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Let's observe

Giorgio Carboni, July 2009
Translated by Sarah Pogue

A couple of eclipses

Figure 1 – How to project an eclipse onto a wall. In this way, my
mother and I observed the total eclipse of the 15th of February 1961.
Information on that eclipse



Eclipses are spectacular astronomical events. Unfortunately, they are often observed using unsatisfactory methods. In fact, many people use a bucket of water to observe the attenuated reflection of the Sun’s image, but the image is too small and it is sufficient that somebody runs nearby or a breath of air to cause ripples on the surface of the water. Even if the water surface reflects only 5% of the incident light, the Sun’s reflection remains very bright and in the end you are half-blinded. With smoked glasses, the light of the Sun is effectively weakened, but the limitation of the small size of the Sun’s image remains. For those who have a pair of binoculars or a telescope, it is better to not even try to look at an eclipse in order to not burn their retinas. In fact, these instruments collect much more light than that which passes through the pupil of the eye. To use a telescope, you must buy a suitable filter. In any case, at the moment of totality it is difficult to avoid the hurly burly to find a place to observe. What I am about to describe is a way to observe eclipses that allows you to overcome many drawbacks. It certainly isn’t an invention, but many people don’t know of it.


If the solar eclipses cannot be observed directly with a telescope because there is a risk of burning the retinas of your eyes, there is however the possibility of using this instrument to project the image of the eclipse onto the wall of a room and to allow your family, neighbours and friends to enjoy the show.

Mount the telescope on a tripod and, without looking into the eyepiece, point it towards the Sun. This will not be easy to do, but when the unfocused image of our splendid star appears on the floor, put it into focus. At this point, you will have a rather small image of the Sun. In order to better observe it, you must project it onto a wall at least a few metres away where it will appear much larger.

To do this, the telescope must be equipped with a prism that allows light to exit at an angle of 90° with respect to the main tube (figure 2). If your telescope does not have a prism, you must procure one. This is a Porro prism, like those found in ordinary binoculars. If you don’t have one at home, you can buy one at low cost at flea market stalls, or, again at low cost, you could buy a pair of normal binoculars and take them apart to obtain the 4 Porro prisms they contain (for this purpose binoculars with straight tubes are to be avoided). In astronomy fairs you can find tubes at 90° equipped with prisms. Alternatively, you can go to an optics store. Mount the prism on the eyepiece using any materials available such as sticky tape etc. Avoid sticking the sticky tape to any useful surfaces (cathetus and hypotenuse of the prism), but stick it only to the sides.


Figure 2 – Telescope pointed towards the Sun. Note that the
eyepiece makes an angle of 90° with respect to the main tube.

Figure 3 – The image of the Sun can be
projected onto a wall or onto white card.


Point the telescope in the right direction until you have an image of the Sun on a wall which is quite far away. In a room, you should be able to obtain an image more than half a metre wide. The position of the Sun, the orientation of the room and the presence of furniture can prevent you from projecting the image of the Sun onto a wall. In this case, project the image onto a white surface (figure 3) placed on an easel or the back of a chair. Focus the image.

The light entering through the window lowers the contrast of the image. To solve this problem, hammer two nails into the blind rail and to these fix a black cloth to cover the entire window. Now pass the telescope under the cover in such a way that the objective sees the Sun and the rest of the telescope is in the room, in the shade (figure 1).

As the Earth rotates on its axis, the Sun moves across the sky and every so often it is necessary to adjust the position of the telescope.

When there is an eclipse you can call your family members, neighbours and friends to observe the show. During a total eclipse, at the moment of the concealment of the Sun, the room will be plunged into darkness and the disk of the Moon will appear black in a blue sky. At the edges of the disk of the eclipse it is often possible to observe orange-coloured solar protuberances.

Partial eclipses are also beautiful to see. This experience is also interesting in the absence of an eclipse. It permits you to observe sunspots, you can see clouds in the sky and swallows in flight. The clouds that pass in front of the Sun are particularly striking to watch.


