4.4 The Calendar
2 min read•june 12, 2024
The , introduced in 1582, is the most widely used civil today. It's a based on Earth's revolution around the Sun, featuring and 12 months of varying lengths.
Calendars play a crucial role in astronomy and timekeeping. Ancient civilizations used astronomical observations to create calendars, while modern timekeeping relies on precise atomic clocks and international standards.
The Gregorian Calendar and Its Origins
Origins of Gregorian calendar
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- Most widely used civil calendar today introduced by Pope Gregory XIII in 1582 as a reform of the
- Solar calendar based on Earth's revolution around Sun with average year length of 365.2425 days
- Features leap years every 4 years, except for century years not divisible by 400 (1700, 1800, 1900 not leap years; 2000 was leap year)
- Consists of 12 months varying from 28 to 31 days (February, April, June, etc.)
- Weeks have 7 days, each named after celestial objects (Monday for Moon, Saturday for Saturn, etc.)
Solar vs lunar calendar systems
- Solar calendars based on Earth's orbit around Sun (Gregorian, Julian, Persian calendars)
- Used in most Western countries and for international trade and communication
- Lunar calendars based on phases of Moon (Islamic and Chinese calendars)
- Important for religious and cultural events (Ramadan, Chinese New Year)
- Lunisolar calendars combine aspects of both solar and lunar calendars (Hebrew and Hindu calendars)
- Keep lunar months in sync with solar years by adding intercalary months
Astronomy in ancient calendar creation
- Ancient Egyptians used of Sirius to mark beginning of annual Nile flooding
- Developed 365-day solar calendar with 12 months of 30 days each, plus 5
- Babylonians used with 12 lunar months and added every few years
- Tracked motion of celestial bodies to predict eclipses and other astronomical events
- Mayans developed several calendars (, , )
- Used astronomical observations to create precise calendars for agricultural and religious purposes
- Stonehenge ancient monument in England believed to be astronomical observatory
- Aligned with summer and winter , possibly used to track seasons and create calendar
Astronomical Time Measurements
- : Time taken for Earth to complete one orbit around the Sun relative to fixed stars
- : Time between two consecutive vernal equinoxes, slightly shorter than sidereal year due to
- : Gradual change in Earth's rotational axis orientation, affecting the position of equinoxes
- : Point in Earth's orbit when the Sun crosses the celestial equator, resulting in equal day and night lengths
Modern Timekeeping
- : Highly precise timekeeping device used to define the second and maintain accurate time standards
- : Imaginary line on Earth's surface where the calendar date changes as one travels east or west
Key Terms to Review (22)
Atomic Clock: An atomic clock is a device that uses the resonant frequency of atoms to keep extremely precise time, with an accuracy that can be better than one second in millions of years. This precision makes atomic clocks essential for applications that require accurate timekeeping, such as navigation systems, telecommunications, and scientific research.
Calendar: A calendar is a system for organizing and dividing time, typically defined by the movements of celestial bodies. It includes days, months, and years to track time periods for various activities and events.
Epagomenal Days: Epagomenal days refer to the extra days added to the end of a year in certain calendars to align the calendar year with the solar year. These additional days are necessary to account for the fact that the solar year is slightly longer than 365 days, the number of days in a standard calendar year.
Equinox: An equinox is an astronomical event that occurs twice a year, around March 20-21 and September 22-23, when the sun is directly above the equator, and the length of day and night are approximately equal. This phenomenon is a crucial concept in understanding ancient astronomy, the seasons, and the calendar.
Gregorian calendar: The Gregorian calendar is the modern calendar used globally, introduced by Pope Gregory XIII in 1582. It refined the Julian calendar to better align with Earth's revolutions around the Sun and to correct seasonal drift.
Gregorian Calendar: The Gregorian calendar is the widely used civil calendar system that was introduced in 1582 by Pope Gregory XIII as a modification of the earlier Julian calendar. It is the calendar system that is most commonly used around the world today for civil purposes.
Haab': The Haab' was the solar calendar system used by the Maya civilization in Mesoamerica. It was a 365-day calendar that was used alongside the 260-day Tzolkin calendar to create a complex and accurate system for tracking time and celestial events.
Heliacal Rising: Heliacal rising refers to the first visible appearance of a celestial object, such as a star or planet, above the eastern horizon just before sunrise. This event marks the object's reemergence from the Sun's glare, making it observable to the naked eye.
Intercalary Month: An intercalary month is an extra month added to a calendar system in order to keep it aligned with the seasons or astronomical events. It is a supplementary month that is inserted periodically to prevent the calendar from drifting out of sync with the solar year.
International Date Line: The International Date Line is an imaginary line that runs from the North Pole to the South Pole, roughly following the 180th meridian of longitude. It is the line that separates one calendar day from the next, allowing for the tracking of time and date across the globe.
Julian Calendar: The Julian calendar is a solar calendar introduced in 46 BCE by Julius Caesar, which established the length of the year as 365 days, with a leap year every four years to account for the extra quarter-day in the solar year. This calendar was a significant improvement over the previous Roman calendar and became the predominant calendar system used in the Western world for over 1,500 years.
Leap Years: Leap years are special years in the calendar system that have 366 days instead of the usual 365 days. This extra day, February 29th, is added to the calendar to keep it aligned with the astronomical year and the seasons.
Long Count: The Long Count is a calendrical system used by the Maya civilization in ancient Mesoamerica to track time over long periods. It is a complex system that utilizes a series of interlocking cycles to measure days, months, and years, allowing for the recording of historical events and the tracking of astronomical phenomena.
Lunar Calendar: A lunar calendar is a calendar that is based on the cycles of the moon, rather than the sun. It tracks the phases of the moon and the number of days between each new moon, typically consisting of 12 or 13 lunar months per year.
Lunisolar Calendar: A lunisolar calendar is a calendar that combines elements of both a solar calendar, which is based on the length of the year as determined by the Earth's revolution around the Sun, and a lunar calendar, which is based on the phases of the Moon. This type of calendar attempts to synchronize the months with the Moon's cycles while keeping the years aligned with the solar year.
Precession: Precession is the slow, conical motion of Earth's rotation axis, which causes the celestial poles to trace out circles in the sky over a period of approximately 26,000 years. This phenomenon affects the position of stars and constellations over long time periods.
Precession: Precession is the gradual shift in the orientation of an object's rotational axis in relation to a fixed reference frame. This phenomenon occurs in various celestial bodies, including Earth, and has significant implications for the study of astronomy, timekeeping, and the calendar.
Sidereal Year: The sidereal year is the time it takes for the Earth to complete one full orbit around the Sun, as measured against the fixed stars in the celestial sphere. This is slightly shorter than the more commonly known tropical year, which is measured against the position of the vernal equinox.
Solar Calendar: A solar calendar is a calendar system that is based on the Earth's revolution around the Sun. It tracks the seasons and the passage of the year by aligning with the solar cycle, which is the time it takes for the Earth to complete one orbit around the Sun.
Solstices: Solstices are the two times of the year when the sun reaches its highest or lowest point in the sky, marking the longest and shortest days of the year. They are important astronomical events that have influenced the development of calendars and timekeeping systems throughout human history.
Tropical Year: The tropical year is the period of time it takes for the Earth to complete one full cycle of seasons, from one vernal equinox to the next. It is the basis for the calendar system used in most parts of the world today.
Tzolkin: The Tzolkin is a sacred 260-day calendar used by the Maya civilization in Mesoamerica. It is one of the key components of the Maya calendar system, which was highly sophisticated and played a central role in their religious, social, and administrative practices.