How do we determine geological time? Before the Calendar was invented, how could the age of the people in the bible be determined? And how many days were in a year?
The measurement of geological time can be divided into two types of measurements–relative and absolute. Concerning the relative time scale, estimates are made both vertically and laterally.
Vertically, things that are found at greater depths in the ground are considered to be older than those nearer the surface. Thus deeper fossil remains are older than those neared the surface.
Laterally, it is assumed that fossils of a given type existed during the same time period. Thus, similar fossils found at different depths in different regions are considered to be of the same age.
The absolute geological time scale has to do with radioactivity. Given radioactive isotopes decay at very specific rates. So from the relative amount of a given radioactive isotope compared with its decomposition stable isotope, rather precise estimates may be made of when the radioactive isotope was introduced into a given location. Absolute time scale estimates were begun shortly after the beginning of the last century (1910 to 1930). Beginning in the 1950s more advanced methods of radio isotope dating began to be employed, so modern estimates are much more accurate.
The estimated age of the earth has been divided up into different time periods with sophisticated names whose meanings are understood only by the geologists. Here is a list of the ages of the different periods of the earth’s history–
Cenozoic Era today to 65 mya (million years ago)
Mesozoic Era 65 to 248 mya
Paleozoic Era 248 to 543 mya
Proterozoic Era 543 to 2500 mya
Archaean 2500 to 3800 mya
Hadean 3800 to 4500 mya
So, according to the above time scale the earth is estimated to be 4 ½ billion years old.
It is vitally important to keep in mind when considering any time estimates of pre-history that they are all based on an assumption that cannot be demonstrated or verified. That assumption is that the aging processes by which these estimates are considered to have taken place over the last 4 ½ billion years have occurred at the same rate that they are occurring today. That assumes that the earth has been in a stable unchanging environment since its beginning. That belief system is called “uniformitarianism.” Uniformitarianism came into vogue at the time of Sir Isaac Newton (1643-1727), who, incidentally, held to the catastrophism theory. The theory of uniformitarianism is held so strongly by the earth scientists of today that evidence to the contrary is generally rejected out of hand.
However, there is ample evidence first published about sixty years ago that a new major planet entered in the solar system in about 1550 BC. This is the planet Venus.
To give just one piece of evidence in support of the “new planet” theory, we look at existing astrological records that are available to times predating the date in question. Before 1550 BC all the astrological records cited three main planets. From the detailed descriptions of their positional relationships with other stars they are identified as Mars, Saturn and Jupiter. After 1550 BC the records refer to four major planets. The new planet is Venus.
There is a growing opinion in astronomy that the four inner planets–Mercury, Mars, Earth and Venus, comprised principally of rock and metal, as opposed to the outer planets that are composed principally of ice and gas–could not have been formed from condensation of matter that resulted in the early solar system, and that they are all intrusions at times after the solar system was formed. (These four inner planets actually rotate within the atmosphere of the sun).
So the entire subject of geological dating is a matter of conjecture, and it would be well to keep an open mind with respect to all the current theories relating to any aspect of prehistory.
Concerning your second question. who knows when the first calendar was invented. From the record of the Old Testament rather precise relative dating is recorded. That relative dating extends into the period when calendars were known to exist. Therefore, absolute time lines can be extended back to the first relative historical dates. Precise dates exist for the birth and death of all the major Old Testament characters, from Adam to Malachi.
However, the calendar has had a rather turbulent history, which itself gives evidence of cataclysmic planetary occurrences. Both Semitic and Mayan early calendars consisted of 10 months of 36 days each. In fact, the earliest Greek calendars named the last four months of the lunar year, Seventh (September), Eighth October), Ninth (November) and Tenth (December).
The calendars in those times were lunar calendars, indicating that the lunar period was 36 days rather the current period of 28 days. However, there is historic evidence that when Venus entered the solar system it caused a major disruption in the orbit of Mars. (In the eighth century in Babylonia, Mars was called “the unpredictable planet”), In 747 BC Mars made a pass near the moon that moved it nearer to the earth and changed its orbit from a 36-day period to a 28 day period.
From that time on, the 36-day calendar did not match the lunar cycle, and frequent adjustments had to be made to have the lunar calendar concur with the solar cycle. To bring the lunar calendar in conjunction with the solar cycle, Julius Caesar, in 46 BC, made a massive change in the lunar calendar, changing the number of days in a month to the number that exist in our modern calendars, and adding two months in the middle of the year, named after Julius and Augustus–July and August.