What observable characteristic made Herschel realize the object he found in 1781 was a planet and not a star?
Answer
It exhibited a discernible disk and shifted position against background stars over successive nights
The object displayed a visible disk and, critically, showed positional change relative to fixed stars over multiple nights, confirming it was orbiting the Sun.
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Sir William Herschel -- "The King's Astronomer" | David Rives
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