A friend of ours across the street has a small front garden, but since we're in Boston, their house faces north, and it's three stories tall the garden gets almost no direct sun. [1] Could we fix this with a mirror?

If I put a static mirror on the front of our house, even on an angled mount, it would only send light to the garden for a short period each day. To be actually useful, the mirror would need to be dynamic, tracking the sun: a heliostat. Since the movement of the sun is well understood you can do this without any sensors, just calibration.

With a bit of looking I found two companies selling heliostats, though I think the main residential use is bringing more light in through windows and not gardening. Possibly building something would be fun, but I'd be nervous about wind loads and breakage filling our own front garden with shards. Has anyone done a similar project?

[1] Boston's latitude is 42°, so the peak height of the sun on the equinox is 48° (90°-42°). The solstices are ± 23° (axial tilt) so at solar noon on the summer solstice it gets up to 71°. The house is ~40ft tall so its shadow is always > ~13ft (40/tan(71°)).

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A town in Norway did it: https://www.bbc.com/future/article/20170314-the-town-that-built-a-mirror-to-catch-the-sun

That's so neat! Seeing the Town Square lit up like that in the middle of so much deep shade looks really computer generated. I wonder in person how quickly it starts to feel normal?

I don't actually have anything to say directly to this topic, but I just want to say I really appreciate how you always make posts about interesting practical things like this. We need more general rationality content, not just AI alignment all the time, and these posts are often thought provoking. I hope you get some good answers!

I am not aware of a heliostat designed for mirrors, but the problem sounds like it's shaped similarly to the problems addressed by solar tracker systems for solar panels. I wonder if a system like that could be repurposed for your needs?

I'd be nervous about wind loads and breakage filling our own front garden with shards

I second the intuition against putting something up in the air that might shatter.

Would a driveway mirror work? I believe that they're commonly made of shatter-resistant materials -- shiny metal, or appropriate plastics.

Would a mylar-faced tarp work for prototyping? It might be ugly and it might be loud in the wind, but "space tarps" sold for survival situations are highly reflective on one side and relatively sturdy. A flexible reflector could be hoisted in good weather and furled in bad with the same basic technologies that have moved sailing ships around the world for centuries. Space blankets are too flimsy to hold up well on their own and would probably require additional support to not tear... although many types of space blanket can be ironed onto a sturdy fabric like a canvas drop cloth if you experiment a bit to find the right setting on your clothes iron.

Would a chromed metal panel, or even bright white metal roofing, work as a mirror without the shatter risk?

Over on Mastodon I got the recommendation to use acrylic mirrors, which sound like a good fit.