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Welcome to our Physics lesson on Calendars, this is the fourth lesson of our suite of physics lessons covering the topic of The Moon's Movement. Eclipses. Calendars, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
Humans invented calendars for the purpose of orientation in time. All calendars are based on the three major cycles that are visible from the Earth:
For many centuries, it has been a very challenging task to find a satisfactory compliance between these three cycles and to form a unique calendar. Thus, the calendar used by the majority of countries (and officially by all countries) in the world is the "Gregorian calendar". It has 365 days except the years divisible by 4, which have 366 days. However, not all years divisible by 4 have 366 days; those years ending with a double zero and with the first two digits not divisible by 4 (for example 1900, 2100, etc.) have 365 days. This is done for compensation purposes, as one solar year does not last exactly 365.25 days but slightly less (365.2422 days).
Years that have 366 days are known as leap years. The additional days corresponds to February 29 (regular years have 28 days in February).
How many days have elapsed between January 1st, 1700 to January 1st, 2024?
From 1700 to 2024 there are 2024 - 1700 = 324 years elapsed. If this number is divided by 4, we obtain 324/4 = 81(1). But 1800 and 1900 have not been leap years, as 18 and 19 are not divisible by 4. Thus, the number of leap years during the given period is 81 - 2 = 79.
Hence, the number of days elapsed during the given period is
Prior to Gregorian calendar introduction in 1582, a similar calendar (Julian calendar) was used in most of the world. The main difference between Julian and Gregorian calendars is that an average year in Julian calendar is 365.25 days while an average year in Gregorian calendar is 365.2422 days. The Gregorian calendar is the normal calendar we currently use to determine the date. The Julian calendar was used from 46 B.C to 1582. Then, it was replaced by the Gregorian calendar. In Christian Orthodox countries of Eastern Europe (except Greece and Cyprus) the Julian Calendar is still in use when performing religious practices. Thus, in these countries Christmas is not celebrated on December 25 but on January 7 due to the shift in dates caused by using different calendars. When this slight difference in time (365.25 - 365.2422 = 0.0078 days is multiplied by 2068 (2024 + 46 = 2070 is the number of years elapsed from the first introduction of Julian calendar to this date), we obtain:
This gap is somehow reduced by corrections made during the last centuries. However, there is still a 13 days gap between these two solar calendars.
Muslim countries on the other hand, use lunar calendars in their religious practices. This calendar, based on the Moon's phases, is about 11 days shorter than solar ones because one lunar month lasts for 29.53 days and when this number is multiplied by 12, it gives 354.36 days. Therefore, the duration of lunar months is 29 or 30 days, unlike solar calendars where a month is 30 or 31 days long (except February).
The common feature all calendars in use share is the number of months (12).
The current year (2024) corresponds to the years 1445 - 1446 in Muslim countries. When did this (lunar) calendar start?
The number of days elapsed since the beginning of Muslim (lunar) calendar is
When converted to solar (tropical) years we obtain
Since we are in 2024, the lunar calendar used in Muslim countries did start in 2024 - 1402 = 622. This is the year according the Gregorian calendar when the Muslim lunar calendar started.
You have reached the end of Physics lesson 22.3.4 Calendars. There are 5 lessons in this physics tutorial covering The Moon's Movement. Eclipses. Calendars, you can access all the lessons from this tutorial below.
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