Earth and Sky: Time, Seasons, Moon Phases, and Eclipses

Vocabulary flashcards covering key terms from timekeeping, celestial coordinates, seasons, Moon phases, and eclipses as presented in the lecture notes.

Apparent solar time

Time as measured by the Sun’s position in the sky (the time that would be indicated by a sundial).

Declination

The angular distance north or south of the celestial equator.

Great circle

A circle on the surface of a sphere formed by the intersection of the sphere with a plane that passes through its center.

International Date Line

An arbitrary line near 180° longitude across which the date changes by one day.

Lunar eclipse

An eclipse of the Moon, when the Moon moves into Earth’s shadow; can occur only at full Moon.

Mean solar time

Time based on Earth’s rotation that passes at a constant rate, unlike apparent solar time.

Meridian

A great circle on the sphere that passes through the poles.

Phases of the Moon

The changing appearance of the Moon’s lighted portion from Earth during its monthly cycle.

Right ascension

The coordinate measuring east–west position, angle eastward from the vernal equinox along the celestial equator to the body’s hour circle.

Sidereal day

Earth’s rotation period defined by the stars; time between successive meridian transits of the same star.

Sidereal month

The Moon’s orbital period about Earth measured with respect to the stars.

Solar day

Earth’s rotation period defined by the position of the Sun; time between successive solar meridian transits.

Solar eclipse

An eclipse of the Sun by the Moon; occurs only at new Moon.

Solar month

The time interval in which the Moon’s phases repeat, from full to full.

Synchronous rotation

When a body rotates on its axis at the same rate as it revolves around another body (e.g., the Moon).

Tides

Alternate rising and falling of sea level caused by the Moon’s differential gravitational pull on Earth.

Latitude

Degrees north or south of the equator (0°).

Longitude

Degrees east or west of the Prime Meridian (0°) in Greenwich, England.

Foucault’s Pendulum

A pendulum experiment that provided clear evidence that the Earth rotates, not the sky.

Cause of Seasons

The 23.5° tilt of Earth’s axis, not its changing distance from the Sun.

Summer Solstice

Around June 21: Northern Hemisphere tilted toward the Sun; longest day.

Winter Solstice

Around December 21: Northern Hemisphere tilted away from the Sun; shortest day.

Equinox

Around March 21 and September 21: Neither hemisphere tilted toward the Sun; equal day and night.

Tropic of Cancer

≈23.5° N; Sun at zenith on the summer solstice.

Tropic of Capricorn

≈23.5° S; Sun at zenith on the winter solstice.

Arctic Circle

≈66.5° N; edge of 24-hour sunlight/darkness on solstices.

Antarctic Circle

≈66.5° S; edge of 24-hour sunlight/darkness on solstices.

Angle of sunlight

More direct sunlight in summer, leading to more heating.

Daylight hours

More hours of sunlight in summer, contributing to warmer temperatures.

Julian Calendar

Introduced by Caesar; leap year every 4 years; average year 365.25 days (slightly too long).

Gregorian Calendar

Our modern calendar; skips leap years on certain century years to correct the Julian error; average year 365.2425 days.

Leap year

A year with an extra day (e.g., 2000 was a leap year; 1900 was not).

New Moon

Moon between Sun and Earth; Sunlit side is away from Earth; not visible from Earth.

Waxing Crescent

Moon becomes visible as a crescent growing after New Moon.

First Quarter

Half Moon with the right half illuminated; about a week after New Moon.

Waxing Gibbous

Moon more than half illuminated, approaching Full Moon.

Full Moon

Moon fully illuminated; opposite the Sun in the sky.

Waning Gibbous

Moon begins to dim after Full Moon.

Third Quarter

Last Quarter; left half illuminated; about three weeks after New Moon.

Wan­ing Crescent

Small crescent illumination before New Moon.

Eclipse season

Period when Sun, Earth, and Moon align near orbital nodes, allowing eclipses.

Umbra

Dark inner shadow of a solar eclipse; where a Total Solar Eclipse is observed.

Penumbra

Lighter outer shadow of a solar eclipse; where a Partial Solar Eclipse is observed.

Coppery red Moon

Often appears coppery red during a lunar eclipse due to sunlight refracting through Earth’s atmosphere.

Moon’s orbital tilt (to the ecliptic)

The Moon’s orbit is tilted about 5° relative to Earth’s orbit around the Sun (the ecliptic).