Abstract

Solar projection using mirrors on a disco ball is a fun and unique way to safely observe the Sun, especially during a solar eclipse. We share our methods of using a disco ball to project a solar eclipse during outreach events and discuss results and best practices from trial runs conducted during the October 25, 2022, and October 14, 2023, partial and annular solar eclipses from various locations across Europe and North America. We encourage any location hosting an event during the April 8, 2024, total solar eclipse to display a disco ball and collect feedback from visitors about their understanding of, and engagement with, the projected images of the solar eclipse.

1. Introduction

Pinhole cameras and projectors (Young, 1989) are commonly used to make solar observations in both professional and educational settings (see for example Heilbron, 1999; Sigismondi & Fraschetti, 2001). They can be used to observe the solar disk, identify sunspots, and safely view solar eclipses. Many resources already exist that provide details for making simple pinhole projectors using index cards, cardboard boxes, and even colanders (e.g. Jenkins, 2008 and https://science.nasa.gov/eclipses/safety/). These incredibly useful tools can help the general public safely observe the Sun, especially during solar eclipse outreach events.

Another unique and potentially eye-catching way to safely observe the Sun is through the use of a disco ball, also commonly known as a mirror ball. Typical disco balls are collections of multiple pinhead mirrors that each have the reflective equivalence of a pinhole camera aperture (Nilsson, 1986; Wood, 1934). The brightness of the projected solar disk image and the distance it comes to a focus are both dependent on the width and area of the mirror (Cumming et al., 2024). Small pinhead mirrors have been used as an educational tool in physics classrooms (Ribeiro, 2016), and as a scientific tool to photograph the Sun (Malpas, 2024; Takeda, 2024)

Disco balls in particular have been used in informal education settings and serve as a beneficial outreach tool during partial or total solar eclipses (Cumming et al., 2024). The individual mirrors have proven effective at projecting clear images of the solar disk, and the presence of the ball sparks great intrigue at public outreach events. Wide use of disco balls in museums, observatories, and at public outreach events could greatly enhance interest in solar science and help engage more people in safe solar observing, especially during solar eclipses. They hold the same educational potential as pinhole projection cards but are arguably more eye-catching. They also have the potential to engage visitors in the science behind pinhole cameras and pinhole mirrors.

In this paper, we share our experiences using a disco ball to observe the October 25, 2022, and October 14, 2023, partial and annular solar eclipses at schools, libraries, and observatories in Europe and North America, respectively. Based on these trial runs, we share tips regarding the type of disco ball to purchase, placement and usage of the ball during the event, and some safety considerations. We also share some qualitative feedback and photos from visitors who attended these outreach events and reflect upon the educational benefits of using a disco ball to observe the Sun. Finally, we provide some additional suggestions for improving the use of a disco ball at outreach events and encourage any site hosting an event during the April 8, 2024, total solar eclipse to display a disco ball and provide feedback on their experience.

2. Using the disco ball as a public outreach tool during solar eclipses

During the October 25, 2022, partial solar eclipse over Europe and the October 14, 2023, annular solar eclipse over the Americas, disco ball solar projections were tested at various public outreach events. These sites include the Leibniz-Institut für Astrophysik Potsdam (AIP) in Potsdam, Germany (2022 partial solar eclipse, first trial run); Round Rock Public Library in Round Rock, Texas, USA; the Carden School of Fresno in Fresno, California, USA; and Palomar College in San Marcos, CA, USA (all during the 2023 annular solar eclipse). All of these sites experienced a partial solar eclipse during their respective events (i.e., none of the USA sites were along the path of annularity). The disco ball projections were just one of many ways visitors could safely observe the Sun during the solar eclipse. Solar glasses and telescopes were used at most sites, along with hands-on astronomy activities. Docents or staff were also available to talk to visitors about the solar eclipse, help with observing, and discuss the projections made by the disco ball.

The diversity in institution type allowed us to test the disco ball in different settings and with a wide range of audiences. Other collaborating schools and universities were unfortunately clouded out and unable to test the disco ball during their events, but they did provide general feedback regarding projecting the Sun with a disco ball on non-eclipse days.

Based on our experiences, we share below some suggestions for the type of disco ball to buy, tips for placement and projection, safety considerations, and the public’s perception and experiences during the solar eclipse events.

