How was time measured for the purpose of Zmanim before we had accurate clocks like we do today? Was it possible to accurately determine Chatzos (at night) or the different watches of the night? I know the Gemara in Brachos suggests some ways to tell, but these seem to have more of a mystical significance than a physical one or at least I do not understand how someone could accurately determine time based on these at night.
For nighttime, they could use water clocks.
[Jastrow (in the introduction to his dictionary, and under s.v. ארפכס) argues that one place in the Midrash (Bereishis Rabbah 4:4) and two in Mishnayos Kelim (14:8 and 30:4) refer to such a device, though most of the commentaries explain it as something like a funnel or a colander.]
Chaim, well said. You can certainly consult a historian of timekeeping and there must have been some decent technology, but I strongly suspect that with regards to many things, it was about best effort; the Chayei Adam says as far as when shabbos ends, "wait until all doubt is gone from your heart."
Anything related to sunlight, people just looked up and had a good sense of it (which most of us no longer have).
If I understand you correctly, your question is whether they were giving general time frames or precise measurements. I think both. There is a precise time that can be measured, even if they didn't have precise enough instruments to do so themselves.
Before modern instruments existed, very advanced systems for measuring time and navigation at sea were developed. Don't underestimate the ancient societies that used astronomy to not only tell time but navigate at sea and in the wilderness.
It's clear from the Mishna in (פסחים (פרק א משנה ד that the population at large had no idea how to accurately tell time.
Which is why on ערב פסח they had to stop eating Chametz 2 - 3 hours before the designated time of midday:
רבי מאיר אומר, אוכלין כל חמש, ושורפין בתחילת שש; רבי יהודה אומר, אוכלין כל ארבע, ותולין כל חמש, ושורפין בתחילת שש
Although we assume that our methods of time-keeping are much more accurate than those of previous periods in history, this is not entirely true. Over the course of history there have been very accurate methods of telling time which could even be used in the absence of the sun.
Some of these methods include:
- Water Clocks - Time is measured by the regulated flow of liquid into or out from a vessel where the amount is then measured to determine the time
- Astronomical Time Telling Methods - Observing constellations, sun and moon location, etc.