Mean Time

Sundials track time based on direct movement of the sun. The lengths of days measured this way differs slightly from day to day. Sundials are not accurate enough to detect this. But, high quality pendulum based mechanical clocks, and the science that used them, needed day lengths that were the same.

Local Time

Time measurement at all places on earth is related to the rotation of the earth in relation to the sun. From observers on the ground the sun is moving overhead, while the earth is the fixed point of reference.

Clocks in this frame of reference are set when the sun is immediately overhead. That point in time is called noon.

Setting clocks this way links the passage of the sun overhead to time on the ground. Setting time on clocks makes those clocks indicate time the same way as on sundials at the same location.

But, there is a problem. Once clocks were accurate enough, and once astronomy had developed enough, time on mechanical clocks diverged from time on sundials.

Solar Time

Sundials are telling time on a system that ultimately became known as solar time.

The length of solar days as indicated on sundials differs by the distance of the sun from the earth. At some times of year the solar day can be as much as 21 seconds less than 24 hours. At other parts of the year the solar day can be as many as 29 seconds longer than 24 hours.

Because Earth's orbit around the sun is an ellipse, there are 2 times in any given year when the sun is near to the Earth. And, there are 2 times in any given year when the sun is far from the Earth. These times do not align with the seasons.

Here is a table from Wikipedia for those times in the year 1998.

Note that these calendar dates and differences in day lengths are not fixed. They shift slightly from year to year. Over the span of 1000s of years, these dates and lengths can land on all dates throughout the calendar year.

Does It Matter?

To anyone telling time with a sundial, even a very large sundial, this difference does not matter. But, to astronomers carefully measuring the time of events in the sky, this difference in day length matters to their calculations.

For the use of telling time in a prophetic sense this is way beyond the accuracy supplied by prophetic events in scripture. So this precision and name change does not matter.

Note that Passion Week has hour accurate precision to the time references given there. But, the schedule of events in Passion Week scales out against other timelines. Those other timelines are often day accurate and transposed and scaled in various ways. But, these other prophetic uses are mostly accurate to the calendar day or longer.

Note the likely exception to this is the battle schedule for events during World War II. There may be a prophetic schedule in that war with accuracy to the hour or better. That era has not yet been studied.

This does beg a different question. Has the rotational speed of the earth changed? Has the orbit of Earth itself changed enough to matter?

So even if scripture has accurate dates within itself, does that accuracy extend to the modern era?

There are enough fulfillment matches against modern day-accurate predicted dates to suggest there have not been Earth changes large enough to make this into a problem.

Sundial accurate time is good enough for normal prophetic uses of Bible Clocks.

Mean Time

To even out the day lengths meant creating a different system of time from that indicated on actual sundials. This was done by averaging out the different day lengths. In other words, to take the mean of the solar day lengths. This new time system became known as Mean Time.

A related problem was to start building references devices and places that all scientific uses of clocks could calibrate against. The place that won out was Greenwich Royal Observatory near London. We turn there next.