Observable Sky and Stellar Motion

Vocabulary flashcards covering astronomical coordinates, motions of celestial bodies, and angular measurements based on the lecture transcript.

Constellations

Groupings of stars totaling $88$ official designations today.

Asterisms

A recognizable pattern on the sky that is not an official constellation, such as the Big and Little Dipper.

Celestial Sphere

A geocentric model of the heavens where stars, sun, and planets are projected onto a sphere.

Latitude

Geographical coordinate that measures north and south of the equator.

Longitude

Geographical coordinate that measures east and west of the Prime Meridian along the equator.

Declination

Equatorial coordinate that measures north and south of the celestial equator.

Right Ascension

Equatorial coordinate that measures eastward along the celestial equator.

Altitude

Horizon coordinate that measures distance above and below the horizon.

Azimuth

Horizon coordinate that measures eastward from due north along the horizon.

Zenith

The point in the sky directly overhead, corresponding to an altitude of $+90^\circ$.

Nadir

The point in the sky directly below the observer, opposite the Zenith.

Meridian

A line that passes from the North Celestial Pole (NCP) to the Zenith to the South Celestial Pole (SCP), dividing the sky into eastern and western halves.

Ante meridiem (a.m.)

Latin phrase meaning "before midday" used in timekeeping.

Post meridiem (p.m.)

Latin phrase meaning "after midday" used in timekeeping.

Ecliptic

The path of the Sun in the sky through the zodiac, reflecting Earth's orbit around the Sun, which is tilted by $23.5^\circ$.

Solar Motion (Relative to stars)

The Sun moves West to East relative to stars at a rate of approximately $1^\circ/\text{day}$.

Lunar Motion (Relative to stars)

The Moon moves West to East relative to stars at a rate of approximately $12^\circ/\text{day}$, rising approximately $50$ minutes later each day.

Stellar Motion (Relative to Sun)

In the daily sky, stars rise $4$ minutes earlier each day relative to the Sun.

Circumpolar Stars

Stars that never set below the horizon from the observer's perspective.

Prograde Motion

West to East ($W \rightarrow E$) motion of planets relative to stars from night to night.

Retrograde Motion

East to West ($E \rightarrow W$) motion of planets relative to stars from night to night, caused by an optical effect of differing orbits and speeds.

Planet

A term derived from the Greek word for "wanderer."

Arcmin (minute of arc)

A unit of angular measurement where $1^\circ = 60\text{ arcmin}$, and is the smallest angle visible to the unaided eye.

Arcsec (second of arc)

A unit of angular measurement where $1\text{ arcmin} = 60\text{ arcsec}$.