This article was taken from the March 2015 issue of WIRED magazine. Be the first to read WIRED's articles in print before they're posted online, and get your hands on loads of additional content by subscribing online.
When teaching astronomy, California-based science teacher John Krieger wanted to show his students the stars. "In the classroom, we're stuck indoors and it's daytime, so I have to try to describe things," he says. Yet the standard displays at commercial planetariums lacked the educational content he was looking for. So Krieger built his own. "You have complete control over the sky, which is a wonderful thing for a teacher," he explains. Here's how you can capture the cosmos for around £300.
What you need
- 55m2 of corrugated cardboard
- 400 bulldog clips
- String and a ruler
- Black and white water-based paint
- Digital projector
- Half-dome security mirror
- Stellarium (open-source star-projector software)
Draw the template
Draw a line 113cm long, 2.5cm in from the edge of the cardboard. "Use the compass-and-ruler technique," says Krieger. "But, instead of a compass, use string." Tie a pencil at one end, measure the string out 113cm and pin the other end on one side of your line. "Stretch it to where the top of the triangle should be, then sling the pencil back and forth in an arc. Repeat from the other side of the line, so the two arcs cross at the triangle's vertex."
Cut out all the pieces
Cut out the triangle leaving 2.5cm around the sides. Trace around this template to make nine more equilateral triangles.
Repeat with another 113cm line, but this time with 100cm of string to make an isosceles triangle -- you'll need 30 of them. Cut ten 100 x 113cm rectangles with a 10cm flap along the long side and 2.5cm round the rest. Paint one side of each piece white -- the interior facing side -- and the other side black to keep the light out.
Assemble the triangles "Assemble the isosceles triangles into six pentagons," says Krieger. "Fill the gaps between them with the equilateral triangles. Build the outer ring first with the bulldog clips, adding the central pentagon last." Have others help with this. "You need something holding all the pieces in place until you add the final keystone -- then it supports itself." Now connect the rectangles around the bottom of the dome to make a wall that raises it up.
Set up the projector
Calibrate your projector against a flat wall so it produces a focused image at a distance of about a metre. Place the security mirror on a table at the dome's edge, so the top of the mirror is about the same height as the top of the wall. Line up the projector so the image fits exactly on to the mirror. "To get the image of the sky on the dome you need the free software Stellarium," says Krieger. "Just click on the spheric mirror distortion option."
This article was originally published by WIRED UK
