Why did the Buddha denounce astronomy as a wrong means of livelihood?

Question

Whereas some ascetics and Brahmins make their
living by such base arts as predicting an eclipse of the moon,
the sun, a star; that the sun and moon will go on their proper ,
course - will go astray; that a star will go on its proper course
- will go astray; that there will be a shower of meteors, a blaze -
in the sky, an earthquake, thunder; a rising, setting, darkening,
brightening of the moon, the sun, the stars; and 'such
will be the outcome of these things', the ascetic Gotama refrains
from such base arts and wrong means of livelihood.

Answer 1

You omitted a part at the start:

'Whereas some recluses and brahmins, while living on the food offered by the faithful, earn their living by a wrong means of livelihood, by such debased arts as predicting: there will be an eclipse of the moon, an eclipse of the sun, an eclipse of a constellation..."  - Brahmajala Sutta

These were individuals who pretended to be holy and gained favors from people by predicting these events. The difference between astronomers and these people is that astronomers are scientists who investigate and study the events outside earth for the purpose of understanding the universe. They do not make a living on having religious followers who think that astronomers are holy beings deserving worship and veneration.

So to say that the Buddha denounced astronomy is misleading.
 

Answer 2

Buddha's way was seeing things internally all his awakenings , realisations came from inside not by seeing externally.