Why do you assume that people who knew astronomy in the times of the gemara didnt know about halleys comet? If it appears evrey 70 years astronomers would have noticed it within the first couple of centurys of recording astronomical calculations (which began far far before the times of the gemara.)
Actually its more like 76 years. Because unless you look at the details of the orbit you would not notice it repeating. Comets pop up on a pretty regular basis. There is a bright one every 10 years or so on average, sometimes more sometimes less. People noticed the comet, but did not really notice, “hey that comet is in the same orbit that the one that my grandfather saw was in”, without that piece you are not going to realize that is a recurring event. And how bright it appears in the sky can vary a good bit depending on how close to earth it gets and other factors.
Actually the period of the comet can vary by up to a year or two depending on various factors.
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