Need for time of day/night: Keep track of time for planning and orientation; later also for navigation purposes. Use the position of the Sun and Moon (including its phase), and the fixed stars.
 Need for time of year: 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. Again, use the motion of the Sun, Moon, and stars.
 Known planets: In the bronze age around 3000 BC, five were known, out to Saturn. Their wandering motion made them less useful for orientation, but they were used for calendars and astrology.

Prehistoric Period - Early Sites and Archeoastronomy

• Earliest evidence: Cave paintings like the ones at Lascaux (16,000 yr old) and others including depictions of celestial objects (stars, 5000-yr old map of the Moon).
• Earliest structures: Many examples with calendar circles, aligned markers to keep track of time of year, like Stonehenge (3100-1550 BC) [Goseck (5000 BC), Nabta (5000-3000 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.
• [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 needs like predicting floods, aligned buildings).
• Greeks: Emphasized need for explanation and understanding; Developed conceptual models based on their naturalistic (as opposed to mystic) philosophy and interpretation of the universe; Used measurements and knowledge of geometry.
• Aristotle (IV cy BC): The most important figure, wanted to develop an understanding of all natural phenomena. In physics, the natural state of an object is rest, heavier objects fall faster than light ones; In astronomy, he argued for a geocentric model (Earth is the center of all motion): we have no feeling of motion, no wind, no stellar parallax; the Earth, Moon, Sun, planets, and stars are spheres; and time is uniform and ever-flowing. Most of his views dominated until the XVII century.

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 (III cy BC): Contrary to most people, believed in heliocentric model [also calculated sizes and distances to the Sun and Moon, and the Earth's tilt].
• [Hipparchus of Rhodes (II cy 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.]
• 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.]
• How do planets move? Their changes of speed and retrograde motion, was explained assuming they moved on large circles and epicycles; [Observations at the time were accurate to about 10'].

  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...]