I’d like to challenge two prepositions in your question. The first is placing early astronomers in the 19th century. People look up the sky for thousands of years, and I’ll explain in a moment, how precise exactly that endeavor already became in the 19th century.

The second is the categorization of their tools as primitive. Useful lenses became available in the time around Galileo, in the early 17th century. They were good enough, that he could spot four of Jupiter’s moons and trace them continuously (so to discover, that they are indeed its moons and not some background stars).

In the following 200 years scientists calculated the distance of the other planets to the sun remarkably well. Of course, doing that only makes sense after accepting a heliocentric point of view. The first important step then was Kepler’s discovery of the planets not running on circles but being on ellipses. The next large building block was Newton’s theory of celestial mechanics.

Equipped with those theories and ever evolving optical tools let me give a specific example of what the scientists of the mid-19th century were discussing. They calculated the course of Mercury with regards to Newton’s theory and found an almost perfect match. The discrepancy was a movement of the perihelion of 43″ per century not being explainable by Newton’s mechanics. 43" is 43 times the 3600th part of a single degree. Per century.

That led them to suggest, that maybe there could be another planet very close to the sun. The proposed planet was named Vulcanus, but it was never spotted.

Only in the early 20th century this conundrum was resolved. With the advent of Einstein’s Special Relativity Theory the discrepancy could be explained in terms of relativistic effects by the gravity of the sun.

Finally, to give you a small glimpse into popular science literature of the 1920s, H.G. Wells wrote a book named “A Short History of the World”, where he muses in the second chapter about those spirally nebulas that astronomers find in large numbers in space. He reports the idea, that these are suns in their early development state while still being gas clouds. As we know now (and was beginning to become scientifically known at that time to the scientists doing the research), what those actually are are complete galaxies on their own.

H.G. Wells, A Short History of the World. https://www.gutenberg.org/files/35461/35461-h/35461-h.htm#chapII