History of Astronomy — The Christians

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History of Astronomy — The Christians


Early Christians. The early Christians used the calendars of their community. In the Roman Empire, they used the Julian Calendar. However, they had one important day to calculate: the crucifixion of Jesus. The Apostles recorded that Jesus died on the first day of Passover. This Jewish holiday is calculated according to the lunar calendar. It is the day of the first Full Moon following the Spring Equinox (the day in Spring where nighttime and daytime have equal lengths).

Christians had to keep both lunar and solar calendars to make this calculation. Some used the Jewish calendar while others used the Julian calendar. In 325 C.E. the Council of Nicea was held (among other reasons) to fix a common calendar for all Christians. These early Christians fixed the date of the Spring Equinox at March 21 (in the Julian Calendar) so that it no longer needed to be computed each year. Note that this is a four-day difference from the date used by Julius Caesar.

Even though Christians no longer needed to compute the Spring Equinox under this newly adopted common calendar, over the following centuries the Spring Equinox was occurring noticeably earlier than 21 March each year. This error was finally corrected by Pope Gregory XIII when he developed the Gregorian Calendar, which we still use today.

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The Gregorian Calendar. The Julian Calendar served as the standard European calendar for about 1500 years. By then, the 11 minute per year error amounted to days. At the time of the Council of Nicea in 325 C.E. the Spring Equinox occurred on March 21. By the time of Pope Gregory XIII in 1575, the Spring Equinox occurred on March 11, an error of ten days. He issued a papal bull (order) that the day after Thursday, October 4, 1582 would become Friday, October 15.

The days of the week stayed the same, only the date changed. Knowing of the error of 10 days in about 1200 years, the length of the year was recaculated to be closer to 365.2425 days. This is much closer to the actual length of 365.242190 days. The error in the Gregorian Calendar is only about 27 seconds per year.

To reduce the average number of days in a year, fewer years were leap years. In the Julian Calendar, every century year was a leap year. In the Gregorian Calendar, only one out of every four centuries is a leap year. The Gregorian Calendar made centuries leap years only if they were evenly divisible by 400. So for example, 1700, 1800, and 1900 were not leap years but 2000 was a leap year. 2100 will not be a leap year.

The Gregorian Calendar was adopted across the Roman Catholic community, but other parts of Europe (such as Protestant England) did not adopt the Gregorian Calendar until much later. For this reason, two calendars existed in Europe for some time: the Julian and the Gregorian. Today our daily calendar is the Gregorian Calendar, but for accurate timing, astronomers now use Atomic Time, discussed in a section below.

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