Ch. 2

PHYS 1062

Altitude

The angular distance of an object above the horizon, measured in degrees

Analemma

The figure-eight shape traced by the Sun's position in the sky at the same time each day over the course of a year. The analemma is caused by the tilt of Earth's axis and the elliptical shape of Earth's orbit.

Angular Distance

The angle that appears to separate a pair of objects in the sky.

Angular Size

The angle an object appears to span in your field of view.

Annular Eclipse

A solar eclipse in which the Moon is too far from Earth to completely block the Sun, leaving a bright ring of sunlight visible around the Moon.

Apparent Retrograde Motion

The apparent westward motion of a planet against the background stars. This occurs when Earth overtakes a planet in its orbit.

Asterism

A recognizable pattern of stars within a constellation. Asterisms are not necessarily defined by the outline of the constellation

Astrometric View

A point of view from outside the plane of the solar system, used to interpret astronomical situations and processes

Azimuth

An object's direction along the horizon, measured clockwise from due north

Celestial Equator

The projection of Earth's equator onto the celestial sphere

Celestial Sphere

An imaginary sphere surrounding Earth, on which all celestial objects appear to lie.

Celestial Poles

The points on the celestial sphere directly above Earth's North and South Poles

Circumpolar Stars

Stars that never set below the horizon as seen from a particular location on Earth.

Constellation

A region of the sky with well-defined borders. Astronomers officially recognize 88 constellations.

Declination

The celestial equivalent of latitude. It is measured in degrees north (positive numbers) or south (negative numbers) of the celestial equator.

Eclipses

Events that occur when the shadow of one celestial body falls on another.

Ecliptic

The apparent path of the Sun on the celestial sphere over the course of a year.

Eclipse Seasons

Periods during which eclipses are possible. They occur when the nodes of the Moon's orbit are aligned with the Sun and Earth.

Equinox

An event that occurs when the Sun crosses the celestial equator. At an equinox, night and day are approximately equal in length all over Earth.

First Quarter Moon

The lunar phase in which the Moon appears half-illuminated, with the illuminated portion on the right side as seen from the Northern Hemisphere.

Full Moon

The lunar phase in which the entire face of the Moon is illuminated as seen from Earth.

Galactic Plane

The plane of the Milky Way galaxy.

Geocentric View

A point of view from Earth.

Gibbous Moon

A lunar phase in which more than half of the Moon's face is illuminated.

Horizon

The boundary between Earth and sky as seen from a particular location.

Latitude

The angular distance north or south of Earth's equator.

Local Sky

The dome of the sky visible from a particular location.

Longitude

The angular distance east or west of the prime meridian.

Lunar Eclipse

An eclipse that occurs when Earth's shadow falls on the Moon.

Meridian

An imaginary line passing through the zenith and connecting the north and south points on the horizon.

Milky Way

The band of light we see in the sky that is our view into the disk of our galaxy.

New Moon

The lunar phase in which the Moon is between the Sun and Earth and is therefore not visible in the sky.

Node

A point where the Moon's orbit crosses the ecliptic.

Partial Eclipse

A lunar or solar eclipse in which the eclipsed object is only partially covered by the shadow of the eclipsing object.

Penumbra

The region of partial shadow surrounding the umbra during an eclipse.

Phases of the Moon

The changing appearance of the Moon as seen from Earth due to the changing relative positions of the Sun, Earth, and Moon.

Precession

The slow wobble of Earth's axis. It takes about 26,000 years for Earth's axis to complete one precession cycle.

Prime Meridian

The meridian that runs through Greenwich, England, and is designated as 0° longitude.

Right Ascension

The celestial equivalent of longitude, measured in hours, minutes, and seconds.

Saros Cycle

An eclipse cycle that repeats every 18 years and 11.3 days.

Solar Eclipse

An eclipse that occurs when the Moon blocks our view of the Sun.

Solstice

An event that occurs when the Sun reaches its most extreme north or south position relative to the celestial equator.

Synchronous Rotation

The state in which a celestial body's rotation period is the same as its orbital period. The Moon exhibits synchronous rotation, which is why we always see the same side of it.

Third Quarter Moon

The lunar phase in which the Moon appears half-illuminated, with the illuminated portion on the left side as seen from the Northern Hemisphere.

Total Eclipse

A lunar or solar eclipse in which the eclipsed object is completely covered by the shadow of the eclipsing object.

Tropic of Cancer

The circle of latitude at 23.5° N, marking the northernmost point at which the Sun can be directly overhead.

Tropic of Capricorn

The circle of latitude at 23.5° S, marking the southernmost point at which the Sun can be directly overhead.

Umbra

The central region of a shadow during an eclipse where sunlight is completely blocked.

Waxing

Refers to the lunar phases from new to full, when the illuminated portion of the Moon is increasing.

Waning

Refers to the lunar phases from full to new, when the illuminated portion of the Moon is decreasing.

Zenith

The point in the sky directly above an observer.

Zodiac

The constellations along the ecliptic.