The needs of modern Astronomers drove the modern clock making industry. Those needs demanded that they use clocks that measured time from noon. Astronomer's clocks eventually leaked out into popular use, replacing the O'Day system used previously. This article explores the reasons for the change.


Consider the world of astronomers like Galileo in the mid 1600s.

Observatories of their day needed to have an accurate reference clock in the building for recording night time observations taken through the telescope.

Because observations are done at night astronomers needed mechanical clocks at least good enough to run for 24 hours. Early astronomers had no access to reference clocks. They had to setup and keep their own clock along with their telescope. Their clocks would be set each day at noon when the sun passed overhead.

All calculations were done long hand. So with pencil on paper or with chalk on slate. There were no computers and no mechanical calculators. So they needed all of their various instruments to create as few calculations as possible.

There were 2 big changes Astronomers made to make their calculations simpler to carry out. The first involved the calendar, the other involved the clock.

Julian Day Numbers

The first source of trouble was the calendar. Astronomical observations do not need the standard year, month, and day divisions of the calendar.

It is far easier to calculate time across days by just assigning a number to each day. Into this need the Julian Day Number system was born. Each that everyone else knows by year, month and day is assigned a single, simple, number.

Day number 1 on the Julian Day Number system is January 1, 4713 OS. This is conceptually simple. Just count days forward from there. That January 1 was chosen because it was before all known day-accurate recorded history.

The only problem with that date is that the Julian Calendar on which it is based does not go back reliably to that era. So the starting day number for the Julian Day Number system is somewhat contrived. Actual astronomical observations did not land in that far back, so this is not a problem.

With day numbers in hand, the calculations between days became easy for Astronomers to carry out by hand. Those calculations are always just a simple difference between 2 different day numbers.

Noon Start

Julian Days on this system are also strange in that they start at noon. They do not start at midnight like dates on most other calendars. This change in start of the day makes all the astronomical observations in any given evening happen within the same calendar day. (This, of course, is for early European Astronomers.)

Using noon as the starting point meant that all hours from noon remained inside the same Julian Day Number. So hours from noon never crossed a day break before reaching 24 hours later the next day at noon. Time within a day eventually becomes some a fraction of the whole day.

Noon was the same time of day when they were already setting their clocks against the sun. This was also the time of day when they were learning the error in the recorded clock times from the previous observation day.

Using noon for the zero point on their clocks thus removed several calculation steps. This was important for reducing the workload of calculation for observed events in the sky.

Clock Makers

Clock Makers, especially from Switzerland, sold their very best, most accurate, most expensive clocks to Astronomers. Those astronomers needed the dials on their clocks set exactly this way in order to reduce their work. So they wanted nothing else on their clock faces.

Other buyers for clocks eventually wanted the same high-precision clocks used by Astronomers. This was especially so for employers who were paying their people by the hour.

So the O'Clock system of time developed for the needs of modern astronomers eventually won out over the O'Day system used in ancient times.

Inspired scripture, though, remains on the old system. This is why we need Bible Clocks.