The Gregorian calendar is a solar calendar that is currently used worldwide. It was introduced in 1582 by Pope Gregory XIII as a modification of the Julian calendar to correct its small error in specifying the tropical year. The Gregorian calendar averages 365.2425 days per year via a rule that 3 years out of every 400 will not be leap years, keeping its drift from the vernal equinox at approximately 1 day every 3,030 years. It consists of 12 months with 28-31 days in a common year and 29 days in February of a leap year, which occurs when the year is divisible by 4, except for centennial years not divisible by 400.

1. A Case Study on
Gregorian Calendar
Submitted By:
L.Ramkiran
Submitted to:
G.S.Gisa

2. WHAT IS A CALENDAR?
• A calendar is a system of organizing days for social, religious, commercial or administrative purposes.
• This is done by giving names to periods of time, like days, weeks, months and years.
• A date is the designation of a single, specific day within such a system.
• A calendar is also a physical record (often paper) of such a system.
• A calendar can also mean a list of planned events, such as a court calendar or a partly or fully
chronological list of documents, such as a calendar of wills.

3. WHY DO WE NEED A CALENDAR?

4. THE MAIN REASONS FOR USING A CALENDAR.
• Planning the daily activates
• Keeping a track of events
• Staying organized and enhancing productivity
• Planning efficiently and meeting the deadlines
• Remembering birthdays and keeping commitments
• Managing the daily schedule
• Remembering important festival dates and many more…..

5. WHEN WAS THE FIRST CALENDAR CONSTRUCTED?
• British archaeology experts have discovered what they believe to be the world's oldest 'calendar', created by hunter-
gatherer societies and dating back to around 8,000 BC.
• The Mesolithic monument was originally excavated in Aberdeen shire, Scotland, by the National Trust for Scotland in 2004.
Now analysis by a team led by the University of Birmingham, published today (July 15, 2013) in the journal Internet
Archaeology, sheds remarkable new light on the luni-solar device, which pre-dates the first formal time-measuring devices
known to Man, found in the Near East, by nearly 5,000 years.
• The capacity to measure time is among the most important of human achievements and the issue of when time was
'created' by humankind is critical in understanding how society has developed.
• Until now the first formal calendars appear to have been created in Mesopotamia c, 5000 years ago. But during this
project, the researchers discovered that a monument created by hunter gatherers in Aberdeen shire nearly 10,000 years
ago appears to mimic the phases of the Moon in order to track lunar months over the course of a year.

6. TYPES OF CALENDAR
• There are umpteen number of calendars as each civilization had their own calendar.
• But they all are based on these below mentioned types:
• Fixed (No. of days varies based on the civilization)
• Lunar
• Solar
• Lunisolar/Seasonal

7. LUNAR CALENDAR
• A lunar calendar is a calendar based upon the monthly cycles of
the Moon's phases (synodic months).
• List of lunar calendars
• Gezer Calendar
• Haida
• Islamic calendar
• Nepal Sambat
• Javanese calendar
• Assyrian calendar
• Yoruba calendar
• Igbo calendar

8. LUNISOLAR CALENDAR
• A lunisolar calendar is a calendar in many cultures whose date indicates both the Moon phase and the
time of the solar year. If the solar year is defined as a tropical year, then a lunisolar calendar will give an
indication of the season; if it is taken as a sidereal year, then the calendar will predict
the constellation near which the full moon may occur.
• As with all calendars which divide the year into months there is an additional requirement that the year
have a whole number of months. In this case ordinary years consist of twelve months but every second
or third year is an embolismic year, which adds a thirteenth intercalary, embolismic, or leap month.
• The Hebrew, Jain, Buddhist, Hindu and Kurdish as well as the
traditional Burmese, Chinese, Japanese, Tibetan, Vietnamese, Mongolian and Korean calendars (in
the east Asian cultural sphere), plus the ancient Hellenic, Coligny, and Babylonian calendars are all
lunisolar.

9. SOLAR CALENDAR
• A solar calendar is a calendar whose dates indicate the season or almost equivalently the
apparent position of the Sun relative to the stars.
• The oldest solar calendars include the Julian calendar and the Coptic calendar. They both have a year of
365 days, which is extended to 366 once every four years, without exception, so have a mean year of
365.25 days. As solar calendars became more accurate, they evolved into two types.
• Bengali calendar (National and official calendar in Bangladesh)
• Iranian calendar (Jalāli calendar)
• Indian national calendar (Saka calendar)

10. FIXED DAYS
• They just have fixed number of days for a year and they repeat in cycles as time proceeds.
• Examples are:
• Egyptian calendar- 365 Days
• Tonalpohualli- 260 Days
• Qumran Calendrical texts- 364 days.
• Pawukon Calendar- 210 days.