2.1 Type and placement of the disco ball

One of our goals for these test runs was to determine what type and size of disco ball would work best for solar eclipse engagement activities. Each site acquired their own disco ball, and they ranged from small, inexpensive balls from party-supply stores (<$20), to larger, more professional style balls ($100+). As long as the disco ball is composed of small flat mirrors, it should function as an efficient pinhead mirror. Thus, the most important properties are the width and quality of the individual mirrors.

Creating a clear and recognizable image of the Sun using the disco ball is important for helping the public recognize the scientific purpose of the disco ball. The small mirrors act as pinhead projectors and reflect the sunlight, ultimately creating an inverted image of the Sun. As you move away from the disco ball, the projected images change from squares to circles, and the Sun will ultimately come into focus. The ideal focal length for light in the middle of the visible range (550 nm) is given by the following expression:

f = 9091 D²

where D is the width of the mirror, and f is the focal length, in cm (Cumming et al., 2024). Most commercial disco balls have mirror widths between 0.4 and 1 cm, which results in an optimal focus distance of 1500 cm and 9100 cm, respectively. However, in practice we have found that disco balls with mirrors in this size range have created small, but recognizable solar projections at a distance of only a few meters away (see Figure 1 and Figure 2).

The quality of the mirrors can also affect how crisp the projection appears. If the back reflective surface and the front glass surface are not parallel, then the resulting projection will have a ghost image. This is due to the reflection of sunlight off the front and back surfaces being misaligned, creating a fainter ghost image and blur effect around the Sun (see diagrams in Cumming et al., 2024). Some disco balls that we tested did show this effect, but there was no correlation between the cost of the disco ball and whether or not it produced ghost images. Testing should be done ahead of time to see if your disco ball projections suffer from ghost images. If the defects are very strong, you may want to purchase a different brand of disco ball. In our experience, the cost of the disco ball has little to no correlation with its effectiveness as an outreach tool, as long as the mirror quality is decent.

Displaying the projected image in a recognizable way is also important. Projection onto a large white piece of paper, foam board, or flat sheet is ideal, provided the projection screen is not in direct sunlight. We have found that placing the screen on the wall of a building under an overhang, inside an alcove, or under a stairwell works very well (Palomar College has an online video illustrating this). This darkens the region enough to make the solar disk projection obvious and the solar eclipse easily visible.

With focal lengths of tens of meters, the disco ball can easily be placed in direct sunlight while the projection screen is in a shaded area. The Round Rock, Texas and Potsdam, Germany sites experimented with placing the disco ball indoors near large windows (Figure 1, Figure 2, and Figure 3). This configuration worked very well and provided a slightly darkened indoor area for people to observe the solar eclipse projections. Rooftop gardens (Figure 4) or building overhangs are also great places to display the disco ball and have a nearby wall for projection.

Since the disco ball is spherical and the mirrors are all at different angles, the exact location of the projected image will change throughout the day. Testing throughout the same time of day as the solar eclipse should be done ahead of time to ensure that as the Sun moves, some of the images are always viewable on the projection screen.

Another consideration is whether to hang the disco ball and let it spin or permanently mount the disco ball in a stationary fashion. At the Potsdam, Germany event, the disco ball was mounted on an old lamp stand using a rope tied to the ball and passed down through the post to the base (Figure 5). This made it easy to move around to find the best location. It also allowed the projected images to be kept stationary so staff could stand near one and explain to visitors what was being shown.

At the Fresno, California, event, middle school students were handed the disco ball and encouraged to walk around and explain the projections to other students and parents nearby. Having the projection device in their hands made the students feel empowered to talk about the solar eclipse and use the tool to its greatest potential. This worked well because the event was relatively small (~60 people), and the attendees were all teenaged students. Walking around with the disco ball may not work as well for large groups, or events with lots of small children.

Hung disco balls can create a fun “spinning Suns” effect, but they are best utilized after staff have pointed out their purpose and explained what the projections are. From an educational standpoint, stationary disco balls worked best, while hung disco balls helped enhance a party-like atmosphere.

2.2 Public perception and educational value of the disco ball

The disco ball was a big hit at all our test sites. It was an eye-catching centerpiece that encouraged visitors to ask questions and consider the many different ways that one can observe the Sun. However, we did find that it was very important to have staff available to discuss the projected images and explain to visitors what they were actually seeing.

