From time to time I've heard it suggested that Isaac Newton was an astrologer (most recently in the comments section here), usually by way of implying that, if Newton thought astrology was OK, then it must be. Now, Newton is viewed as being one of those thinkers responsible for sparking the Age of Enlightenment and a significant contributor to the 'Scientific Revolution', so it seems a bit unlikely that he'd be deeply into the rather unscientiific tenets of astrology – but not impossible. As the late Stephen Jay Gould was fond of pointing out, it's rather unfair to view thinkers who lived hundreds of years ago through lenses focused on today. So I looked a bit more deeply.
Fortunately it's fairly easy to check these claims out. For example, Cambridge University holds a very large collection of Newton's papers, available in digitisal format. The astronomical section, which might reasonably be expected to contain notes or commentary on matters astrological, doesn't appear to do so; nor does the section of 'notes' & copies of letters on general astronomical topics. (I hadn't realised Newton was into the chronology of 'ancient kingoms': Greeks, Medes & Persians, & so on.) Cosmography and astronomy =/= astrology.
In addition, Robert van Gent has looked into the issue rather carefully. He notes that
One of the foremost Newton scholars, the English historian of science Derek Thomas Whiteside, has stated that he never found any reference to astrology among the 50 million words which have been preserved from Newton's hand.
Newton had a sizeable library of at least 1752 books – but surprisingly, while we rightly remember him for his major contributions to mathematics, physics, and astronomy, van Gent found that there were just
126 (7.2%) on mathematics, 52 (3.0%) on physics and only 33 (1.9%) on astronomy.
There were also just four books about astrology, one of which was a rebuttal of its claims. It's also noteworthy that Newton himself apparently commented to his nephew, who was gathering material for a biography, that he
was "soon convinced of the vanity & emptiness of the pretended science of Judicial astrology"
after his early studies in geometry and calculus.
Was Newton an astrologer? No.