As mentioned earlier, the day is an obvious unit of time for people. But what about weeks, months and years?
Years are fairly straightforward. Man created the concept of a year because seasons repeat on a yearly basis. The ability to predict seasons is essential to life if you are planting crops or trying to prepare for winter. Most plants sprout and bear fruit on a yearly schedule, so it's a natural increment.
A year is defined as the amount of time it takes for the Earth to orbit the sun one time. It takes about 365 days to do that. If you measure the exact amount of time it takes for the Earth to orbit the sun, the number is actually 365.242199 days (according to Encyclopedia Britannica). By adding one extra day to every fourth year, we get an average of 365.25 days per year, which is fairly close to the actual number. This is why we have leap years that are one day longer than normal years.
To get even closer to the actual number, every 100 years is NOT a leap year, but every 400 years IS a leap year. Putting all of these rules together, you can see that a year is a leap year not only if it is divisible by 4 -- it also has to be divisible by 400 if it is a centurial year. So 1700, 1800 and 1900 were not leap years, but 2000 was. That brings the average length of the year to 365.2425 days, which is even closer to the actual number. Occasionally, we have leap seconds added or subtracted to keep things exact.
The problem with the concept of a year is that it is hard to determine the exact length of a year unless your society has fairly good astronomers. Many cultures that lacked astronomers relied on the cycles of the moon instead. A moon cycle lasts approximately 29.5 days (29.530588 days is the exact number), and it is easy for almost anyone to track the moon's cycle simply by looking at the sky every night.
The moon is where the concept of a month comes from. Many cultures used months whose lengths were 29 or 30 days (or some alternation) to chop up a year into increments. The main problem with this sort of system is that moon cycles, at 29.5 days, do not divide evenly into the 365.25 days of a year.
When you look at the modern calendar, the months are extremely confusing. One has 28 or 29 days, some have 30 days and the rest have 31 days. According to the "World Book Encyclopedia," here is how we got such a funny calendar:
This little history explains why we have 12 months, why the months have the number of days they have, why leap day falls at such an odd time and why the months have such funny names.
- The Romans started with a 10-month calendar in 738 B.C., borrowing from the Greeks. The months in the original Roman calendar were Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November and December. The names Quintilis through December come from the Roman names for five, six, seven, eight, nine and 10. This calendar left 60 or so days unaccounted for.
- The months Januarius and Februarius were later added to the end of the year to account for the 60 spare days.
- In 46 B.C., Julius Caesar changed the calendar. Ignoring the moon but keeping the existing 12 month's names, the year was divided into 12 months having 30 or 31 days, except Februarius at the end with 29 days. Every fourth year, Februarius gained an extra day. Later, he decided to make Januarius the first month instead of Martius, making Februarius the second month, which explains why leap day is at such a funny point in the year.
- After Julius' untimely death, the Romans renamed Quintilis in his honor, hence July.
- Similarly, Sextilis was renamed to honor Augustus, hence August. Augustus also moved a day from Februarius to Augustus so that it would have the same number of days as Julius.
What about weeks? Days, months and years all have a natural basis, but weeks do not. They come straight out of the Bible:
Remember the sabbath day, to keep it holy. Six days shalt though labor, and do all thy work but the seventh day is the sabbath of the Lord thy God. (Exodus 20:8)
This fourth commandment, of course, echoes the creation story in Genesis.
The Romans gave names to the days of the week based on the sun, the moon and the names of the five planets known to the Romans:
These names actually carried through to European languages fairly closely, and in English the names of Sunday, Monday and Saturday made it straight through. The other four names in English were replaced with names from Anglo-Saxon gods. According to Encyclopedia Britannica:
Tuesday comes from Tiu, or Tiw, the Anglo-Saxon name for Tyr, the Norse god of war. Tyr was one of the sons of Odin, or Woden, the supreme deity after whom Wednesday was named. Similarly, Thursday originates from Thor's-day, named in honour of Thor, the god of thunder. Friday was derived from Frigg's-day, Frigg, the wife of Odin, representing love and beauty, in Norse mythology.