During the October 2022 event in Potsdam, people were particularly captivated by the light reflections combined with the grand architecture of the AIP building (see Figure 5). The ball was set up in the hallway that led to the telescopes, so all visitors passed by and noticed the projections. However, they didn’t always make the connection that the reflected light off the disco ball was a projected image of the Sun. This was true even during the solar eclipse when the dark crescents were clearly visible. Once it was brought to their attention that these were actually solar projections, people examined them up close and naturally migrated to images farther away that were larger projections. People even placed their hands on the wall to see what the projection would look like on their own hands. Sunspots were also clearly visible before and after the solar eclipse. These spots were pointed out to visitors and led to great discussions about the mechanics of sunspots and solar science.

Similar experiences occurred at the October 2023 events. Some visitors recognized that the circular projections near the disco ball were indeed images of the Sun, especially once the solar eclipse started, while others needed it briefly explained to them. Regardless, once the visitors recognized the eclipse, they were instantly intrigued. It may be that more of these visitors had observed a recent previous solar eclipse, (e.g. the Great American Solar Eclipse of August 2017) or because they had seen pictures or videos on the web of solar eclipse projections. In most interactions, staff did not discuss the details of how the optics worked. Instead, they just shared that the sunlight reflected off the disco ball creating a projection of the Sun.

At the Round Rock, Texas event, many young children were in attendance. They enjoyed photographing the solar eclipse images falling inside their shadows (Figure 6), examining the images on the walls, and even capturing images of the eclipse on their clothing (Figure 7). Similarly, guests enjoyed putting their hands inside a 2-lens safe solar viewer and photographing the solar projection on their hand. Families also enjoyed being able to view the solar eclipse projections together at the same time, which often sparked discussions between kids and adults about what they were seeing. There seemed to be something special about seeing the solar images at the same time as opposed to sharing glasses and wondering whether each person was experiencing the solar eclipse in the same way. Concurrent viewing of the solar eclipse with a disco ball seemed to spark more curiosity and discussions amongst visitors when compared to other viewing devices. It also allowed staff to overhear questions, participate more in discussions, and clear up any misconceptions. Overall, both visitors and event staff had a more wholistic and immersive experience with the disco ball present.

Parents and children at both the Round Rock, Texas, and the Fresno, California, events also praised the ease and accessibility of the disco ball for viewing the solar eclipse. Young children didn’t have to worry about wearing the dark glasses and finding the Sun with them, nor did they have to struggle to look through a solar telescope eyepiece. Since the ball produced multiple projections at once, classes and families could observe the solar eclipse together, and more easily discuss what they were seeing.

One interesting observation that event staff at multiple locations made was related to whether visitors really understood what they were looking at in the solar eclipse projections. Many young visitors insisted that they were looking at the bright Moon instead of the Sun. This is a common misconception, especially when using solar glasses. Since the Sun is the same angular size as the full Moon, people intuitively think it looks a lot like the full Moon through solar glasses. Most young children have never seen the Sun directly before, so their most familiar reference to a large white object high in the sky is the Moon. Even during the actual solar eclipse, children generally see the crescent shape of the Sun and associate that with a crescent Moon. This misconception is exacerbated by the fact that the Moon looks like a dark circle during a solar eclipse, and there are no discernable craters or gray coloring to help children identify the Moon versus the Sun.

Slater & Gelderman (2017) point out a variety of misconceptions related to solar eclipse viewing that are experienced by children and adults alike. They suggest using a variety of active learning techniques to help people better understand the Moon’s orbit, and how solar eclipses work. Pictures, demonstrations, and kinesthetic activities are likely to help counteract this Sun vs. Moon misconception and could be done in classrooms prior to the solar eclipse, or as activity stations during an eclipse event.

Finally, the disco ball observations just seemed to spark joy and excitement in all of the visitors. It provoked a party-like atmosphere and made the solar eclipse events feel more fun and extra special. Lots of exclamations of “Wow!” and “That’s so cool!” were heard at all the different events, which made the disco ball setup completely worth the effort. It’s not every day that visitors get to walk into a room with a glowing disco ball that’s projecting science!

2.3 Safety considerations

Overall, we found minimal safety concerns associated with the disco ball projections. Projected images are completely safe to observe, and the disco ball provides a lot of them all at once. Even if someone were to look directly at the disco ball or stand such that they were in the line of sight of a projected image, the reflected light would be like that of sunlight bouncing off a shiny surface. Event staff should keep watch over the disco ball to ensure that people are not looking directly into the mirrors at close range where the light would be more concentrated, but that was not an issue we encountered at our test sites. Since visitors don’t have to handle a pinhole projector box or camera on their own, there is a lower chance of misuse and eye damage.

