Prehistoric Period - Early Sites and Archeoastronomy

• Importance of astronomy: Season changes, marked by equinoxes and solstices (celebrated with decorated trees, Yule logs, mistletoe and communal celebrations, for example), were important dates for agriculture and hunting; Motion of the Sun, the Moon and fixed stars also used to keep track of the time of day, for orientation, and later navigation needs.
• Earliest evidence: Cave paintings like Lascaux (16,000 yr ago) and others (stars, 5000-yr old map of the Moon).
• Earliest structures: Many examples with aligned markers to keep track of time of year, like Nabta (5000-3000 BC), Stonehenge (2800-1100 BC), Seahenge (2050 BC).
• America: Caracol Temple by Mexico's Mayas in the Yucatán peninsula; Aztec Templo Mayor in Tenochtitlán; Big Horn Medicine Wheel by the plains Indians in the US.
• Planets: In the bronze age around 3000 BC, the planets were known out to Saturn. Their wandering motion made them less useful for orientation, but they were used for calendars and astrology.

[More speculative: Did catastrophic events like comet impacts occur around 3000-2000 BC and contribute to the demise of ancient civilizations?]

  Classical Period - Early Development of Ideas

• The Middle East: Rich societies at commercial crossroads after 3000 BC.
• Mesopotamians: Astrology and prediction; [sexagesimal number system].
• Egyptians: Practical applications, predicting floods and aligning buildings.
• Greeks: Explanation and understanding, based on conceptual models; Related to Greek philosophy, and their naturalistic (as opposed to mystic) interpretation of the universe; Use measurements and knowledge of geometry.
• Aristotle (IV cy BC): The most important figure, wanted to understand all phenomena. In astronomy, he believed that Earth is the center of all motion; that the Earth, Moon, Sun, planets, and stars were spheres (at a range of distances from the Earth); and that time is uniform and ever-flowing. Most of his views dominated until the XVII cy.

Classical Period - Early Solar System Models

• Setting: Alexandria, founded by Alexander in 332 BC, became Egypt's capital with Ptolemy I (one of his generals), and an international cultural center.
• Eratosthenes (III cy BC): Believed the Earth is a sphere, and measured its radius.
• Aristarchus of Samos (310-230 BC): Heliocentric model [also calculated sizes of and distances to the Sun and Moon, and the Earth's tilt].
• Hipparchus of Rhodes (190-120 BC): Wrote a star catalog in which he introduced the magnitude system [also calculated the length of the year and discovered the precession of the equinoxes].
• Most other models: Geocentric. Why? No feeling of motion, no wind, no stellar parallax. (How can you explain this, based on present understanding?)
• How do planets move? Their changes of speed and retrograde motion, was explained assuming they moved on large circles and epicycles; [Observation at the time were accurate to about 10'].
• Ptolemy (II cy AD): Followed Aristotle's ideas; Perfected a geocentric model which survived for 1300 years, preserved and refined by Arab astronomers; Wrote the "Almagest"; [latitude and longitude; North = up, South = down; stellar magnitudes.]

  Later Developments

• Medieval ideas: Stars are "holes" in the surface of the celestial sphere, that let light from behind shine through.
• Arabs (VIII-XV cy): New instruments, mathematical tools, many names of stars; contacts with Eastern scholars (mainly Indian and Chinese); Studied, criticized and improved Aristotle's and Ptolemy's ideas; Reintroduced them to European thinkers during the XII and XIII centuries, paving the way for the Renaissance.
• Chinese: As part of the imperial establishment in ancient China, an astronomical observatory was usually built inside the capital city. Trained astronomers were appointed to keep a diligent watch of the sky day and night, and recorded important events (eclipses, comets, "new" stars...) for astrological reasons.
• Renaissance Europe: Beginning of development of science; Took over Eastern knowledge and accumulated observational records, used technological advances.
• [Other cultures: Temples with astronomical alignments and images in Asia; The Great Enclosure in the archaeological site of Great Zimbabwe from the 1300's...]