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  2. The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar.

    • The original goal of the Gregorian calendar was to change the date of Easter. In 1582, when Pope Gregory XIII introduced his Gregorian calendar, Europe adhered to the Julian calendar, first implemented by Julius Caesar in 46 B.C.
    • Leap years don’t really occur every four years in the Gregorian calendar. The Julian calendar included an extra day in February every four years. But Aloysus Lilius, the Italian scientist who developed the system Pope Gregory would unveil in 1582, realized that the addition of so many days made the calendar slightly too long.
    • The Gregorian calendar differs from the solar year by 26 seconds per year. Despite Lilius’ ingenious method for syncing the calendar with the seasons, his system is still off by 26 seconds.
    • Some Protestants viewed the Gregorian calendar as a Catholic plot. Though Pope Gregory’s papal bull reforming the calendar had no power beyond the Catholic Church, Catholic countries—including Spain, Portugal and Italy—swiftly adopted the new system for their civil affairs.
    • Overview
    • The Gregorian calendar

    The Julian calendar year of 365.25 days was too long, since the correct value for the tropical year is 365.242199 days. This error of 11 minutes 14 seconds per year amounted to almost one and a half days in two centuries, and seven days in 1,000 years. Once again the calendar became increasingly out of phase with the seasons. From time to time, the problem was placed before church councils, but no action was taken because the astronomers who were consulted doubted whether enough precise information was available for a really accurate value of the tropical year to be obtained.

    By 1545, however, the vernal equinox, which was used in determining Easter, had moved 10 days from its proper date; and in December, when the Council of Trent met for the first of its sessions, it authorized Pope Paul III to take action to correct the error. Correction required a solution, however, that neither Paul III nor his successors were able to obtain in satisfactory form until nearly 1572, the year of election of Pope Gregory XIII. Gregory found various proposals awaiting him and agreed to issue a bull that the Jesuit astronomer Christopher Clavius (1537–1612) began to draw up, using suggestions made by the astronomer and physician Luigi Lilio (also known as Aloysius Lilius; died 1576).

    The Julian calendar year of 365.25 days was too long, since the correct value for the tropical year is 365.242199 days. This error of 11 minutes 14 seconds per year amounted to almost one and a half days in two centuries, and seven days in 1,000 years. Once again the calendar became increasingly out of phase with the seasons. From time to time, the problem was placed before church councils, but no action was taken because the astronomers who were consulted doubted whether enough precise information was available for a really accurate value of the tropical year to be obtained.

    By 1545, however, the vernal equinox, which was used in determining Easter, had moved 10 days from its proper date; and in December, when the Council of Trent met for the first of its sessions, it authorized Pope Paul III to take action to correct the error. Correction required a solution, however, that neither Paul III nor his successors were able to obtain in satisfactory form until nearly 1572, the year of election of Pope Gregory XIII. Gregory found various proposals awaiting him and agreed to issue a bull that the Jesuit astronomer Christopher Clavius (1537–1612) began to draw up, using suggestions made by the astronomer and physician Luigi Lilio (also known as Aloysius Lilius; died 1576).

  3. The Gregorian calendar was introduced as a refinement of the Julian calendar in 1582, and is today in worldwide use as the "de facto" calendar for secular purposes. Etymology [ edit ] The term calendars itself is taken from the calends , the term for the first day of the month in the Roman calendar , related to the verb calare "to call out ...

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