For how much Lovecraft loved Astronomy, he found Astrology completely ridiculous calling it a pseudo-science. In fact, he actually felt stronger than that. Check out this quote, the first paragraph in his letter “Science versus Charlatanry”, from Joshi’s Collected Essays, Volume 3 (2005):
“It is an unfortunate fact that every man who seeks to disseminate knowledge must contend not only against ignorance itself, but against false instruction as well. No sooner do we deem ourselves free from a particularly gross superstition, than we are confronted by some enemy to learning who would set aside all the intellectual progress of years, and plunge us back into the darkness of mediaveal disbelief.” HPL
This letter was submitted to The Evening News and was specifically addressed to Mr. Hartmann, a proponent of Astrology.
As described by Joshi, the Evening News published a series of letters by both Lovecraft and Hartmann on Astrology and its value (if any, other than a footnote of human superstition that was originally entwined with Astronomy).
As sort of an extreme Astronomical version of Richard Dawkins, not only did Lovecraft say that Astrology is not a true science since it does not follow the same methodology and logic, he also stated that those who preach or push Astrology will drag humanity into a “new dark age”, which is funny because in the opening quote from Call of Cthulhu, Lovecraft said that if we are not careful science will do the exact same thing! However, two major differences, Call of Cthulhu is fiction and second the opening quote is a warning about how the misuse of science can negatively impact humanity, where with Astrology Lovecraft says taking it serious will undermine all of our scientific and technological effort to date (ceria the early 20th century).
To say Lovecraft had absolutely no tolerance for people who believed in Astrology or any other concept of Superstition would be an understatement. In his notes for “The Cancer of Superstition”, which was to be a book arguing against superstition to be commissioned by Harry Houdini, Lovecraft said “Lack of all scientific basis – persists only through mental indolence of those who neglect to assimilate and correlate results of modern science.” Again, this is from Joshi’s Collected Essays Vol. 3.
Based on that quote above, you get the impression that according to Lovecraft if you could not use or understand science, you were being lazy. Thus, the “lazy alternative” to the hard work of science was superstition. Given Lovecraft’s love for Astronomy, one can certainly understand how the popularity of Astrology must have really annoyed him. This gave rise to his scathing letters criticizing both Mr. Hartmann and Astrology as a whole.