Early Observations of the Sky

Ancient Astronomers The earliest astronomers were found in: China Egypt Mesopotamia. All at about the same time Thousands of years later (but completely independently) the Maya of the new world developed similar astronomical knowledge.

Mesoamerica The Maya’s knowledge passed onto the Aztec, who developed a calendar system that was accurate to within 1 hour over 100 years. That’s more accurate than the Julian calendar that was used contemporaneously in Europe!

Ancient Mesoamerican Observatory

Babylonia The Babylonian calendar, though, was the first true calendar. Developed 5,000 years ago, it featured 29 and 30 month days based on the lunar cycle. This is called a Lunar Calendar The time from new moon to new moon is 29.53 days. Because there are 12.36 lunar months in a 365.24 day year, some years the Babylonians had 12 months, the others they had 13 months. The Hebrew calendar (foundation for Judaism and Christianity) is based on the Babylonian calendar.

Hebrew Calendar

Egypt The Egyptians developed a different calendar, based on the sun. This is called a solar calendar Their year had 12 months of 30 days each, for a total of 360 days each year. The remaining 5 days were at the end of the calendar, and not part of a specific month. The length of the Egyptian week was 10 days, giving 3 weeks each month.

Rome Julius Caeser took the Egyptian Calendar and renamed the months after Roman Gods, or simply just called it their number. December = deci = 10th month October = oct = 8th month January = janus = roman god March = mars = roman god (original first month on the calendar) Also, Caeser added a leap day at the end of february every four years. We call this calendar “The Julian Calendar”

Julian Calendar The problem with the Julian Calendar was this: The length of the solar year is NOT 365.2500 days The length of the solar years IS 365.2422 days Over time, too many leap days were used, and the calendar drifted out of alignment with the equinoxes. The equinox should fall on the 21st-22nd of march, but it was now happening on the 11th of march.

Julian Calendar

Gregorian Calendar The equinox should fall on the 21st-22nd of march, but it was now happening on the 11th of march. In 1582, Pope Gregory ordered the Catholic Church to lose 10 days (starting March on March 11th, rather than March 1st) and changed how leap days were done.

Gregorian Calendar Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.

Gregorian Calendar In the new Gregorian calendar, the length of a year is 365.2425 days, which is much closer to the true value of 365.2422 days. This means in the year 3000 AD, the Gregorian calendar will be off by 1 day. In the year 12,000 AD, the Gregorian calendar will be off by 12 days.

Gregorian Calendar

Back to Egypt! The Egyptians made careful observations of the motions of the planets, the sun, and the moon. They developed a lot of skill in predicting solar and lunar eclipses. Even today, their data and predictions are used to help modern astronomers determine the rate of slowing of the earth’s rotation.

Stonehenge While the Egyptians were building the pyramids, the Celts of northern Europe built massive stone structures called Standing Stones. The most complete and famous is Stonehenge.

Stonehenge Many of the lines of sight at Stonehenge correspond to the rising and setting of the sun and moon. Every summer solstice the sun rises over one particular stone (called the heelstone) when viewed from the center of the main stone circle.

China The Chinese took copious records. We have complete records dating back to 1000 BC, and incomplete records that stretch back to 3000 BC. They recorded eclipses, comets, and novae. They called nova and supernova “guest stars” One “guest star” first observed in 1054 AD is now the Crab Nebula, a remnant of the supernova long ago.

China The Chinese created the system of altitude and angular distance that is still used today in astronomy.

India The Moghul emperors of India, circa 1600s AD, built observatories for the Sun, Moon, and planets. Some of these were of impressive size and built entirely of stone.

Ancient Indian Observatory

Timeline Activity Tape 3 pages of computer paper together in landscape format Draw a line straight down the center of the pages Make notches at both ends. The far left notch should read 700 BC The far right notch should read 2000AD Make a notch every 10 cm and add 400 years Remember that 700 BC goes to 0, then the number increases to 2000