LS: Speaking of Science Columnn November 17, 2008

Calendars are plotted on lunar and solar cycles

Throughout recorded history people have endeavored to devise a method for recording and predicting important events and special dates. For the Egyptians, it was necessary to be able to predict when the annual floods of the Nile would occur in order to know when to plant crops.

Several religions needed a way to predict events of importance and significance to their particular belief system. With some calendars, such as with the Chinese calendar, years are counted in cycles, with no particular cycle being the first.

A calendar also can chronicle past dates of importance (such as your birth date).

Calendars are constructed around two observable astronomical events: the lunar cycle and the solar cycle. One lunar cycle would be from one full moon to the next full moon and is called a synodic month. The ancient Egyptian calendar had 12 months of 30 days. This gave a year in the Egyptian calendar that was 360 days long. To keep the calendar somewhat in sync with the solar year, five days were added to the end of the calendar, and they became feast days to honor particular deities of their time.

The synodic month is averaging 29.5305889 days long. It is gradually increasing to where, at the present rate, it will be one day longer in about a million years. The solar cycle encompasses one complete journey around the sun by the planet Earth. The time from a fixed point like a solstice or equinox back to that point comprises a tropical year. A tropical year is 365.242190 days but is getting gradually shorter in number of days because of the Earth’s rotation gradually slowing.

As the Roman influence began to spread, they attempted to devise a calendar that would suffice for their world. At the time, the Romans believed that even numbers were unlucky so their months were 29 or 31 days long with the exception of February, which was 28 days long. This gave 355 days in their year. This did not match with the tropical year, so a month called Mercedonius of 22 or 23 days was invented and added every second year.

The added month was not sufficient to keep the calendar in sync with the tropical year. Julius Caesar in 46 B.C. made a year that was 455 days long to bring the calendar back in step with the seasons.

Then, using the length of the solar year as 365 days and six hours, a new calendar was formulated.

The months were 30 or 31 days in length and every four years an extra day was added to take care of the 6 hours. This made a calendar that was 11 1/2 minutes longer than the actual solar year and, after a few centuries, it got out of sync with the seasons.

The seven-day week was most likely devised from the Genesis account of creation where God made the earth in six days and rested on the seventh.

In 1545 A.D., the Council of Trent authorized Pope Paul III to make corrections to the calendar to bring it more closely in harmony with the seasons. The revisions were completed in 1582 A.D. and Pope Gregory XIII decreed that Thursday, Oct 4, 1582, was to be the last day of the Julian calendar and the next day would be Friday, Oct. 15.

Every fourth year is a leap year unless it is a century year that is not divisible by 400. The year 1700 would not be a leap year, but the year 1600 would.

Even with the new leap year rule, the calendar year is still about 26 seconds longer than the Earth’s orbital period but will take more than 3,000 years to build up to a single day.

And where did we get the names of the months? January through August are named after gods or festivals, and the months September through December by the prefixes for the numbers seven through 10.


Jack Costello taught in School District 51 for 37 years, the last 32 years at Fruita Monument High School, where he taught mainly physics and chemistry.

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