Astronomy 30500 Study Guide

A series of flashcards covering key concepts and vocabulary from the Astronomy 30500 course study guide.

Light Year

A unit of DISTANCE: how far light travels in one year (~9.46 × 10^{12} km).

Celestial Sphere

An imaginary sphere surrounding Earth onto which stars appear projected.

Ecliptic

The apparent annual path of the Sun across the celestial sphere.

Seasons

Caused by Earth's axial tilt of 23.5°, not its distance from the Sun.

Right Ascension (RA)

Like celestial longitude; measured eastward in hours (0h – 24h) from the vernal equinox.

Declination (Dec)

Like celestial latitude; measured in degrees (–90° to +90°) from the celestial equator.

Geocentric Model

Earth-centered model of the universe proposed by Ptolemy using epicycles to explain planetary motion.

Heliocentric Model

Sun-centered model proposed by Copernicus and championed by Galileo and Kepler.

Rapidity of Retrograde Motion

Apparent shift of a planet westward relative to background stars, caused by Earth 'lapping' an outer planet.

Kepler's First Law

Planets orbit in ELLIPSES with the Sun at one focus.

Newton's First Law of Motion

An object at rest stays at rest, and an object in motion continues in a straight line unless acted on by a net force.

Mass vs. Weight

Mass is the amount of matter in an object; weight is the force of gravity on that object and varies with local gravity.

Law of Universal Gravitation

Gravitational force is inversely proportional to the square of the distance between two masses.

Photon Energy

E = hf = hc/λ, where a shorter wavelength corresponds to higher frequency and more energy.

Doppler Effect

The shift in wavelength of light from a moving source: redshift when moving away, blueshift when moving toward.

Special Relativity

Einstein's theory stating that the laws of physics are identical in all inertial frames, with c being constant in all frames.

Twin Paradox

A scenario where one twin traveling at near-light speed ages slower than the twin who remains on Earth.

Adaptive Optics

Technology that corrects for atmospheric distortion in real time for ground-based telescopes.

Inferior Conjunction

When an inferior planet is between Earth and the Sun, rising at sunrise.

Opposition

When a superior planet is opposite the Sun, rising at sunset and visible all night.