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!

  • $\begingroup$ Galileo accidentally documenting a just-turned-retrograde Neptune in his notes on the Galileian moons is probably a contender. $\endgroup$ – notovny May 18 '20 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 '20 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 '20 at 14:35

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