This answer to If there's nothing special about Pluto, why was it discovered so early? says:

In fact, often objects are discovered to be in pictures from long ago, such as Orcus, discovered in 2004, but pictures were taken of it in 1951.

This led me to wonder about solar system objects that were in fact discovered in very old data.

Answers mentioning notable discoveries that are old but may not necessarily be the "oldest" are also are welcome!

Different but related: Examples of the oldest "precoveries"; objects that were first discovered in recent data but then looking back are confirmed in much older data

  • $\begingroup$ Galileo accidentally documenting a just-turned-retrograde Neptune in his notes on the Galileian moons is probably a contender. $\endgroup$
    – notovny
    May 18, 2020 at 12:37
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    $\begingroup$ By " discovered in old data" do you mean "The initial data used for the discovery is old at the time of discovery," or "We noticed that an object in old data is an object we now know exists from other sources." $\endgroup$
    – notovny
    May 18, 2020 at 14:15
  • $\begingroup$ @notovny the first one; "The initial data used for the discovery is old at the time of discovery." For example, somebody is looking at an old plate and suddenly says Hey! What the heck is that? $\endgroup$
    – uhoh
    May 18, 2020 at 14:35
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    $\begingroup$ Isn't there a famous painting that includes a supernova which was then thought to be a regular star? That is, it appeared to be just another star in the night sky to the naked eye, but was in fact a supernova. I don't recall how long afterwards the connection was made, but do recall it was long after the death of the artist. $\endgroup$ Aug 31, 2021 at 16:20
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    $\begingroup$ @honeste_vivere while I'd envisioned discoveries looking at emulsion on glass plates, if the ultimate answer turns out to be from an "oil on canvas plate" that would be good to know about. $\endgroup$
    – uhoh
    Aug 31, 2021 at 22:35


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