Forty Students, One Unfiltered Telescope and a 78 Per Cent Eclipse
A secondary school in Nepalgunj plans to let 40 Grade 8 students watch a partial solar eclipse whose maximum obscuration at that place will be 78 per cent at 10:12 a.m., with the event lasting 2 hours 40 minutes. The school owns a 60 mm refracting telescope donated by an alumnus; it has no solar filter, and the small finder scope cannot be removed quickly. The teacher has brought one sheet of exposed X-ray film, a piece of smoked glass, and a smartphone. The plan is to point the telescope at the Sun and let each student look through the eyepiece for five seconds, holding the X-ray film in front of the eyepiece as a 'filter'. A parent has also told the class that food cooked during an eclipse spoils and that pregnant women should stay indoors.
1. Assess the teacher's plan and explain the physical reason it is dangerous.
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The plan must not be carried out.
The retina has no pain receptors. Damage from focused sunlight is not felt at the moment it happens; the student notices a permanent blind spot in the centre of vision hours later. The 60 mm objective collects light over roughly 2,800 mm², hundreds of times the area of the dark-adapted pupil, and the eyepiece concentrates that into a tiny image on the retina. A few seconds is enough.
The filters offered do not protect. Developed X-ray film and smoked glass are not rated for solar viewing. They may look dark in visible light but transmit infrared radiation, which is what causes the thermal burn, and they can crack in the concentrated beam.
The eclipse does not help. At 78 per cent obscuration the uncovered part of the Sun is as bright as ever; the danger is greatest when the Sun looks dimmest, because the pupil dilates.
The finder scope is a second, unfiltered aperture aimed at the Sun - a classic cause of injury. Correct practice is a certified solar filter over the objective, or projection, and never a filter at the eyepiece.
2. Design a safe programme for 40 students within the 2 hours 40 minutes, using only what is available.
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Use projection only. Nobody looks at the Sun and nobody looks through the telescope.
- Pinhole projectors station - each group of five makes a 1–2 mm hole in a card and projects the Sun's image onto white paper or the inside of a carton. The crescent shape visibly changes as obscuration grows; two cards held one behind the other demonstrate the same effect and keep hands busy.
- Telescope projection station - cap the finder, aim the tube by making its shadow as small as possible, and let the eyepiece throw the image onto a white card held 20–30 cm behind it. Nobody stands at the eyepiece. The smartphone photographs the projected image, never the Sun itself.
- Rotation - eight groups of five, 15 minutes per station, all in shade, with water available and a teacher at each station.
- Record and observe - air temperature, light level, sharpness of shadows, and the crescent images formed under a leafy tree.
- Whole-school briefing so that no other student looks up, and a clear statement that even at 78 per cent the Sun is never safe to view directly.
3. Respond to the two statements the parent made, and answer a student's question: why is there not a solar eclipse at every new moon?
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On food. There is no physical mechanism. The Moon's shadow does not alter the chemistry of cooked food. Food spoils because of time and temperature, and an eclipse changes neither; any illness after an 'eclipse meal' is ordinary food poisoning, and the correct prevention is cooking, covering and refrigeration as on any other day.
On pregnancy. The eclipse poses no special risk to a pregnant woman or a fetus. The only hazard is the same eye damage that faces anyone who looks at the Sun, plus practical risks from crowding in a dark place, and that is easily avoided.
Why not every new moon? The Moon's orbital plane is inclined by about 5° to the plane of Earth's orbit. At most new moons the Moon passes slightly above or below the Sun as seen from Earth, so its shadow misses us. A solar eclipse occurs only when the new moon happens near one of the two nodes where the orbital planes cross, and the alignment is close enough for the Moon's shadow to fall on Earth. That is why there are only a few solar eclipses a year, and why a total one is rare at any particular place.