Light fever: bringing disco to astronomy

23 Jan 2024 Laura Hiscott

As well as bringing sparkle to the dance floor, could disco balls become a new educational tool in astronomy? Laura Hiscott investigates

Reflecting the universe The tiny mirrors on a disco ball act like pinhead mirrors, allowing us to view multiple reflected images of the Sun. (iStock/Liubov Motavschuk

Flared trousers, light-up dance floors and John Travolta’s infamous dance moves.

What else could we be talking about but disco – a dance genre and subculture that emerged in the 1970s, and has made comeback after comeback ever since. Even in recent years artists have been making retro albums, #DiscoTok has a growing presence on social media, and multiple documentaries about the genre and its history have aired, from Love to Love You, Donna Summer to the BBC’s Disco: Soundtrack of a Revolution docuseries.

And adorning much of the media surrounding disco are pictures of the most emblematic object of the era: the disco ball.

Aside from their musical associations, these ornaments speak to our love of dancing light – also evident in everything from sun catchers to firework displays. And, although they may not be obviously related to astronomy, a new study (soon to be published in Physics Education) says that these glitzy globes could in fact help us to view the universe’s own natural light shows.

Disco balls are simply spheres covered in tiny mirror fragments, which, if small enough, can work as “pinhead mirrors”. To understand the effect, it’s useful to think about the much better-known pinhole camera, or camera obscura. These simple optical devices, documented as early as 500 BCE, are essentially closed boxes with a tiny hole in one side.

They work by restricting light; a ray from any point on an object will only get through the aperture if it is incident at the correct angle, so the light rays are kept “in order” to produce an inverted image of the object. Similarly, a tiny mirror will only receive a small number of rays from any point. Those rays are reflected, rather than transmitted, but they also produce a recognizable image.

Robert Cumming, astronomer and communications officer at Onsala Space Observatory in Sweden, chanced upon the effect after hanging up a disco ball he’d bought for a New Year’s Eve celebration. “I got a text from home saying: ‘the flat looks like this!’” he recalls. “The ball was illuminated and the place looked magical. I realized that each bright patch on the walls was an image of the Sun.”

Inspired by this serendipitous encounter, Cumming and an international team of astronomers with an interest in public outreach decided to test the ornaments’ potential for helping people engage with astronomical events. Their results are compelling. They successfully observed the partial solar eclipse of 25 October 2022, during which the disco ball clearly projected a changing crescent shape as the Moon’s shadow swept over the Earth. They also demonstrated a strong effect using other obstacles, such as tree leaves.

On the theoretical side, the scientists’ study outlines why disco-ball mirrors are the right size to work. As with the aperture in a pinhole camera, the optimum size for a pinhead mirror is a compromise between getting too much light from the wrong places if it’s too large, and getting diffraction patterns if it’s too small. The best size therefore depends on the wavelength of light and the distance of the imaging surface from the reflector.

Typical disco balls can have mirrors as small as 40 mm and, taking the average wavelength of visible light to be 550 nm, the optimum imaging distance is 15 m. That might sound like a large distance for projecting an image inside a room, but, as the results show, perfect focus is not necessary to offer a clear enough picture. In fact, the researchers were even able to produce images of the solar disc with a few discernible sunspots at just 6 m from the disco ball.

From an outreach and education perspective, the astronomers point out that disco balls have several advantages. Besides being unexpected – and therefore intriguing – objects to associate with astronomy, they are also cheap and widely available. Like other methods of indirectly viewing the Sun, the images are safe for our eyes; but unlike most, a disco ball projects multiple images around a room, enabling a group of people to experience an eclipse collectively rather than having to take turns. And of course, as Cumming says, “A disco ball adds a bit of celebratory sparkle to anything!”

“The [pinhead mirror] phenomenon tends to get rediscovered every decade or so, and the pinhead mirror was even patented for a while,” notes Alexander Pietrow, co-author and solar physicist at the Leibniz-Institut für Astrophysik Potsdam in Germany. “Still, our paper is the first to describe the effect as an educational eclipse viewing tool.” Excitingly the solar eclipse of April this year offers a timely chance to see it in action.

Furthermore, according to the scientists’ calculations, the disco-ball effect should be strong enough to display a transit of Venus, although they couldn’t check this out, because the last visible transit occurred in 2012, and the next one isn’t until 2117. We can’t possibly know if disco music will be going through another renaissance in 93 years but hopefully there will be some disco balls on hand to bring some sparkle to that spectacle.

Laura Hiscott is a freelance science journalist

FROM PHYSICSWORLD.COM    28/1/2024