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Building a Camera Obscura — Projecting the Sun and Eclipses Safely
A camera obscura (Latin for 'dark chamber') is the simplest optical device possible: a dark box with a small hole in one wall. Light from an outside scene passes through the pinhole and projects an inverted image on the opposite wall. The principle was described by the Chinese philosopher Mozi in the 5th century BC and by Aristotle, and was used extensively by the Arab polymath Ibn al-Haytham (Alhazen) around 1020 AD in his groundbreaking Book of Optics. For astronomers, the camera obscura serves a critical purpose: it projects an image of the Sun onto a screen inside the box, allowing safe observation of sunspots and solar eclipses without looking at the Sun directly. Galileo used a camera obscura to study sunspots, and astronomers throughout history have used them to time the phases of eclipses. This blueprint builds a portable camera obscura from a wooden box, suitable for projecting sunspot detail and safely observing solar eclipses.
初学者
2-4 hours
说明
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Understand the pinhole principle
Understand the pinhole principle
Light travels in straight lines. When sunlight passes through a small hole in an opaque barrier, rays from different parts of the Sun pass through the hole at slightly different angles and project an inverted image of the Sun on any surface behind the hole. The image is dim but sharp — the smaller the hole, the sharper the image (up to a point, where diffraction begins to blur it). The optimal pinhole diameter depends on the distance from the hole to the screen: for a screen 50 cm from the hole, a pinhole of about 0.5-1.0 mm gives the best balance of brightness and sharpness.
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Build the box
Build the box
Build or find a sturdy box about 30-60 cm long, 20-30 cm wide, and 20-30 cm tall. A longer box projects a larger (but dimmer) image. The box must be completely light-tight — seal every joint and seam with black tape. One end of the box will hold the pinhole; the opposite end (the screen wall) will be the viewing end. Paint the entire interior flat black to prevent stray light reflections from washing out the projected image.
此步骤所需材料：
Hardwood Block1 个所需工具：
Hand Saw3
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Make the pinhole plate
Make the pinhole plate
Cut a square hole about 3 cm across in the centre of the front wall. Cover this hole with a piece of thin brass sheet or heavy aluminium foil, taped firmly in place. Using a fine sewing needle, pierce a single clean hole through the centre of the metal. The hole should be as round and clean-edged as possible — ragged edges cause a fuzzy image. For the sharpest pinhole, place the foil on a hard surface and push the needle through with a gentle twisting motion. The resulting hole will be about 0.3-0.5 mm in diameter.
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Install the viewing screen
Install the viewing screen
On the inside of the back wall (opposite the pinhole), mount a sheet of white paper or card — this is the projection screen where the Sun's image will appear. The paper should be smooth and flat. Alternatively, replace the back wall entirely with a sheet of translucent paper (tracing paper or greaseproof paper) stretched over a frame — this allows you to view the projected image from behind, which is more comfortable than looking inside the box.
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Add a viewing port
Add a viewing port
If using an internal white screen (not a translucent back wall), cut a small viewing hole in the side or top of the box near the screen end, just large enough to look inside and see the projected image. Shield this hole with a flap of card to block ambient light when not viewing. The viewing port must be positioned so your head does not block the sunlight entering the pinhole.
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Project the Sun
Project the Sun
Point the pinhole end directly at the Sun. NEVER look through the pinhole at the Sun — the camera obscura is safe precisely because you look at the projected image, not at the Sun itself. When properly aimed, a bright circular disc of light appears on the screen — this is the image of the Sun. The image will be about 5 mm in diameter for every 50 cm of box length (the Sun subtends about 0.5 degrees). Adjust the aim until the disc is centred and as bright as possible.
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Observe sunspots
Observe sunspots
With the Sun's image projected on the screen, look carefully for small dark spots on the bright disc — these are sunspots, regions of intense magnetic activity that are cooler (and therefore darker) than the surrounding solar surface. Large sunspot groups are visible even in a simple camera obscura. Trace the disc and the sunspot positions on paper each day — over a week, you will see the spots migrate across the disc as the Sun rotates (it completes one rotation in about 27 days). Galileo used exactly this technique to prove that the Sun rotates.
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Upgrade with a lens for a brighter image
Upgrade with a lens for a brighter image
To produce a much brighter and sharper image, replace the pinhole with a convex lens. A lens gathers far more light than a pinhole, projecting a brilliant image of the Sun. Mount a convex lens (focal length 30-50 cm) in a hole cut in the front wall. Adjust the box length or add a sliding extension until the screen sits at the lens's focal distance — the image will snap into sharp focus. Caution: a lens-based camera obscura concentrates enough solar energy to scorch paper. Never place anything flammable at the focal point.
此步骤所需材料：
Convex Lens1 个9
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Observe a solar eclipse
Observe a solar eclipse
During a solar eclipse, the camera obscura shows the Moon's disc slowly crossing the Sun's disc in perfect safety. Set up the camera obscura well before the eclipse begins and project the Sun's image onto the screen. As the Moon begins to cover the Sun, you will see a dark bite appear on one edge of the projected disc and grow over the course of an hour. During a total eclipse, the brilliant disc shrinks to a thin crescent and finally disappears entirely — and the projected image goes dark. This is the safest and simplest way to observe a solar eclipse, and it has been used by astronomers for over a thousand years.
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Record and measure
Record and measure
To make scientific observations: tape a sheet of paper on the screen and trace the Sun's disc at regular intervals during an eclipse, noting the time for each tracing. Between eclipses, trace sunspot positions daily. Measure the diameter of the projected Sun disc and use the known angular size of the Sun (about 0.53 degrees) to calibrate your instrument — the ratio of image size to box length gives you the angular scale in degrees per millimetre. You are now performing observational solar astronomy exactly as it has been done since Ibn al-Haytham first described the camera obscura principle in 1020 AD.
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