Astro Exam 1

Day

One rotation of Earth relative to the Sun (~24 hours)

Lunar month

One full cycle of Moon phases (~29.5 days)

Year

One complete orbit of Earth around the Sun (~365.25 days)

Seasons

Caused by Earth’s axial tilt combined with its orbit around the Sun

Eclipse cycle

Repeating alignment of Sun, Earth, and Moon (~18 years, Saros cycle)

Astronomy and food

Predicts seasons and planting times

Astronomy and water

Predicts floods and seasonal rainfall

Astronomy and navigation

Uses stars and the Sun to determine direction

Astronomy and timekeeping

Creates calendars and predictable schedules

Astronomy and safety

Tracks eclipses, comets, and space hazards

Sundial

Uses the Sun’s shadow to track hours

Lunar calendar

Uses Moon phases to define months

Solar calendar

Tracks the Sun’s yearly path to define the year

Star calendar

Rising and setting of stars mark seasons

Water clock

Steady water flow measures time intervals

Stone alignments

Structures aligned with solstices and equinoxes

Moon phases

Caused by changing viewing angle of the Sun-lit Moon

New moon

Moon is between Earth and Sun

Full moon

Earth is between Sun and Moon

Solar eclipse

Moon blocks sunlight from reaching Earth

Lunar eclipse

Earth’s shadow falls on the Moon

Summer warmth

Sun is higher in the sky and sunlight is more direct

Winter cold

Sun is lower and sunlight is less direct

Longer summer days

Earth’s tilt keeps one hemisphere facing the Sun longer

Solstice

Sun reaches highest or lowest position in the sky

Equinox

Day and night are equal length

Retrograde motion

Apparent backward motion of a planet against the stars

Geocentric explanation

Planets move on epicycles around Earth

Heliocentric explanation

Relative motion as Earth passes other planets

Parallax

Apparent shift in an object’s position due to viewpoint change

Observer separation

Greater distance between observers gives larger parallax

Nearby objects

Show large parallax

Distant objects

Show little or no parallax

Tycho’s conclusion

Comets and supernovae are farther than the planets

Copernican model

Heliocentric (Sun-centered) system

Earth’s motion

Earth rotates daily and orbits the Sun yearly

Retrograde explanation

Apparent motion caused by Earth’s movement

Importance of Copernicus

Simplified planetary motion and challenged geocentrism

Kepler’s First Law

Planets orbit in ellipses with the Sun at one focus

Kepler’s Second Law

Planets sweep out equal areas in equal times

Kepler’s Third Law

Orbital period² is proportional to orbital distance³

Galileo: Imperfect heavens

Moon mountains and sunspots

Galileo: Not everything orbits Earth

Moons orbit Jupiter

Galileo: Evidence for heliocentrism

Full phases of Venus

Galileo: Falling objects

Inertia keeps Earth’s motion

Galileo: Stellar parallax

Stars too distant to detect at the time