As a boy, I was very interested in astronomy and microscopy. I also liked to construct optical instruments. It was in this way that I arrived at the by now distant eclipse of the 15th of February 1961 with a telescope at my disposal. It was about 9:30 in the morning and the eclipse was about to begin. I knew that I couldn’t observe the event directly with the telescope as it would have burned my eyes. I didn’t have a suitable dark filter, therefore I tried to project the eclipse onto a wall of the living room. I began by placing a dark cover over the window, fixing it to the blind rail with two nails. I placed the telescope mounted on a tripod under the cover. After pointing it towards the Sun, I managed to project the image of our star onto the wall. I focused the image, which obviously also contained the Moon. At that point, the two celestial bodies were almost completely superimposed. The image of the Sun on the wall had a diameter of approximately half a metre, while the field of the image on the wall was approximately two metres in diameter. The show was amazing. There were clouds in the sky and one of these partly entered into the image field and I was worried that this would have hidden the eclipse but fortunately this was not the case. I saw birds in flight. When the eclipse was almost total, I called my mother who was left speechless. When the last segment of Sun disappeared behind the Moon, the room became even darker. The sky became deeper and bluer. The images of the clouds and the birds were sharper. On one side of the eclipse, a jet of solar plasma bright orange in colour stretched out before falling back again on the Sun. On almost the opposite side another smaller jet which didn’t fall back was visible. The blue sky, the clouds, the birds, the celestial bodies... everything was clear and amazing (figure 1). As soon as a small portion of the Sun emerged, the room brightened up again and we recovered from the amazement.

Many years later, in May 1984 and during another solar eclipse, I organised a "screening" like the one just described. This time I knew that I would obtain a spectacle that many would enjoy. So, I called my wife, my daughter and a couple of neighbours. Unfortunately, this eclipse was partial and the show was not comparable to that of 23 years before. Nevertheless, it was interesting enough to make someone run outside to call in other people, such that in the end there was a crowd of people, amongst which there were also some strangers. I had to project the eclipse onto the ceiling and by means of a ladder people took it in turns to move closer to the image to see it better. It was almost sunset, the Sun was low on the horizon, birds were particularly abundant and from their size it was possible to guess the distance. Like the previous time, every so often I had to correct the position of the telescope because the image of the eclipse tended to exit from the field. At a certain point, after once again regulating the position of the instrument, the trees on the hill opposite “entered” into the Sun. This was a group of cypress trees that concluded the event in a very striking manner.


With this system, you can also observe other celestial phenomena such as, for example, the transit of Venus across the Sun. An event of this type occurred on the 8th of June 2004. That morning, careless as usual or still not completely awake, I arrived in the office without remembering about the celestial phenomenon that was about to begin and thus without bringing with me the necessary instruments to organise a screening of the event. When a colleague reminded me I panicked because I thought that I wouldn’t be able to show the passage of Venus across the Sun. Fortunately, in the office I had a Porro prism while in the car I had a small pair of binoculars with straight tubes that I always keep in case of necessity or to observe the scenery on mountain roads. In the office I also had a small spare tripod. Thus, using a bit of sticky tape I fixed the binoculars to the tripod and the prism to the binoculars (figure 4). It was in this way that I succeeded in projecting the image of the transit of Venus in front of the Sun (figure 5). Many colleagues arrived to admire the event and this also proved to be the occasion for astronomical reflections.


Figure 4 – Binoculars mounted on a tripod and a prism
mounted on the eyepiece by means of sticky tape.

Figure 5 – The author of the article indicates the planet Venus
during its transit across the Sun on the 8th of June 2004.


On the wall it was possible to compare the dimensions of the Sun (approximately half a metre) with those of Venus (approximately 14 mm) and of the Earth which is a little larger than Venus. The Sun appeared enormous in size (figure 5). Since the Earth rotates approximately 150 million kilometres from the Sun and Venus at approximately 108 million kilometres, if Venus was at the same distance as the Sun what size would it have been on the wall? Are 4.3 mm ok? And for the Earth are 4.6 mm enough?


The references that follow will serve to let you know when there are new solar eclipses, transits of Venus or Mercury and to provide information on past events. I am not sure if it is possible to see the transit of Mercury across the Sun with this technique because this planet is rather small. If the image of the Sun on the wall was a half a metre in diameter, Mercury would measure 2.4 mm, taking into account the fact that it orbits at approximately 58 million kilometres from the Sun.  Past and future solar eclipses (from NASA) General information on the transit of Venus across the Sun. Past and future transits. Transits of Mercury across the Sun.

Don’t miss the transit of Venus on the 5-6 June 2012, visible from many locations of the world. It will be best visible in its entirety from Hawaii, Alaska, Australia, the Pacific and eastern Asia.

The next transits of Venus across the Sun will occur more than a century from now in December 2117 and December 2125.

Finally, the next transits of Mercury across the Sun are expected for the 9th of May 2016 and the 11th November 2019. Recent and future transits of Mercury.


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