THE ORIGIN OF CALANDER

The calendar mainly based on the motions of celestial bodies.

We know the time taken by the earth to go round the sun once is taken as one year. We also know the time for the earth to spin is taken as a day. It happens to be 365+ days in a year. That is why, a circle is divided into 360 degrees.

Moon takes roughly 29+ days to go round the earth. Hence 30 days average is taken as the month. The moon revolves around the earth 12 times a year.

Seven days makes a week. Because only seven heavenly objects were known then. Hence the week days were named after them.
Sun - Sunday, moon - Monday, mars - Tuesday, mercury - Wednesday, Jupiter - Thursday, Venus - Friday, Saturn - Saturday.

In the past, sundials (measuring time using shadow) were used to measure daytime. It had 12 equal marks. So we have "12 hours day and 12 hours night" system still.

Each hour is split into 60 minutes and a minute into 60 seconds. Similarly a degree (angle) is split into 60 minutes and 60 seconds.

English calender count the years from the birth of Christ. Other calenders follow their own system.

We can learn some mathematics from calender system.

Modular arithmetic: 10 days have gone by from last Sunday. what is the day now?
'10 modulus 7' is 3. So after 3rd day, 4th day-Wednesday is running now .
Modulus means 'divide by 7 and get the reminder only'.

Base 60 : How much time is between 9:45 a.m. and 12:30 a.m?
12:30-9:45 = 11:90-9:45 = 2:45
The answer is 2 hours and 45 minutes. Here we should bear in mind, the hour and the minute has the base of 60.

The calender becomes meaningless at the north and south poles of earth. The sun never rise or set and it only circles there. Because of the tilting of the earth every six months, "6 month day and 6 month night" prevails there.

If we happen to live in moon, we have to design a new lunar calender because a lunar day is equal to one terrestrial month.

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