Ancient Astronomy and Celestial Events

• Review of Ancient Astronomy, Phenomena Before Telescopes, and Planetary Motion.

Earth's Circumference

• Eratosthenes, a Greek philosopher, first computed Earth's circumference.
• Eratosthenes calculated it to be 250,000 stadia.
• 250,000 stadia is equivalent to 40,000 kilometers.

Earth's Shape

• The assumption that every portion on Earth has a tendency to be compressed and form a sphere is not an argument given by ancient Greeks.
• Earth's shape, as described by modern astronomy, is an oblate spheroid.

Celestial Events and Phenomena

• Lunar eclipses, phases of the Moon, and daily and annual motion of stars were observed as phenomena before the telescope's invention.
• Diurnal motion causes the apparent shift in the location of the stars.
• Equinoxes occur when the Sun's ecliptic intersects with the celestial equator, resulting in equal day and night length.
• A solar eclipse happens when the Moon is between the Sun and Earth.

Ancient Observations

• Ancient people and astronomers saw designs or groupings in the stars, naming them after religious figures, animals, and objects.
• This practice did not support the discovery of visible planets before telescopes.

Kepler's Laws

• Kepler's 2nd Law of Planetary Motion states that an imaginary line segment connecting a planet to the Sun sweeps out equal areas in equal time intervals.
• Accurate observation of stars and planets was Tycho Brahe's biggest contribution to astronomy
• Kepler modified Brahe's data, determining that the shape of celestial bodies' orbits is elliptical.
• Kepler's 3rd Law of Planetary Motion includes the formula P² = a³ to compute the period of orbit of a planet around the Sun, where P is the period and "a" is the semi-major axis.