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Secrets of the Calendarby Devon Hamilton PhD - Senior Scientist / Physics
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| Inside Secrets of the Calendar |
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January the first provides a good opportunity to take a look at our calendar and how we mark off the days. Most people don't realize it, but we have a calendar available to us just outside our door—the sky. Our modern calendar is directly derived from observations of our sky, extending deep into human history.
Often astronomers refer (perhaps egotistically) to astronomy as the "first" science, although agricultural science may have an equally valid claim. The two are intertwined in a compelling combination of Earth and sky. Early civilizations owed their existence to the development of farming, which allowed people to live together in one place and begin developing technology. However, the success of these early farmers depended upon their knowledge of the night sky.
We have no records of when our ancestors first began to notice the cyclic changes that were occurring over their heads, but there are ancient artifacts predating the earliest of humanity's first farming communities: bones marked with different shaped holes, perhaps counting off the stages in the monthly lunar cycle.
Archaeologist and writer Alexander Marshack has examined numerous bone artifacts dating back up to 30,000 years; and has concluded that they mark off the days of the lunar cycle. Marshack has suggested they were used as "remembering marks", perhaps connected to the telling of stories about the Moon.
Our ancestors could have used the slowly cycling night sky as a calendar to predict seasonal animal migrations—associating the appearance of particular patterns (the first constellations) with the migrations of prey, or perhaps to predict the beginning of the rainy season or the appearance of particular plants.
In later ages, the first farmers could have used these patterns to determine when it was best to plant and harvest. We know the ancient Egyptians depended on the flooding of the Nile for their agriculture, and kept meticulous records of when the floods happened using the night sky.
Using the annual changes to the night sky, our ancestors developed early yearly calendars that actually came very close to measuring the true year. Our modern month is directly tied to the lunar cycle—which can be a cycle especially important to early hunters (such as the carver of the bone) since the full moon provides an excellent light source for night hunting.
In our deep past, we had no need for a calendar that was more sophisticated than one which predicted seasonal changes and the lunar cycle. Our ancestors could determine these patterns simply by counting days, and if the cycles didn't quite match up, so be it.
One of the oldest calendars still in use is Stonehenge. When Stonehenge was constructed over four thousand years ago, it was carefully aligned to mark off the moment of summer solstice, the longest day of the year.
Other cultures have built similar monuments to the Sun related to the calendar:
Many different cultures used monument calendars to mark the occurrence of four important days of the year:
These days marked the beginning of the seasons and came to occupy very important roles in different cultures, especially the winter solstice.
The winter solstice marks the shortest day of the year, during the harshest of all seasons. In the northernmost latitudes, the sun doesn't even peak over the horizon. At locations such as Stonehenge (and here at the Ontario Science Centre), the Sun remains close to the horizon.
Our ancestors welcomed and celebrated the arrival of the winter solstice because they realized it foretold the lengthening of daylight, and the strengthening of sunlight as the Sun appeared higher over the horizon for longer periods of time.
It's no wonder that this date has become closely associated with a number of the most important celebrations in the different northern cultures of the world: the ancient Celts, Greeks, Egyptians and Romans all had celebrations of rebirth and renewal, and today we celebrate Hanukkah, Kwanzaa, Yule, Christmas, and of course New Year's during the same time period.
Interestingly, the days of the solstices and equinoxes (those two days, one in the spring the other in the fall, when the sun is in the sky for exactly 12 hours) actually change, and this change was discovered by the ancient Egyptians.
Thanks to their meticulous records, the Egyptians discovered that the actual date of the equinox changed slightly, about one quarter of a day each year. So over four years, the equinox (and all the other days of the year) would have shifted by one day.
This was eventually corrected for in the European calendar by Sosigenes (Julius Caeser's astronomer, and one of my favourite names) by the addition of a leap day every four years, which almost completely adjusted for the fact that it takes slightly longer than 365 days to orbit the Sun, giving us the Julian year
This, however, did not completely compensate for the shift, because the true year is actually 11 minutes shorter than the Julian year.
Even the addition of leap years didn't completely compensate for the differences. Over time the date of the equinox (and associated holidays that were marked from it, such as Easter) shifted.
Eventually a solution was arrived at in the 16th century, when Pope Gregory XIII convened a council in 1582, that derived a new system, replacing the Julian calendar. Leap days would be added every four years except in those century years that had the first two digits not divisible by four so:
This became the Gregorian calendar, which is our modern calendar that is used by most, but not all the cultures of the world. For example, the Islamic calendar uses 12 lunar months, and is about 11 days shorter than the Gregorian solar calendar—so the calendars do not match up year to year.
Read Devon's next column to learn more secrets of the calendar.
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