11. SO WHICH CALENDAR DO YOU THINK GREGORIAN
CALENDAR IS?
Solar Calendar

12. HISTORY OF GREGORIAN CALENDAR
• It is named after Pope Gregory XIII, who introduced it in October 1582. The calendar spaces leap
years to make the average year 365.2425 days long, approximating the 365.2422-day tropical year that
is determined by the Earth's revolution around the Sun.
• The calendar was developed as a correction to the Julian calendar, shortening the average year by
0.0075 days to stop the drift of the calendar with respect to the equinoxes. To deal with the 10 days'
difference (between calendar and reality) that this drift had already reached, the date was advanced so
that 4 October 1582 was followed by 15 October 1582.
• There was no discontinuity in the cycle of weekdays or of the Anno Domini calendar era. The reform
also altered the lunar cycle used by the Church to calculate the date for Easter (computus), restoring it
to the time of the year as originally celebrated by the early Church.

13. THERE’S A RULE FOR A LEAP YEAR IN THIS CALENDAR
• Every year that is exactly divisible by four is a leap year, except for years that
are exactly divisible by 100, but these centurial years are leap years if they are
exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not
leap years, but the year 2000 is.

14. MONTHS IN GREGORIAN CALENDAR• Months
• The Gregorian calendar continued to employ the Julian months, which have Latinate names and irregular numbers of days:
• January (31 days), from Latin mēnsis Iānuārius, "Month of Janus", the Roman god of gates, doorways, beginnings and endings
• February (28 days in common and 29 in leap years), from Latin mēnsis Februārius, "Month of the Februa", the Roman
festival of purgation and purification, cognate with fever, the Etruscan death god Februus ("Purifier"), and the PIE word
for sulfur
• March (31 days), from Latin mēnsis Mārtius, "Month of Mars", the Roman war god
• April (30 days), from Latin mēnsis Aprīlis, of uncertain meaning but usually derived from some form of the verb aperire ("to
open") or the name of the goddess Aphrodite
• May (31 days), from Latin mēnsis Māius, "Month of Maia", a Roman vegetation goddess whose name is cognate with
Latin magnus ("great") and English major
• June (30 days), from Latin mēnsis Iūnius, "Month of Juno", the Roman goddess of marriage, childbirth, and rule
• July (31 days), from Latin mēnsis Iūlius, "Month of Julius Caesar", the month of Caesar's birth, instituted in 44 BC as part of his
calendrical reforms
• August (31 days), from Latin mēnsis Augustus, "Month of Augustus", instituted by Augustus in 8 BC in agreement with July
and from the occurrence during the month of several important events during his rise to power.
• September (30 days), from Latin mēnsis september, "seventh month", from its position in the Roman calendar before 153 BC
• October (31 days), from Latin mēnsis octōber, "eighth month", from its position in the Roman calendar before 153 BC
• November (30 days), from Latin mēnsis november, "ninth month", from its position in the Roman calendar before 153 BC
• December (31 days), from Latin mēnsis december, "tenth month", from its position in the Roman calendar before 153 BC

15. A SONG ON HOW TO
REMEMBER THE NO. DAYS
IN EACH MONTH.


16. WEEKS IN GREGORIAN CALENDAR
• In conjunction with the system of months there is a system of weeks. A physical or electronic calendar
provides conversion from a given date to the weekday, and shows multiple dates for a given weekday
and month. Calculating the day of the week is not very simple, because of the irregularities in the
Gregorian system. When the Gregorian calendar was adopted by each country, the weekly cycle
continued uninterrupted. For example, in the case of the few countries that adopted the reformed
calendar on the date proposed by Gregory XIII for the calendar's adoption, Friday, 15 October 1582, the
preceding date was Thursday, 4 October 1582 (Julian calendar).
• Opinions vary about the numbering of the days of the week. ISO 8601, in common use worldwide,
starts with Monday=1; printed monthly calendar grids often list Mondays in the first (left) column of
dates and Sundays in the last. In North America, the week typically begins on Sunday and ends on
Saturday.

17. ACCURACY
• The Gregorian calendar improves the approximation made by the Julian calendar by skipping three
Julian leap days in every 400 years, giving an average year of 365.2425 mean solar days long.
• This approximation has an error of about one day per 3,030 years with respect to the current value of
the mean tropical year.
• However, because of the precession of the equinoxes, which is not constant, and the movement of
the perihelion (which affects the Earth's orbital speed) the error with respect to the astronomical vernal
equinox is variable; using the average interval between vernal equinoxes near 2000 of 365.24237
days implies an error closer to 1 day every 7,700 years.
• By any criterion, the Gregorian calendar is substantially more accurate than the 1 day in 128 years error
of the Julian calendar (average year 365.25 days).

18. CALENDAR SEASONAL ERROR