How did they do that?
Today I was teaching about binary stars to my second year SL class. It's often interesting to think about how some of these discoveries were made without the help of modern technology, In the case of astronomy the Greeks simply looked at the stars but by the early 1900 astronomers looked at photographs of stars. What Henrietta Leavitt ( I wonder if she is related to Randy Leavitt the famous climber?) did was to first take a photograph of the stars, this would be black dots on a white background since the photo plate turned black when exposed to light. The size of the dots is related to the brightness of the star but how did she notice which ones had variable brightness? What she did was to make a positive out of the negative and lay it over the original. The effect is that the black and white dots cancel each other out and you get a totally black sky unless one of the dots has changed size. I thought I'd try to simulate this using paint, it works quite well I think.