astronomy ch 3

Practical value of ancient astronomy

Timekeeping, tracking seasons, creating calendars, and navigation.

How central Africans predicted rainfall

By observing the tilt and orientation of the waxing crescent moon.

Months with fewer than 100 mm of rainfall

January, February, March, September, October, November, December.

Names of the seven days of the week

Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn.

Reason for attention to ancient Greek science

They explained nature using models based on math and logic rather than myth.

How Eratosthenes estimated Earth's size

By comparing angles of shadows cast by sunlight at noon in two different cities.

Description of the geocentric universe

A model where Earth sits motionless at the center of the universe.

Reason Greeks rejected Earth's movement

They could not detect stellar parallax.

Ptolemy’s contribution

He built a complete geocentric model using epicycles.

Location of the Sun in Ptolemy’s model

Orbiting Earth, placed between Venus and Mars.

Reason for 'circles upon circles' (epicycles)

To explain retrograde motion while keeping circular orbits centered on Earth.

Scholars who preserved Greek work during the Dark Ages

Islamic scholars.

Copernicus’s main contribution

He introduced a mathematical heliocentric model.

Flaw in Copernicus’s model

He assumed orbits were perfect circles.

Tycho Brahe’s contribution

Extremely detailed and accurate naked-eye observations of planetary positions.

Astronomer who angered the Pope

Galileo Galilei.

5 discoveries Galileo made with his telescope

1. Four moons orbiting Jupiter 2. Phases of Venus 3. Sunspots 4. Mountains and craters on the Moon 5. The Milky Way is made of vast numbers of individual stars.

Galileo's discoveries about the Sun and Moon

The Moon has a rough surface, and the Sun has dark, moving spots.

Kepler’s First Law of Planetary Motion

Orbits are ellipses with the Sun at one focus.

Kepler’s Second Law of Planetary Motion

A planet sweeps out equal areas in equal times, moving faster when closer to the Sun.

Kepler’s Third Law of Planetary Motion

p² = a³, where more distant planets take longer to orbit the Sun.

Error in the planetary orbit picture

The Sun is drawn in the middle of the orbit instead of at one focus.

Point where planet travels fastest

Point closest to the Sun.

Northern Hemisphere season when Earth moves fastest

Winter, in January.

Terms in p² = a³

p = Orbital period in years; a = Average distance from the Sun in astronomical units.

Comet orbital calculation at 4 AU

8 years (p² = 4³ = 64, so p = 8).

Perihelion

The point in an orbit closest to the Sun.

Aphelion

The point in an orbit farthest from the Sun.

Purpose of scientific models

To explain and predict real-world observations.

Truth about scientific models

They must make testable predictions and can be changed or discarded if disproven.

Scientific definition of a theory

A well-tested, powerful explanation supported by extensive evidence.

Ancient goal of astrology

To predict human events and fortunes using the positions of celestial objects.