the diameter of the sun's image on the wall will be a little under 6 inches, ample to see the eclipse. The device may be used at other times to view sunspots.

You will need to set the thing up in advance of the eclipse to make sure you have full control when moving the mirror to keep the image on the wall or screen. It is not too difficult (I have used a clothes peg to clamp the mirror to a camera tripod).

The image will remain in position long enough for the operator to go indoors and enjoy the eclipse on the wall: better, have a companion and take it in turns.

Other methods of projection.

If you have a pair of binoculars or telescope, then you can get a much larger and clearer image by projecting and focusing an image onto a nearer surface (suitably screened from direct sunlight) from the eyepiece.

Fig. 2 Using a small telescope to project the Sun’s image

When using a binocular, cover one of the main lenses so that the two images do not overlap.

DO NOT ALLOW CHILDREN TO CROSS THE LIGHT BEAM IN THE SET-UP ILLUSTRATED BELOW. DO NOT OBSERVE THE REFLECTED LIGHT BEAM DIRECT. NEVER OBSERVE THE SUN WITH A TELESCOPE OR BINOCULAR DIRECT.

Other considerations.

In general one is not particularly conscious of any change in the Moon's apparent diameter as seen from Earth during the course of a lunar month. This change is significant, however. Whereas the Sun's apparent diameter ranges between only 31'.47 (early July) and 32'.63 (early January), the Moon varies between 29'.73 and 33'.5. This is because the Moon's path about the Earth is more eccentric than is the orbit of the Earth about the Sun.

When a full Moon occurs close to apogee its small size is readily apparent. So also in the case of the present eclipse the "smallness" of the Moon will be obvious: an eclipse is, in fact, the only chance that one has of seeing the Moon at zero phase; and this raises an interesting point.

When the Moon is eclipsing the Sun there can be no light reaching it from the Sun that we see other than that reflected back from the Earth (for at this phase of the Moon the Earth appears "full" from our natural satellite). But this earth-shine as it is called (and readily seen when the Moon is a thin crescent - referred to in the case of a new Moon as the old Moon in the new Moon's arms) could not possibly contribute to our seeing the Moon at eclipse. No, what we are observing is in fact a black object against a brilliant background (the Sun itself). Incidentally, this is not the same as when one observes the Gallilean satellites of Jupiter in shadow transit.

Many will have observed the Moon at various phases during daylight, usually quite by chance. Hunting the new or old Moon in twilight has led to some interesting claims that we will not go into at this time. However, the smallest phase at which the Moon may be seen with the unaided eye in full daylight has received less attention.

Many remark on the Moon's apparent "largeness" when viewed close to the horizon but fewer comment on the smallness of a full Moon at high altitude when the Moon is close to apogee. The former is purely a psychological "trick" on the senses. [We ignore here the limiting effects of refraction at moonrise (and sunrise)]. This may also be likened to the perceived difference when one looks up as against looking down.

Climbers of the "Old Man of Hoy" will know the difference between looking up at the commencement of the climb compared to looking down from a position well short of the top! As a child in London I had the same sensation when taken to the "Monument": people in the cage at the top looked small enough from the ground, but from the top looking down one had a quite different impression of size and sense of height.

The full Moon in summer occupies the same sections of the zodiac followed by the Sun in winter, and vice versa. From Orkney at the summer solstice the Sun's altitude at local noon is only 54.4º. At this time of year a full Moon will languish in the constellation Sagittarius where it may rise no higher than 15º above the horizon.

A full Moon is most apparent, therefore, in the winter months when it spends more than twelve hours above the horizon. It is interesting to compare incidents of full moon when close to apogee or perigee especially in the winter months when this event takes place with the Moon comparatively high in the sky.

The full Moon in December 2003 occurs 32 hours following apogee and should be seen as a typical "small Moon".

JV