The Rest Is Science: Could We Put Sunglasses on the Sun A Deep Dive into Angular Diameter and Space Sunshades

The Rest Is Science tackles a cartoonist’s dream with serious science: can sunglasses ever fit on the sun and how big would they have to be to work for viewers on Earth or across London? The episode blends geometry, eye physiology, and space-engineering speculation to build up a plausible, if wildly ambitious, answer.

• Core idea: sunglasses on the sun are impossible on the surface but entertaining as an optical illusion.
• Key physics: angular diameter, light pressure, and safe viewing standards for looking at the Sun.
• Scaled experiments: from arm’s-length pencils to city-scale skylines and finally to a 1.3 million kilometer wide disc located far from Earth.
• Ethics and practicality: why a global sunshade would affect life on Earth and whether such projects should be pursued.

Overview and premise

The Rest Is Science embarks on a playful yet scientifically informed exploration of a seemingly silly question: could we make the Sun look like it is wearing sunglasses? The hosts acknowledge early hurdles: the Sun lacks ears and a face, and its extreme heat makes placing real sunglasses on its surface impossible. Yet the discussion quickly shifts from a toy idea to a serious analysis of what would be required to produce a convincing illusion visible to people on Earth and across large regions such as London. The episode hones in on angular diameter as the central tool to crack the problem, then scales the concept from a pencil-sized occluder at arm’s length to a planetary-scale sun shade in space. The episode also interweaves related topics such as solar probes, the safety standards for solar viewing, and the physics of light momentum that would affect any large structure designed to shade or reflect sunlight.

Angular diameter and the geometry of sight

Angular diameter describes how large an object appears from a given vantage, not its actual size. The Sun’s apparent size in the sky is about 32 arc minutes, or roughly half a degree. The hosts explain arc minutes and degrees, and how the Earth’s geometry translates real-world sizes into visible angles. They use a concrete example: if an object is held at arm’s length, to cover the Sun completely it must be at least 1/107th as wide as its distance from the observer. With an approximate arm length of 70 cm, a block around 65 cm wide would fully obscure the Sun as seen from Earth, which is clearly dangerous for the eye. The discussion emphasizes that any such in-sky cover must dim the Sun far more than mere full-occlusion to be eye-safe, referencing ISO solar viewing standards that require a reduction in brightness by about 100,000 times.

From pencil to planetary scales: two- and multi-eye viewing

To understand how sunglasses could appear on the Sun, the hosts analyze binocular vision. They consider how the pupil size and inter-eye distance affect the necessary size of an occluding disc to maintain the illusion for both eyes simultaneously. The math evolves from a 0.65 cm disc for single-eye coverage to a roughly 7 cm boundary when accounting for the average human eye separation. They conclude that, for two viewers at a distance, the required disc grows, and the geometry becomes more stringent. They also point out that the Earth’s curvature and differing observer positions would complicate simultaneous perception of the illusion across a city, so a more global approach is needed for London-scale viewing.

Scaling up: distance, width, and city-wide viewing

The episode then escalates to larger distances. If you wanted the illusion to be seen across all of London, a disc width of tens of kilometers would be necessary, and the disc would need to sit at a distance where the Sun’s angular size aligns with that width. They explore options including a disc in space (e.g., at a geostationary orbit of about 35,786 km, or at a Lagrange point L1 roughly 1.5 million kilometers away), and even further at 140 million kilometers, effectively placing the shade between Earth and Sun. The numbers shift dramatically with distance: a far-away disc needs to be proportionally huge to cover the same angular size. A recurring theme is the tension between feasibility and the scale of the required structure, along with the danger of blocking too much sunlight and harming Earth-based life that relies on regular insolation.

Specific locations and final blueprint proposals

After weighing various placements, the hosts settle on a bold concept: a disc of safe solar viewing film that dims the Sun by 100,000 times, with sunglasses drawn on its surface. The proposed dimensions are staggering: a disc roughly 1.3 million kilometers across placed about 140 million kilometers from Earth. In this arrangement, the Sun would appear to wear sunglasses to anyone able to see it, revealing a playful monument to human curiosity and ingenuity. They acknowledge that such an artifact would profoundly dim sunlight and disrupt photosynthesis, making it a cultural and ecological faux pas as well as a technological fantasy. The discussion culminates in a nuanced takeaway: while the sunglasses-on-the-sun thought experiment is a delightful way to learn about angular size, optics, and space engineering, real-world impact on life-supporting processes would be severe, and the project belongs squarely in the realm of a thought experiment or a museum-like imaginary sculpture rather than a practical plan.

Related concepts, ethics, and alternatives

The conversation transitions to related ideas such as space sunshades, dust-dimmed shields, and space mirrors. They reference past and current proposals like dust-laying or dust-shield concepts, and NASA-style starshades with petals to avoid optical fringes such as the Arago spot. They discuss existing real-world efforts to manipulate sunlight in space for other purposes, including solar sails, which rely on light momentum to propel spacecraft. The episode also touches on the broader ethics of geoengineering, the risk of giving moral license to pollute, and the importance of collective decision-making in climate interventions. The sponsor, Cancer Research UK, is acknowledged as part of the episode, and the hosts reiterate the value of sound science and public engagement in discussing large-scale, potentially risky ideas.

Final reflections

Ultimately, the hosts present a rigorous blueprint for a sun wearing sunglasses as a thought exercise, not a plan for action, and they emphasize the educational value of chasing such questions. The episode closes with an invitation to the audience to bring equally absurd questions, underscoring the podcast’s philosophy that curiosity, combined with careful reasoning, yields valuable insights, even from whimsical inquiries.