Astronomical Cycles

Vocabulary practice flashcards for the Earth and Space Science unit on astronomical cycles, covering Earth's rotation and revolution, models of the solar system, seasons, lunar phases, tides, and eclipses.

Geocentric Model

An early astronomical model developed in ancient Greece that positioned Earth at the centre of the universe, based on the assumption that gravity pulled all bodies, planets, and stars toward the centre of Earth.

Epicycle

A smaller circular orbit along which a planet travels while revolving around a larger circular orbit, introduced by early Greek astronomers to account for apparent retrograde motion (where planets appear to move backwards).

Heliocentric Model

An astronomical model published by Nicolaus Copernicus in $1543$ that placed the Sun at the centre of the solar system rather than Earth.

Kepler's Orbit Theory

Johannes Kepler's discovery in $1609$ proving that planetary orbits around the Sun are elliptical rather than circular, determined through mathematical analysis of observational data.

Law of Universal Gravitational Attraction

Isaac Newton's law published in $1687$ stating that every object with mass exerts an attractive force on every other object with mass, which increases with larger mass and weakens with increasing distance.

Earth's Rotation

The spin of Earth on its axis once every $24\text{ hours}$, creating alternating periods of daylight and nighttime.

Earth's Revolution

The orbital motion of Earth travelling around the Sun in an elliptical path, which takes approximately $365.25\text{ days}$ (one year) to complete.

Axial Tilt

The angle of Earth's rotational axis, tilted at approximately $23.5^\circ$ relative to its orbital plane and pointing in a constant direction in space.

Terminator Line

The boundary line separating the illuminated daylight side of a celestial body from the shaded nighttime side (the twilight zone).

Solar Intensity

The amount of solar radiant energy delivered to a specific area of Earth's surface; it is strongest at the equator where rays are concentrated and decreases toward higher latitudes where rays are spread across larger areas.

Solar Altitude Angle

The angle at which sunlight strikes the surface of the Earth, which governs the intensity of solar radiation and the length of shadows throughout the day and year.

Summer Solstice

The astronomical event occurring when a hemisphere's pole is tilted closest toward the Sun, resulting in the longest daylight hours and shortest night of the year (December $21\text{st}$ in the Southern Hemisphere).

Winter Solstice

The astronomical event occurring when a hemisphere's pole is tilted farthest away from the Sun, resulting in the shortest daylight hours and longest night of the year (June $21\text{st}$ in the Southern Hemisphere).

Equinox

The two dates each year when Earth's axis is perpendicular to the Sun, placing the Sun directly overhead at zenith ($90^\circ$) above the equator and resulting in approximately $12\text{ hours}$ of day and $12\text{ hours}$ of night globally.

Arctic Circle

The line of latitude marking the northernmost point where the Sun can remain below the horizon for an entire day during the winter solstice and continuously above the horizon during the summer solstice.

Waxing Moon

The phases of the lunar cycle in which the illuminated fraction of the Moon visible from Earth is growing; in the Southern Hemisphere, the visible illumination increases starting from the left side.

Waning Moon

The phases of the lunar cycle in which the illuminated portion of the Moon visible from Earth is shrinking or fading.

Crescent Moon

A lunar phase in which less than one quarter of the Moon's visible surface is illuminated by sunlight.

Gibbous Moon

A lunar phase in which greater than one quarter but less than the full surface of the Moon is illuminated as seen from Earth.

Sidereal Month

The time required for the Moon to complete one full $360^\circ$ revolution around Earth, lasting approximately $27.3\text{ days}$.

Synodic Month (Lunar Month)

The period of $29.5\text{ days}$ between identical lunar phases (such as New Moon to New Moon), which is longer than a sidereal orbit because Earth advances around the Sun during that time.

Tidal Bulge

The outward deformation of ocean water on opposite sides of the Earth produced by the gravitational pull of the Moon and the Sun, which Earth rotates through to create high and low tides.

Spring Tide

A tide with the greatest difference between high and low water, occurring twice monthly during New Moon and Full Moon when the gravitational pulls of the Sun and Moon align.

Neap Tide

A tide with the smallest tidal range, occurring when the Sun and Moon are positioned at right angles ($90^\circ$) relative to Earth during first quarter and third quarter moon phases.

Perigee

The location in the Moon's elliptical orbit where it reaches its closest distance to the Earth.

Apogee

The location in the Moon's elliptical orbit where it is at its farthest distance from the Earth.

King Tide (Perigean Spring Tide)

An exceptionally large high tide occurring $2\text{ to }4\text{ times}$ a year when a Spring Tide coincides with the Moon being at perigee (closest point to Earth).

Syzygy

A straight-line astronomical alignment of three celestial bodies in a gravitational system, specifically the Sun, Earth, and Moon.

Umbra

The dark, central, inner cone of a celestial shadow where all direct light from the illuminating body is completely blocked.

Penumbra

The lighter outer region of a shadow where the light source is only partially obscured.

Total Lunar Eclipse

An eclipse that occurs at Full Moon when the Moon passes entirely into Earth's umbra, appearing reddish (a Blood Moon) because the only sunlight reaching it has passed through Earth's atmosphere.

Partial Lunar Eclipse

A lunar eclipse occurring when only a portion of the Moon enters Earth's umbral shadow while the remainder remains in the penumbra.

Penumbral Lunar Eclipse

An eclipse occurring when the Moon passes only through the Earth's outer partial shadow (penumbra), resulting in a subtle dimming of its surface.

Total Solar Eclipse

An eclipse that occurs at New Moon when the Moon passes directly between the Sun and Earth, completely covering the Sun's face and revealing its outer corona along the path of totality.

Annular Solar Eclipse

A solar eclipse occurring when the Moon is aligned between the Sun and Earth but is located near apogee (farther away), making it appear too small to fully obscure the Sun and leaving a visible outer 'ring of fire'.

Partial Solar Eclipse

A solar eclipse occurring when the Sun, Moon, and Earth are not in perfect alignment, causing the Moon to obscure only a fraction of the Sun's disk.

Path of Totality

The narrow corridor on Earth's surface (typically $150\text{ to }250\text{ km}$ wide) traced by the Moon's umbra where a total solar eclipse can be fully seen.

Lunar Nodes

The points where the Moon's orbital plane intersects Earth's orbital plane (the ecliptic); eclipses can only take place when New or Full Moons occur near these nodes due to the Moon's $5^\circ$ orbital inclination.