With thanks to Hank Green and _3Jane, I fell down a rabbit hole: https://bsky.app/profile/hankgreen.bsky.social/post/3lefcief4kk2m
Venus is bright enough to cast visible shadows
This struck me as both obvious and also very surprising. Obvious because "visible" is load bearing -- anything that emits visible light casts a shadow, you just need sensitive equipment to see it and get rid of unwanted light. On the other hand, if "visible" here means "with the naked eye", then that's wild. I had to know more.
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So, off to my favourite search engine I go. My main question: Can you see the shadow of Venus with the naked eye?
By gut, I knew it would be a rare event. You'd need Venus visible during astronomical night, during the new Moon, you'd need to be well away from light pollution, and you'd need a clear sky.
My first hit (that wasn't about Astrology >.<) was http://astroblogger.blogspot.com/2005/11/searching-for-venuss-shadow.html, which mostly confirmed what I had suspected. After Astronomical twilight, clear skies, etc.
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That gave me the answer I was looking for: You can see Venus' shadow and gives a procedure for doing so:
when the sky is dark but Venus is still up. you will also need a white screen of some sort to see a shadow against. Then turn your back on Venus, hold an object such as your hand in front of the screeen, and if your eyes are dark adapted you should see a shadow.
But more than that, it gave a link to DigitalSky... someone had apparently photographed the effect!
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Unfortunately, it was a dead link: http://www.digitalsky.org.uk/venus/shadow-of-venus.html. However, the Internet Archive has it!
https://web.archive.org/web/20111220075632/http://www.digitalsky.org.uk/venus/shadow-of-venus.html
As a side bar, I love the Wayback Machine for this. This is incredibly interesting stuff, someone went out of their way to make photos of something genuinely fascinating, and the IA saved it.
Not only does this blog have photos -- some enhanced or long exposure -- but the opening line is yet another lead:
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When I was a young boy, I recall reading a book written by Patrick Moore in which he mentioned the fact that three normal (excluding bright fireballs and perhaps artificial satellites) bodies in the night sky were capable of casting a shadow on Earth*.
And later:
- Apparently it's also possible to see a shadow cast by the light of Jupiter and Mars.
Although I recognized Sir Moore's name immediately, I didn't realise he'd written any books. The rabbit hole continues!
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I found Sir Moore's Wikipedia page, which had a bibliography. which listed a 1956 book "The Planet Venue". Off I went again to see what he had written about it.
Again, the Internet Archive had the book in question, so I got it and started reading through.
Shadows cast by the planet have also been recorded since olden times. Pliny noticed them;[6] so did the Greek astronomer, Simplicus, in the sixth century,[7] [...] I have myself seen the shadow cast by Venus when at maximum brilliancy -p.22
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That reference, [6] is to Pliny's "Naturalis Historia", Book 2, parts 6 and 8. Using Wikisource's copy of Holland's 1847 translation, in part 8, discussing the various "wandering stars", this is in the description of Venus:
Now this planet, in greatnesse, goeth beyond all the other five: and so cleere and shining withall, that the beames of this one starre cast shaddowes upon the earth
Shadows cast by Venus, documented by Pliny the Elder, circa 77 CE.
https://en.wikisource.org/wiki/The_Natural_History_of_Pliny_(Holland)/Book_2#8
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This fully answers my question: Yes, you can see the shadow of Venus with the unaided eye, you just need the right conditions.
This opens several new questions: When will the right conditions happen for me? Also shadows from Jupiter and Mars?
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