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Comment by burlesona | Hacker News Reader

Right at the top of this article I found myself surprised by the idea that people in the 1600s had figured out a way to safely look at the sun through a telescope and make observations about sun spots.

I don't know much about this, except that you can do it on a telescope today by putting in a solar lens, which looks basically opaque, as that's what it takes to reduce the light of the sun to something we can handle. I spent a while googling to figure out how they did it and got basically nothing.

So, HN, does anyone know how the early telescope users figured this out? How many went blind trying different filtering approaches before they got a "just right" filter that let enough light through to observe the sun without hurting themselves?

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I believe it was done by projection onto a wall or screen, for example through a pinhole projection, rather than looking through a lens directly.

I, and many others, learned a lot about the sun when during the last solar eclipse.

I held a spice shaker lid (ie a disc with several round holes) into the sun and observed several crescent-shaped shadows on the ground [0]. It turns out the light that passes through the holes is a projection of the sun, which is kinda a neat realization. It was pretty unintuitive before I saw this, so I’m impressed by the 17th century astronomers.

Anyone who was near a tree during the eclipse saw that, too. The effect was very clear in spots of light passing through leaves. Definitely weird to see.

Weird, but wonderful. I laid down a large (butcher paper) white sheet under a tree and took a picture. Much later on, I took that same picture and a pencil I had at the time, and projected the picture back onto the butcher paper and traced the picture back onto it. Pretty cool art!

Check this out: https://youtu.be/X9_6EbucCPQ

Cover your windows with opaque garbage bags, cut a small hole into one (<1in), and voila! Camera obscura!

I first discovered this for myself as a child when walking along a tree-lined path during the solar eclipse and seeing that the sunlight through the trees was crescent-shaped! Looking online, lots of people saw this too!

I was in Kentucky for the eclipse in 2017. I was looking around for a pin and some paper to do that, before I realized I was wearing a straw hat. It's a pinhole projector factory.

In searching for the info... Eye injuries of early solar observers - https://aty.sdsu.edu/vision/others.html

https://en.wikipedia.org/wiki/Thomas_Harriot#Sunspots

> Harriot observed the sunspot with the use of a telescope in a direct and hazardous way. ... Specifics as to how Harriot's telescope was set up remains largely unknown.

This lead me to http://galileo.rice.edu/sci/observations/sunspots.html

> Scheiner, a Jesuit mathematician at the university of Ingolstadt (near Augsburg), wished to preserve the perfection of the Sun and the heavens and therefore argued that sunspots were satellites of the Sun. They appeared as black spots when they passed in front of the Sun but were invisible at other points in their orbits. Their orbits had to be very close to the Sun for their shapes were foreshortened as they approached its edge. Scheiner observed sunspots through a telescope equipped with colored glasses.

> ...

> It was Castelli who developed the method of projecting the Sun's image through the telescope, a technique that made it possible to study the Sun in detail even when it was high in the sky.

This was a very rapid development. https://en.wikipedia.org/wiki/Benedetto_Castelli

> In December 1610, Galileo received a letter from Castelli, asking if the phases of Venus were observable through Galileo's new telescope.

This then lead lead me to https://brunelleschi.imss.fi.it/itineraries/multimedia/Helio...

> In his work on sunspots dated 1613, Galileo refers to the method invented by Benedetto Castelli for observing through the telescope and drawing—without damage to his eyes—the spots that he had discovered on the Sun's surface.

> Galileo darkens the room and aims the telescope at the Sun. At about one meter from the eyepiece, he places a sheet of white paper. The solar disk is thus projected on the sheet. To heighten the contrast of the image and thus make it easier to observe, Galileo places a darkening screen around the sheet. He now draws a circle on the sheet with the compasses and, moving the sheet closer or farther away, he finds the position where the Sun's image exactly overlaps the circle. He then marks the sunspots on the sheet. Since the Sun moves, the helioscope itself must be moved continuously in order to keep the solar image in the circle.

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