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| MEASURING CALENDAR TIME |
Ancient Calendars
Humans have always had a need to tell time -- in ever shorter periods. Everywhere in the world we find the remains of monuments and buildings designed to measure and predict the changes in day and night, phases of the moon, and the cycle of yearly seasons. Some ancient calendars even predicted lunar and solar eclipses accurately.
Aztec Calendar Stone and Stonehenge
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The ancient people of Central and South America developed a complex calendar system founded on the natural cycles of time they observed. This Aztec calendar stone, carved from a piece of basalt rock weighing over 22 metric tons, has sun gods at its center, to represent past and present eras. They are encircled by a number of segmented rings, that show the divisions of time important to Aztec culture and daily life.
As for Stonehenge, in England, no one is really sure who built it or why. Early theories were that it was constructed by the Druids or even the Romans, but they were disproved in the 20th century, when archeologists discovered that construction on Stonehenge began over 2,000 years before the Celts or Romans had even reached England. Today we believe that Neolithic peoples of the British Isles started building the Stonehenge monument around 5,000 years ago.
We're almost certain that Stonehenge was a place for religious rituals and ceremonies. The construction leads us to believe that the early architects were sun worshipers, because the entrance faces directly towards the sunrise on June 21st, the summer solstice or first day of summer. Newgrange, an Irish monument built around the same time was lined up with the sunrise on December 21st, the winter solstice or first day of winter.
American astronomer Gerald Hawkins suggested in the early 1960s that Stonehenge was an observatory and a calendar. He suggested the builders used it to predict the phases of the moon and the seasons. This theory is still widely held today, even though there are a number of uncertainties. For more information on Stonehenge, do a Web search.
All these ruins and ancient calendars teach us one thing. Time measurement is and always has been based on the cycle of natural events that happen day after day and year after year as our planet travels around the sun and our moon travels around us.
Units of Time we Measure on a Calendar
Short units of time -- Days, Hours, Minutes and Seconds are measured on a clock.
Longer intervals are measured on a calendar.
A year is the time it takes the Earth to orbit around the sun.
A day is the time it takes the Earth to rotate once on its axis.
An hour is one twenty-fourth of a day.
A month is one-twelfth of a year,
a season is 3 months (really 13 weeks) or ¼ of a year.
A week is the only calendar time interval that is not based on a natural cycle,. There is no heaven or earth event that takes 7 days. The week is based on the tradition of the Sabbath -- the 7th day is a day of rest -- so people divided each month into about 4 weeks. Thank goodness they did or we wouldn't have weekends!!
The first day of the week is Sunday -- named for the Sun. Monday is named for the moon. In French, Monday is Lundi -- and moon is Lune. The word "Month" comes from moon as well. Lunar calendars like that of the Chinese and the Jews are based on the Moon. A lunar month is either 29 or 30 days because the moon orbits the Earth in 29.5 days. Since a solar year is 365¼ days, it takes only 2 and ½ years for the Lunar calendar to be a full month behind the solar one. To avoid confusion, we use the familiar solar calendar thoughout the world today, in daily life as well as commerce and trade. Religious organizations still use ancient lunar calendars to determine the correct dates for certain holidays such as Easter, that date back to ancient times and events.
The Calendar Year
A year or the time it takes the Earth to orbit around the sun is equal to 365¼ days -- a number that doesn't divide by 12. But because people had used periods of 12 units to measure the hours in a day, they wanted to do the same with months in a year. This is why we have 12 months in a year but we don't have the same number of days in every month. The longest are 31 days, the shortest 28. The rhyme to remember which months are long and which are short goes:
30 days has September, April, June and November,
When short February's done, all the rest have 31.
Writing Days and Dates
We label every day in the calendar with a name and number for the day, a month and a year.
The 7 days of the week,
Sunday, Monday, Tuesday, Wednesday, Thursday, Friday and Saturday
are named after planets and gods from many cultures. So are the 12 months.
