How did they know that hourglasses measured an hour, and how did they know how much sand to put in it?

Trial and error.

“An hourglass is a piece of blown glass that is pinched in the middle. Sand is sealed inside the glass. When the glass is tipped over the sand pours slowly through the pinched center at a constant rate until all of the sand in the top flows to the bottom which is of equal size and shape. The turning of the hour glass sends the grains again from top to bottom at the same rate, thus the timer or glass will measure time equally whichever side is turned up. If the sand is measured, one can use the glass to measure the passage of time.

hour glass image.
An hour glass can be somewhat precise because the pressure at the base of a pile of sand does not increase as the height of the sand increases. Thus, sand grains will trickle through the narrow opening of an hourglass at a constant rate no matter how many are above, pushing down. This means the hour glass can be graduated to signify the passing of minutes as well as hours.”

How Clocks Work

@marinelife You must have been a reference librarian in a former life.

@janbb I spent many years as a researcher. As I told you, I love librarians!

Well, pondering this question with raised eyebrows and perhaps pursed lips…there’s probably less sand in your hour glass if you live in New York City or LA…and, lots more sand if you live in a laid-back small town. I don’t know about the sand in DC. What do you think?

I think what you may really be asking is how did ancients calibrate the first hour glasses. One site I found indicated that the precursor to the hour-glass (after sun-dials that didn’t work too well at night) was a clepsydra (water clock). A guy, Prince Amenemhet took a big bucket of water and filled it up to a specific line. Then he cut a small hole in the bottom and marked off lines on the bucket as each hour passed (probably using a sundial). All he would have to do is fill the water to the top line and he could keep track of a number of passing hours.

I imagine that the first sand guy used a similar approach. Over time, folks learned that the size of the sand grains would impact the speed of movement (and rough grains would etch the glass, making the hole bigger and speeding up time).

@Kayak8 Is that where they got the song, “Livin’ on Tulsa Time”?