An additional safety benefit the disco ball offers is a little bit of crowd control. Since the disco ball is creating multiple images at large distances, people will likely be drawn away from the ball – and therefore concentrated reflected light – and towards the projected images which are spread out. This can help disperse large crowds and offers multiple different locations for people the observe the Sun, rather than just one. If any signage about the disco ball is used, it should be hung near the projected images, not the disco ball itself. This will help draw visitors away from any concentrated light, and draw their focus to the projected images rather than the ball itself.

3. Using the disco ball for solar eclipse observing in 2024 and beyond

All of our test sites experienced great success with the disco ball, and those of us who will get to experience a partial or total solar eclipse on April 8, 2024, will definitely use the disco ball again. From our previous experience, we have a few suggestions for ways to enhance the party-like atmosphere and capitalize on all the disco ball’s educational potential.

The disco ball works best when it is used with other outreach tools. Solar glasses, telescopes, and other pinhole projectors should be used in conjunction with the disco ball. This offers visitors multiple ways to view the solar eclipse, and broadens the potential for rich, educational discussions.

Placement of the disco ball should be in a centralized, obvious location, like the entrance to a building or near a large outdoor common area, away from a place where people naturally congregate. This will stop people from accidentally blocking the sunlight from hitting the disco ball, especially if you are using a small ball that is easy to overshadow. It will also mitigate any safety concerns regarding the reflection of sunlight into someone’s eyes.

A plethora of royalty-free disco music is available on the internet (see, for example, the song Disco Medusae on YouTube). Playing some disco music in the vicinity of the ball can enhance the party vibe and draw more attention to the disco ball. It may also help visitors realize that the disco ball is there purposefully and encourage them to study the projections more closely.

Signage can play a useful role in helping visitors understand the disco ball’s purpose as well as the science behind pinhole projections and pinhead mirrors. Infographics designed for a general audience are currently in the works by our team and are available for anyone to download and print prior to the April 8 total solar eclipse1.

Our test sites also found it helpful to train event staff on the basics of how the disco ball works, so that they can explain it to any interested visitors. Having staff ask visitors questions such as “Why do you think there is a disco ball at this event?” or “What are those circles projected on the wall actually displaying?” can also increase engagement and scientific understanding. For educators or amateur astronomers who attend your event, it’s also interesting to point out that the disco ball can theoretically be used any day to observe large sunspot groups, or even a bright Moon. Observing sunspots may require a darker space for projection, and a screen placed closer to the true focal point of the mirrors so that a larger image is shown. Previous tests have successfully displayed sunspots on the solar disk and images of the full Moon (Cumming et al., 2024).

4. Conclusions and future work

Disco balls are an inexpensive, safe, and easy way to engage the general public in solar observing, especially during a solar eclipse. They bring an additional fun component to these events.

By using a disco ball, large numbers of people can simultaneously see the projected solar disk, which sparks discussion among and enhanced curiosity in people of all ages. The disco ball projections are easy for all visitors to see and understand, and the ball itself creates a wonderfully fun atmosphere to promote science education.

We encourage anyone who plans to host an outreach event for the April 8, 2024, total solar eclipse and beyond to use a disco ball as part of their observing plan, even if it’s just for the purpose of making a scientific event feel more like a disco party. We are curious to learn more about how the disco ball is received at these events, and how much educational potential it has for explaining solar science or optics. If you are interested in providing some qualitative data on the effectiveness of the disco ball at solar eclipse events, consider taking notes on the following questions:

•             How do visitors interact with the disco ball and its images at public outreach events?

•             How do visitors talk about the ball and the images with each other?

•             What questions do visitors ask the event staff or each other?

•             How can you help people to understand what they see (e.g., signage, verbal explanations)?

•             How does the disco ball compare to other solar eclipse viewing equipment when it comes to engagement and scientific understanding?

Reflections on these questions will help us gauge how useful the disco ball is for teaching solar science to the general public, and how beneficial it is for understanding solar eclipses. Please email author Alex Pietrow ([email protected]) with the term “Disco Ball” in the subject line either before or after your event with any details you’d like to share.