In 1933, a crowd gathered in Chicago for the opening of a World's Fair, and no one flipped a switch. Instead, a telescope was pointed at a star. Its light struck a photocell, the cell produced a tiny current, and the current turned on the lights of the fairgrounds. The star was Arcturus. The story the organizers told was that this light had left the star around the time of the previous Chicago fair, in 1893, and had been traveling ever since.

Most people who walk outside on a spring evening have looked straight past that star. It is one of the brightest in the northern sky, and it is easy to find once someone shows you the trick.

The trick: arc to Arcturus

Start with the Big Dipper. Find the three stars that form the curved handle. Now follow that curve, the way you would follow the bend of a rope, away from the bowl.

About three fists held at arm's length from the end of the handle, you run into a bright star with a distinctly warm, orange-gold tint. That is Arcturus. Astronomers' mnemonic for it is arc to Arcturus, and it has helped people for generations because it asks you to do only one thing: keep curving.

There is a second half to the rhyme. Keep going along the same sweep, and you reach Spica, a bluer star, about another fist and a half to two fists farther on. Arc to Arcturus, then speed on to Spica. With one gesture you have navigated across a quarter of the sky.

The usual mistake is to draw a straight line from the handle. The line is a curve. If you draw it straight, you will end up in empty sky and decide the whole thing is a myth.

Why it looks orange

A star's color is its temperature on display. Hot stars burn blue-white; cooler ones glow orange and red. Arcturus is a red giant, a star that has used up the hydrogen in its core and swollen to roughly 25 times the Sun's diameter. Its surface is cooler than the Sun's, which is why it reads as gold-orange rather than white.

You don't need a telescope to see this. Put Arcturus and Spica in the same glance and the difference registers: one warm, one cool. Our eyes are not very sensitive to color in dim light, but stars this bright still give up their tint to anyone who looks for a few seconds instead of one.

A star that is passing through

Arcturus is only about 37 light-years away, which makes it one of the closer bright stars. It is also fast. Most stars drift slowly against the background, but Arcturus has one of the larger apparent motions of any bright star. Edmond Halley noticed in 1718 that it had shifted from where ancient catalogs placed it, which helped overturn the idea that the stars were fixed in place forever.

It also moves differently from the Sun and its neighbors. Arcturus belongs to an older population of stars that orbit the galaxy on tilted, more eccentric paths, and it is passing through our neighborhood rather than living in it. We are looking at a visitor on a long trip, and in a few hundred thousand years it will have moved on.

The part about the World's Fair

About that 1933 story: the Century of Progress exposition really did use Arcturus to start its lights. Light from the star was collected by telescopes at several observatories, including Yerkes, and converted to electrical signals. The organizers liked the idea that the light was as old as the previous fair, because astronomers at the time believed Arcturus was about 40 light-years away. The modern figure is a little closer, so the symbolism was roughly right but not exact.

What is exactly right is the idea behind it. Every point of starlight is an old message. You are not seeing Arcturus as it is tonight. You are seeing it as it was in the late 1980s, and that has been true for every person who ever pointed at it.

Older names for the same star

The name comes from Greek and means roughly "guardian of the bear," because it seems to trail Ursa Major across the sky. The constellation it anchors, Boötes, is usually pictured as a herdsman or bear-driver, with Arcturus near the knee or waist of the figure.

In Hawaii, the same star is Hōkūleʻa, the "star of joy." Arcturus passes directly overhead at the latitude of the Hawaiian Islands, which makes it a zenith star there. Navigators used that fact: when Hōkūleʻa stood overhead, they knew roughly where they were. A star can be a decoration, a story, or a position fix, depending on who needs it.

Your next moves

  • Find the Big Dipper first, then the curve. On the next clear spring or early-summer evening, locate the Dipper and trace the handle's bend. Don't aim for a particular star yet. Just feel the direction of the arc.
  • Measure with your fist. Hold your closed fist at arm's length; it spans roughly 10 degrees. Count about three fists from the end of the handle along the curve. That is where to look.
  • Compare Arcturus and Spica. Once you have both, spend a full ten seconds on each. Say aloud which one looks warmer. Training your eye to notice color is the quickest way to start telling stars apart.
  • Check the date. Arcturus is a spring and summer evening star in the Northern Hemisphere. If you're looking in late autumn, it is lost in the Sun's glare or only visible before dawn. Don't conclude you got it wrong.
  • Tell someone the Chicago story. Say it out loud while you are looking at the star. Sharing the story makes it much more likely you will remember how to find it next time.

One more star you can now find on your own

Learning the sky works less like memorizing a map and more like learning a path you can walk. The Dipper gets you to Arcturus, Arcturus gets you to Spica, and each new landmark makes the next one easier. A phone can confirm what you are looking at, and Astra is built for exactly that moment: you point at the orange star you think is Arcturus and it tells you whether you were right. If you'd like to try it, you can find it at astra.lumenlabs.works. But the real reward is the first time you find it without any help.