English poem

• when I heard the learn'd astronomer: apostrophe is used to show omission of letters: meaning very knowledgeable.
• astronomer: a scientist who studies the stars and planets
• when I sitting heard the astronomer where he lectured with much applause in the lecture-room: it tells us that the lecture was good
• how soon unaccountable became tired and sick: he doesn't not know why he is tired-unexplainable
• till rising and gliding out I wander'd off by myself: choice of words, emphasises how quietly he left the lecture room