Astronomy Chapter 1: Celestial Sphere, Constellations, and Coordinates

Vocabulary flashcards covering constellations, celestial coordinates, Earth's rotation and revolution, and solar position changes throughout the year.

Summer Triangle

A group of three bright stars in the summer sky consisting of Deneb, Altair, and Vega.

Constellations (Modern Astronomy)

The $88$ non-overlapping sky regions that cover the entire sky and have definite boundaries.

Ursa Major

The constellation (also known as the Big Dipper) whose stars are used as pointers to the north celestial pole in the northern hemisphere.

Constellation Pattern Time Scale

The time scale over which the pattern of stars in a specific constellation appears to change from Earth, which takes thousands of years because of motions of individual stars.

Zenith

The line or point directly over an observer's head, defining a direction vertically above the observer relative to their horizon regardless of location on Earth.

Right Ascension (RA)

The celestial coordinate equivalent to longitude, measured along the celestial equator on the great circle joining the north and south celestial poles upon which an object is located.

Declination

The celestial coordinate in the sky that is equivalent to latitude upon the Earth.

Zero Point of Right Ascension

The point where the Sun crosses the celestial equator moving northward in its path across the sky.

Celestial Equator

The line in the sky that is perpendicular to Earth's spin axis, dividing the celestial sphere so that exactly one-half of the sky is north of it.

Diurnal Motion

The gradual nightly motion of constellations from east to west across the sky, caused by the rotation of the Earth on its axis.

Circumpolar Stars

Stars near the celestial pole that move in circles around the pole; the number of these stars increases as an observer moves toward increasing latitude.

Observer Motion Direction

Eastward with respect to the stars, resulting from both the rotation of the Earth on its axis and the revolution of the Earth in its orbit around the Sun.

Ecliptic

The Sun's apparent path across our sky against the background stars, along which the Sun appears to move eastward at a rate of $1^\circ$ per day.

Tilt Angle of Earth's Spin Axis

A fixed angle of $23.5^\circ$ relative to the direction perpendicular to the ecliptic (known as the ecliptic pole).

Right Ascension of the Sun (March 21)

The approximate value of right ascension of the Sun on March 21, which is $0\text{ hours }0\text{ minutes}$.

Right Ascension of the Sun (June 21)

The value of right ascension of the Sun on June 21, which is $6\text{ hours }0\text{ minutes}$.

Right Ascension of the Sun (December 21)

The approximate value of right ascension of the Sun on December 21, which is $18\text{ hours }0\text{ minutes}$.

Sun's Declination Change (March 21 to June 21)

The change in the declination of the Sun between March 21 and June 21, which is approximately $23.5^\circ$.

Sky Visibility at the Equator

An observer standing on the equator is able to see $100\%$ of the overall sky over a period of one year.

Sky Visibility at the Poles

An observer at the North Pole or South Pole can see exactly $50\%$ (half) of the celestial sphere over the course of a year.