There are a number of ways to write and read dates. On most official documents, the order is set by the letters D - day, M - month and Y - year. On checks, legal and business documents, the accepted format doesn't include the weekday -- just the digits. The 14th of October, 2004 would appear as 14/10/04. But, in normal conversation, we usually say the month name before the number of the day, so we could mistake 2/11/05 for February 11th, 2005 -- when it really means November 2nd, 2005. The proper way to write it in words or digits is Day, Month and Year. When asked for the date, we answer February 22, 2007 in words, and 22/2/07 in digits. If asked for day and date, we can only answer like this: Thursday, February 22, 2007.
When we write the date with words instead of just digits, we put a comma between the name of the weekday, the month and date, and the year. The first Monday in the second month of this calendar is Monday, February 5, 2007. One week before it, was Monday, January 29, 2007. The last day of August is Friday, August 31, 2007.
date format: May 7, 2000 or 7/05/00
day and date format: Sunday, May 7, 2000
Why and When Do We Have Leap Years?
Because the Earth takes 365 and ¼ days to complete a turn around the sun, after four times around, our calendars will have missed counting a whole day. That's why, every 4 years, we have a leap year, in which February lasts for 29 days. Leap years only happen in even numbered years (a multiple of 4 is an even number). 2004 was a leap year. People born on February 29th -- known as "leap day" -- celebrate their birthday on March 1st most of their lives.
Elapsed Calendar Time
Banks, businesses and other commercial institutions always need to know how long it is between this date and that. Because there is no "standard" month, without the help of a computer, we have to either count the elapsed time on a calendar or figure it out month by month and year by year -- remembering to count "leap days". Many computer programs will quickly tell us how long it is from March 5th, 1999 to November 7th, 2006 because they have built in calendars.
Changing or Converting Calendar Units
As always, when we change from big units to small, we multiply -- because one big unit contains lots of small ones. When we change from small units to big ones, we divide -- because a whole lot of small units count as 1 big one.
Calendar Equivalences
7 days = 1 week
365 days = 52 weeks = 12 months = 1 year
Examples: Fill in the blanks
| a) 23 weeks = ___ days 1 week = 7 days, so 23 × 7 = 161 days |
b) 28 days = ____ weeks 1 week = 7 days, so 28 ÷ 7 = 4 weeks |
c) ½ year = ___ months 1 year = 12 months, so ½ × 12 = 6 months |
Now get a pencil, an eraser and a note book, copy the questions,
do the practice exercise(s), then check your work with the solutions.
If you get stuck, review the examples in the lesson, then try again.
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Practice Exercise
1) Convert these units of time:
| a) 5 days = ____ hours | b) 48 months = ____ years | c) 3 years = ___ weeks |
| d) 15 years = ___ months | e) 112 days = ___ weeks |
2) Use the calendar to answer these questions:
a) Write the date in words and digits for the second Friday in April.
b) Write the day and date for the first Thursday in September.
c) Write the day and date for 3 days after the last Monday in February.
3) Write these dates in words:
| a) 07/12/02 | b) 25/11/04 | c) 10/10/05 |
4) What is the biggest number that can replace the ? question mark?
| a) 07/ ? /02 | b) ? /11/04 | c) 10/10/ ? |
Solutions
1) Convert these units of time:
| a) 5 days = ____ hours 1 day = 24 hours, so |
b) 48 months = ____ years 12 month = 1 year, so |
c) 3 years = ___ weeks 1 year = 52 weeks, so |
| d) 15 years = ___ months 1 year = 12 months, so |
e) 112 days = ___ weeks 7 days = 1 week, so |
2) Use the calendar to answer these questions:
a) April 13, 2007 in words, 13/04/07 in digits.
b) Thursday, September 6, 2007.
c) Thursday, March 1, 2007.
3) Write these dates in words:
| a) 07/12/02 December 7, 2002 |
b) 25/11/04 November 25, 2004 |
c) 10/10/05 October 10, 2005 |
4) What is the biggest number that can replace the question mark?
| a) 07/ ? /02, ? = 12 | b) ? /11/04, ? = 31 | c) 10/10/ ?, ? = this year |
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