How did hourglass happen? How did they even get the right amount/size of each sand particle/other physical aspects to be aligned with the time?
In: Physics
Even today, most are done with calibration. You form the basic shape, put in some sand, and see how long it takes to run through. Then you pour out excess of add a little extra to get the right time.
They calibrated them against sundials which are pretty accurate and can be constructed anywhere. A sundial uses the movements of a shadow across the floor to measure time. An hourglass is more convenient in that it is portable. They also used various other portable devices, like candle clocks and water clocks etc., all ultimately calibrated and verified against a stationary sundial.
Traditional hour glasses are open at the end and capped by a wooden cap. So you can make an hour glass but leave one end open. Then you put it next to a sundial and fill it with sand exactly when the shadow on the sundial crosses a mark. Then at the next mark you stop the hourglass and poar out the excess before you cap the other end as well. That leaves you with an hourglass that have exactly enough sand to last an hour and no more.
In ancient times water clocks were used. You would have a bowl that floated in water. Then punch a small hole in the bowl so that water would slowly leak in. It would take time for the bowl to fill up with water and sink. You would time things by the bowl. For instance you might pay for a massage that would last one bowl.
When these were made, there was no precised short units of time and no way of measuring them. You did not make a bowl that lasted 5 minutes because there was no such thing as 5 minutes.
You made a bowl that lasted a time that seemed right. Or you would measure how long a bowl took to fill compared to another bowl and slowly increase the size of the hole until they took the same amount of time.
Hourglasses were the same. The amount of time they measured was the amount of time they measured. There was no precise minute to make an hourglass that time a minute.
When mechanical clocks were invented and time got rigidly defined with precise hours, minutes, and seconds (in Roman times, the length of an hour varied by time of day and year), then hourglasses were calibrated to a particular time by adjusting the amount of sand inside them.
There’s a great HI101 podcast about timekeeping. Lots of time spent on sundials and water clocks.
Some of the things humans invented and figured out in the past just blows my mind to this day. No bullshit it’s more impressive to me that they figured out astronomy and telescopes and shit than like the some dude inventing the internet idk. Some dude was just sitting around one day and somehow figured out astronomy cause he was curious. That shits crazy, from like NOTHING. I feel like humans used to be a different breed lmfao
Edit: I am massively oversimplifying this example obviously but it still applies.
With trial and error, using different particles for a more or less constant volume. Simple as that.
They used ground up eggshells in the really early ones. Hence the german name “Eieruhr”-“Eggclock”.
I’ve got a related question: do hourglasses run faster over time as the sand inside abrades itself?
Edit: Since this seems unlikely to be answered here, I researched it and found it a bit outside of ELI5 territory. Anyway there is this thing called the Hagen-Beverloo law on the outflow of granular solids that seems to indicate that yes, provided the total mass of the particles and the size of the funnel aperturue remains constant, the hourglass will speed up if the diameter of the grains were assumed to decrease over time due to wear. Specifically given `d.I` and `d.F` representing initial and final grain diameter and `a` the funnel aperture size, the ratio of total time will change by `T(d.I)/T(d.F) = (a-d.I / a-d.F)^(2/5)` leading to the conclusion, that as seems to be intuitive, the hourglass will speed up if the sand was able to wear itself down and become smaller or alternatively to wear the aperture to become larger.
Anyone with kids: this would be a solid science fair project.
I have owned several hourglasses (and similar timepieces) and I can tell you from my personal experience, they all vary. None are exactly the amount of time they claim to measure (hour, 5 minute, etc) and they all vary due to a lot of factors. Even an hourglass that actually runs 52 minutes one day might run 57 minutes tomorrow, or it might get a clog that needs to be cleared (I give it a little shake).
So, they’re NOT accurate or precise measurements of time AT ALL. They’re more like “in the ballpark of” so getting worried about how they’re made to exacting specifications doesn’t really make sense. It’s all just “good enough” engineering.
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