Given the theme of “When I Heard the Learn’d Astronomer,” why does the speaker call the astronomer “learn’d”?

The astronomer is called “learn’d” because he has formal knowledge of astronomy and explains it through lectures, charts, and calculations. The speaker is not interested in experiencing the stars through those methods; he leaves to look at them quietly for himself. So the astronomer’s knowledge is real, but it does not appeal to the speaker.

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