The Night Sky - Chapter 2 (PHYS 405/406)

Vocabulary flashcards covering key concepts from Chapter 2: The Night Sky, including coordinates, constellations, celestial sphere, phases, eclipses, and the Sun’s path.

Celestial Sphere

A model of Earth-centered sky with stars fixed on the surface; used to describe positions of celestial objects. In reality, stars are not fixed.

Dome of the Sky

A visualization of the sky as a dome over the Earth, used to imagine the sky’s appearance from a given location.

Celestial Sphere vs Dome of the Sky

Celestial Sphere is a constant, mathematical construct; the Dome of the Sky is the observable portion of that sphere from a location.

Local Sky

The portion of the celestial sphere visible from a specific location at a specific time.

Zenith

The point directly overhead; altitude = 90°.

Horizon

The boundary where the sky appears to meet the ground; altitude = 0°.

Meridian

The imaginary great circle passing north–south through the zenith; divides east and west in the sky.

Altitude

Angular height above the horizon, measured from 0° (horizon) to 90° (zenith).

Azimuth

Compass direction along the horizon, usually measured clockwise from North.

Latitude

Angular distance north or south of the Earth's equator; 0° at the equator, ±90° at the poles.

Longitude

East–west position relative to the Prime Meridian; measured in degrees.

Right Ascension (RA)

Celestial longitude measured in hours along the celestial equator from the vernal equinox; 24h around the sky.

Declination (Dec)

Celestial latitude measured in degrees north or south of the celestial equator; range from −90° to +90°.

Celestial Equator

Projection of Earth's equator onto the celestial sphere.

Ecliptic

The Sun’s apparent path on the celestial sphere; inclined 23.5° to the celestial equator.

Vernal Equinox

Around March 21, Sun crosses the celestial equator moving north; RA = 0h.

Autumnal Equinox

Around September 21, Sun crosses the celestial equator moving south.

Summer Solstice

Around June 21, Sun reaches its northernmost declination; longest day.

Winter Solstice

Around December 21, Sun reaches its southernmost declination; shortest day.

Solstices vs Equinoxes

Solstice: extreme Sun declination; Equinox: Sun on celestial equator with day and night equal.

Seasons

Driven by Earth’s axial tilt and changing day length, not by distance to the Sun.

Lunar Phases

The Moon’s appearance changes through New Moon, First Quarter, Full Moon, Last Quarter; waxing/waning describes brightness over time.

Moon’s Rotation (Synchronous Rotation)

The Moon always presents the same face to Earth; it does rotate, but its orbital period matches its rotation relative to us.

Lunar Eclipse

Moon enters Earth’s shadow; can be total, partial, or penumbral; involves umbra and penumbra.

Umbra

The dark inner part of a shadow where the Sun is completely obscured.

Penumbra

The outer part of a shadow where the Sun is only partially obscured.

Nodes (Lunar Orbit Crossing Points)

Points where the Moon’s orbit crosses the ecliptic; eclipses occur near these nodes.

Solar Eclipse

Moon blocks the Sun; can be total, partial, or annular; involves umbra and penumbra; path of totality is a narrow line on Earth.

Annular Eclipse

Moon’s umbra misses Earth, Sun appears as a bright ring (annulus) around the Moon.

Big Dipper / Ursa Major

An asterism in Ursa Major; useful for locating Polaris via pointer stars.

Little Dipper / Ursa Minor

Contains Polaris; useful for finding north.

Polaris

The North Star, near the celestial north pole; part of Ursa Minor.

Summer Triangle

Asterism consisting of Deneb (Cygnus), Vega (Lyra), and Altair (Aquila).

Winter Triangle

Asterism formed by Procyon (Canis Minor) and Castor and Pollux (Gemini).

88 Modern Constellations

The 88 official constellations defined by the IAU that cover the entire sky.

Ecliptic Inclination (Earth’s Tilt)

The Earth’s axis is tilted ~23.5° relative to its orbital plane.

Star Trails

Apparent circular/arc paths of stars in long-exposure photos, caused by Earth’s rotation.

Daily Motion of the Stars

Stars rise in the East and set in the West due to Earth’s rotation; patterns vary by latitude.

Circumpolar Stars

Stars near the celestial pole that never set for observers at high latitudes.