Astronomy Vocabulary Flashcards

Vocabulary flashcards covering topics from axial tilt and celestial coordinates to historical models, Galileo’s observations, and eclipses.

Precession of the equinoxes

The slow wobble of Earth's axis that shifts the positions of the background stars and the zodiac relative to the Sun over a ~26,000-year cycle.

Zenith

The point directly overhead in the sky; its declination equals the observer’s latitude.

Declination

A celestial coordinate that measures angular distance north or south of the celestial equator.

Circumpolar star

A star that never sets for a given latitude; it remains above the horizon year-round.

Stars that rise and set

Stars that move above and below the horizon, rising in the east and setting in the west.

North Celestial Pole

The point in the sky around which northern stars appear to rotate; near the star Polaris.

South Celestial Pole

The analogous point in the southern sky around which southern stars rotate.

Nadir

The point directly opposite the zenith, i.e., directly below the observer.

Equinox

Two yearly moments when the Sun crosses the celestial equator, yielding roughly equal day and night.

Solstice

Two yearly moments when the Sun reaches its extreme declination, giving the longest and shortest days.

Vernal (Spring) Equinox

Around March 21; marks the start of spring in the Northern Hemisphere.

Autumnal (Fall) Equinox

Around September 21; marks the start of autumn in the Northern Hemisphere.

Winter Solstice

Around December 21; the shortest day of the year.

Summer Solstice

Around June 21; the longest day of the year.

Tropic of Cancer

Approximately 23.5° N; the northernmost latitude where the Sun can be overhead at noon.

Tropic of Capricorn

Approximately 23.5° S; the southernmost latitude where the Sun can be overhead at noon.

Arctic Circle

Approximately 66.5° N; region with polar day/night phenomena.

Antarctic Circle

Approximately 66.5° S; southern equivalent of the Arctic Circle.

Lahaina Noon

The moment when the Sun is directly overhead, causing shadows to point straight down; occurs within the Tropics.

Planet

From Greek for 'wanderer'; a celestial body that moves relative to the fixed stars.

Retrograde motion

The apparent backward motion of a planet as observed from Earth, caused by differences in orbital speeds.

Epicycle

In Ptolemy’s geocentric model, a small circle around which a planet moves to explain retrograde motion.

Geocentric model

Earth-centered view of the cosmos with celestial objects orbiting Earth.

Heliocentric model

Sun-centered view of the solar system with planets orbiting the Sun.

Copernicus

Renaissance astronomer who proposed the heliocentric model.

Occam’s Razor

The principle that the simplest explanation with the fewest assumptions is preferred.

Galileo

Pioneer of modern observational astronomy; used the telescope to study the heavens, discovering features like lunar craters, Galilean moons, rings of Saturn, sunspots, and Venusian phases.

Telescope

An instrument that magnifies distant objects for astronomical observation.

Galilean moons

Io, Europa, Ganymede, and Callisto—the four largest moons of Jupiter discovered by Galileo.

Moon’s mountains and craters

The Moon has topographical features similar to Earth—mountains and impact craters.

Rings of Saturn

Saturn’s ring system, first detailed through telescopic observation.

Sunspots

Dark blemishes on the Sun’s surface, indicating the Sun is not a perfect, featureless sphere.

Phases of Venus

Observations showing Venus exhibits phases (crescent to full), supporting a heliocentric model.

Phases of the Moon

The cycle of illuminated Moon as seen from Earth, from new moon to full moon and back.

Lunar eclipse

Earth blocks sunlight from reaching the Moon when the Moon is in Earth’s shadow during a full Moon.

Umbra

The dark, central shadow in an eclipse (Earth’s shadow on the Moon or Moon’s shadow on Earth during a solar eclipse).

Penumbra

The partial shadow surrounding the umbra during an eclipse.

Solar eclipse

Moon blocks all or part of the Sun from Earth’s view; types include total, partial, and annular.

Total solar eclipse

The Sun is completely obscured by the Moon along the path of totality.

Partial solar eclipse

Only part of the Sun is blocked by the Moon.

Annular solar eclipse

Moon covers the Sun’s center, leaving a bright ring (annulus) around it.

Path of totality

The narrow path on Earth’s surface where a total solar eclipse is visible.