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Mercury, Venus, Mars, Jupiter, and Saturn have been identified by ancient astronomers as they are visible with naked eye at night. The planet Uranus, despite being visible during very clear nights, wasn't recognized as such. Why?

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    $\begingroup$ It is very very faint. It seems entirely unsurprising that it wasn't noticed as a planet. Put yourself in their shoes. Would you notice it? $\endgroup$
    – James K
    Commented Mar 5 at 23:51
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    $\begingroup$ I'm guessing a combination of its low visibility and very long orbital period (~84 years, so a very long lifetime by ancient standards) means that nobody noticed it was "wandering" around the background of stars like the other planets did. $\endgroup$ Commented Mar 5 at 23:52
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    $\begingroup$ An astronomer in the ancient world stands a good chance of seeing Saturn complete an entire revolution, and maybe even two. It is very unlikely that even a single astronomer would ever have seen Uranus complete a cycle. $\endgroup$ Commented Mar 6 at 11:42
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    $\begingroup$ Add to that the fact that Saturn, the faintest ancient planet (comparing maximum magnitudes), is still brighter than all but two nighttime stars (Sirius is significantly brighter and Canopus is just a small bit brighter). There's no missing Saturn in the night sky if you're looking around, and seeing that it has moved over the course of a year (~12 degrees per year) is very easy. (Mercury is much more difficult to see but it moves so obviously even day to day and its brightness changes dramatically in such a way that anyone who was able to see it regularly would know it was weird.) $\endgroup$ Commented Mar 7 at 2:26

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The source of light that is Uranus was observed as far back as 128 BC. However it was misidentified as star as late as the 1760's. Then it was observed by Herschel in 1781, who also misidentified it, but as a comet. He had a telescope he built himself which would have been an impressive instrument for the day. He contacted another astronomer, one Nevil Maskelyne, who first suggested it was not a comet, but a planet. Further analysis by other astronomers made this more certain. It was essentially the orbit (not very elliptical) which seems to have been the main reason for identifying it as a planet. That and the apparent lack of a tail.

It required advances in observational accuracy and mathematical analysis of the observations to work out the orbit to work out it was a planet. These things simply were not possible to the required accuracy (or theoretical understanding) until Herschel's time.

From comments and by way of comparison, Saturn was considered a planet in ancient times. It has a shorter orbital period and is a great deal brighter than Uranus. It's relatively easy to spot this bright object whose (apparent) motion can be noticed and is easily distinguished from stars (unlike Uranus). Saturn is the 8th brightest natural object in the night sky. Uranus is the 350th. All the objects considered planets in ancient time were easier to see and notice the apparent motion of compared to stars. Uranus was just beyond that "natural" threshold where this could be done without relatively advanced technologies and theory.

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    $\begingroup$ And then there was a big to-do about how the planet should be named, because in all recorded history, nobody had ever discovered a new planet. $\endgroup$ Commented Mar 6 at 13:10
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    $\begingroup$ ...and he HAD to be accurate, because stating a body to be a planet requires absolute precision and truth and if misidentified, the person would have a become a joke especially during those times. $\endgroup$ Commented Mar 6 at 13:31
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    $\begingroup$ @A.R. You mean they could name it anything and they literally chose Uranus? $\endgroup$
    – pipe
    Commented Mar 6 at 17:48
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    $\begingroup$ @pipe Yep. Many other names were considered, including "Herschel" (imagine if the planets were Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Herschel; just doesn't feel right), "Neptune" (a fine name, but one that just punts the problem down the road a few decades), and "Georgium Sidis" (a completely ridiculous name which would have honored the British monarch at the time). $\endgroup$ Commented Mar 6 at 18:11
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    $\begingroup$ I feel like this answer doesn't actually respond to the question. It provides interesting context for the discovery of Uranus, but it doesn't really explain why it wasn't identified as a planet earlier. It didn't require advanced orbital calculations to determine that the other planets were planets, and it wouldn't have done for Uranus had it been readily visible and moved a significant amount night-to-night. Uranus is near the edge of visibility, even in dark skies. It's easily mistaken for any one of thousands of similar stars, and it moves very slowly. That's why it wasn't recognized. $\endgroup$ Commented Mar 7 at 2:20
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I like ScienceSnake's comment - a combination of faintness and its comparatively small apparent motion. It was observed in 1690 by John Flamsteed on several occasions, but because it moved so little from night-to-night, he thought it a star and identified it as 34 Tauri. Had it moved more he almost certainly would've recognised it as a local body. It's also possible he was observing around the time it went into retrograde, when it would move even less.

For the ancients to have identified Uranus as "wandering" they would have had to record the precise location of every naked eye object (4000+), and then done so again after some time, and identified Uranus as having moved. This isn't easy. It depends on where it is relative to the Earth, but Uranus moves at roughly 42 arc seconds per day. It takes roughly 45 days to move the same distance in the sky as the apparent diameter of the moon. If you made two star charts a month apart, you'd have to accurately identify that an object at the limit of visibility had moved less than the diameter of the moon - and to have done so without any instruments.

I believe had it been brighter - say as bright as Saturn - they would have discovered it. As it was, it was just too dim and too slow to notice.

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It would require a lucky coincidence

As noted by others, Uranus is faint and easy to misidentify as a fixed star. However, Bourtembourg 2013 claims that Uranus was observed already in 128 BC by Hipparchus and recorded in his star catalogue (that's the source for this claim on Wikipedia). They mention that the goal of the catalogue was to notice changes in the sky. So if somebody followed up on the (likely extremely tedious) observations by Hipparcus a couple times in the following years, they could have plausibly noticed the movement. Many later astronomers did notice that the star from the catalogue was missing, so it doesn't seem such a stretch that movement could have been noticed as well if the followup observation was made early enough.

Bourtembourg even argues that at the time of observation, Uranus must have been apparently almost motionless (due to just transitiong from apparent prograde to apparent retrograde motion), because otherwise Hipparchus would have noticed Uranus moving over consecutive observations. They then use this claim to support a particular dating of the observation time.

So taking Bourtembourg at their word, it seems that if either: a) Hipparchus did his observations at a different time or b) somebody actually tried to use the catalogue to check for motion in the following years, Uranus would at least rise suspicion and with a bit more luck it could have been recognized as a planet.

EDIT: But note that Hipparchus's original observation and his creation of a catalogue is already quite a lucky coincidence, so I'd say the ancients would need huge luck to classify Uranus as a planet.

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    $\begingroup$ Reading the paper it seems to me to contain an lot of supposition and assumption rather than being a truly convincing demonstration that Uranus really was the observation. I wonder how this is viewed by more qualified scholars of astronomical history? $\endgroup$ Commented Mar 6 at 13:49
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    $\begingroup$ (the "more qualified scholars" there was meant to be in comparison me, reading it back I fear that wasn't clear) $\endgroup$ Commented Mar 6 at 14:32
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    $\begingroup$ @JackAidley I am not a scholar of history, neither of astronomy, but my impression is that whether the Uranus observation was really made or not, there is little doubt that Hipparchus and other ancient astronomers were in principle able to measure the position of objects as faint as Uranus with sufficient precision to notice the movement. However, there are many such objects and observations would have to be timed well and record keeping would have to be good, etc. so overall it is quite unlikely, but not completely inconceivable. $\endgroup$ Commented Mar 6 at 14:42

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