There are two very bright stars in the southern skies. One is the bright white Spica, brightest star in the constellation Virgo. The other is the yellowish Arcturus in Bootes. Spica lies almost due south and about halfway between the horizon and the zenith (point directly overhead.) Arcturus is slightly above and a bit to the east.
All that nice information aside, you might ask, but what does all that have to do with the 1933 worlds fair? Well, what it has to do with is some creativity on the part of the Chicagoans, and the distance to Arcturus. Oh, and photocells, we can't forget the part the photocells play in all this! Okay, you say, now totally perplexed, (and just a little hesitant) what have photocells got to do with it?
Well, the explanation starts out with the light we see from distant stars. The fastest thing we know of in the universe (so far) is light. In one second, light travels 186,000 miles. Pretty impressive. It serves a purpose to measure the immense distances in the universe. For example, our Sun is roughly 8 light minutes away, or 480 seconds. Multiply that by 186,000 miles and you get the average distance to the Sun, 93 million miles. It also, of course, means that that patch of sunlight that is warming up your face when you go outside left the Sun 8 minutes ago. A light-year is the distance light travels in a year, nearly 6 trillion miles. That's only one light-year, but stars and galaxies may be dozens, hundreds or even billions of light years away. Space is incredibly vast!
At the time of the 1933 Chicago world's fair, astronomers had been measuring distances to objects in the universe. They believed Arcturus to be 40 light years away. Which meant that the light that we saw when we looked up at Arcturus in the sky, had left the star 40 years ago. Well, guess when Chicago last had a world's fair? 40 years before in 1893. Somebody planning the fair must have been an astronomy buff! They decided to use that little astronomical tidbit to spice up the 1933 fair. While most people had a passing interest in such astronomical knowledge, it just didn't generate the kind of excitement needed to open a world's fair, so it was necessary to get creative.
Now comes the part about the photocell. Photocells absorb light and turn it into a primitive electrical energy, not much energy mind you, but enough to do something like turn on a small switch. Photocells were new and exciting technology back in 1933, and it was decided to demonstrate this new wonder by using it to turn on all the lights at the opening ceremony of the world's fair.
In the end what they did was point a telescope at the star Arcturus. The light from the telescope was aimed at a photocell. The photocell was connected to a small switch, which was, in turn, connected to other larger switches. When enough light from Arcturus (which had left the star 40 years before when the last world's fair was at Chicago in 1893) had gathered in the photocell, it tripped the switch that opened the 1933 world's fair. So now, as you can see, all the pieces fit together. People were impressed, the world's fair was a success.
You may not be quite as awed as the 1933 crowd, and you may be even less impressed when you find that they didn't have the distance quite right. While the actual distance is 36 light-years, you could argue it was close enough for the purpose. But it does give you something novel to think about if you go out and look at the night sky.