Atronomy for the Slays

Meridian

A great circle in the sky passing through the local zenith, nadir, and celestial poles; separates east and west skies.

Prime Meridian

Zero longitude on Earth, passing through Greenwich, England.

Horizon

The apparent line separating Earth and sky.

Azimuth

Following the Horizon, North to East. It creates an angle

Zenith

The point directly overhead from any location on Earth.

Nadir

The point directly beneath any location on Earth.

Altitude

Angular position measured upward from the horizon toward the zenith.

Celestial Sphere

Imaginary sphere on which the Sun, Moon, stars, and planets appear fixed.

Celestial Equator

Projection of Earth's equator onto the celestial sphere.

Celestial Poles

Points in the sky where Earth's axis intersects the celestial sphere; stars appear to rotate about them.

Ecliptic

An imaginary Path of the Sun on the celestial sphere over a year (~1° per day eastward).

Sidereal Day

Earth's rotation period relative to the stars (23h 56m 4s).

Solar Day

Time for the Sun to return to the meridian (24h).

Equinox

Points where the ecliptic crosses the celestial equator (vernal and autumnal).

Right Ascension (RA)

Celestial equivalent of longitude, measured eastward from RA = 0 (March equinox).

Declination (Dec)

Celestial equivalent of latitude, angular distance north/south of celestial equator.

Astronomical Unit (AU)

Average distance from Earth to Sun.

Parsec (pc)

Distance at which Earth's orbit subtends 1 arcsecond (~3.1×10¹⁶ m).

Obliquity

Earth's axial tilt (23.4°) relative to its orbital plane.

Analemma

Figure traced by the Sun's position at the same clock time across a year.

Satellite

Any object that orbits a planet.

Impact Craters

Bowl-shaped depressions formed by asteroid/comet collisions.

Lunar Basins and Maria

Dark, smooth plains formed from ancient lava flows.

Lunar Highlands

Rugged, bright terrain with many craters; older than maria.

Lunar Regolith

The loose, dusty layer of rock fragments covering the Moon.

Sidereal Month

27.3 days; time for the Moon to orbit once around the celestial sphere.

Solar Month

29.5 days; time between full moons.

Umbra

Full shadow of the Moon or Earth, where eclipses are total.

Annular Eclipse

Solar eclipse where the Moon appears too small to cover the Sun fully.

Apogee

Farthest point in the Moon's orbit from Earth.

Perigee

Nearest point in the Moon's orbit to Earth.

Totality

Period of complete eclipse.

Solar Corona

The Sun's outer atmosphere, visible during total solar eclipses.

Ecliptic Plane

The plane of Earth's orbit around the Sun.

Synchronous Rotation

State where the Moon's rotation period equals its orbital period (tidal locking).

Fission Theory

Hypothesis that the Moon split off from Earth.

Co-formation Theory

Hypothesis that the Earth and Moon formed together from the solar nebula.

Capture Theory

Hypothesis that Earth's gravity captured the Moon.

Large Impact (Giant Impact) Hypothesis

Moon formed from debris after a Mars-sized body (Theia) struck Earth.

Tides

Differential gravitational forces causing periodic ocean rises and falls.

Spring Tide

Extra-strong tide when Sun and Moon's gravitational pulls align (new/full moon).

Neap Tide

Weaker tide when Sun and Moon's pulls are at right angles (first/last quarter).

Geocentric Model

Ancient model with Earth at the universe's center.

Epicycles

Small circular motions superimposed on larger orbits to explain retrograde motion in geocentric theory.

Heliocentric Model

Copernican model placing the Sun at the universe's center.

Tychonian System

Model where planets orbit the Sun, but the Sun and Moon orbit Earth.

Sunspots

Dark patches on the Sun caused by cooler gases and magnetic activity.

Perihelion

Closest orbital point of a planet to the Sun.

Aphelion

Farthest orbital point of a planet from the Sun.

Eccentricity

Measure of how elongated an ellipse is (0 = circle).

Kepler's First Law

Planets orbit the Sun in ellipses with the Sun at one focus.

Kepler's Second Law

A line joining a planet and the Sun sweeps equal areas in equal times.

Kepler's Third Law

The square of a planet's orbital period is proportional to the cube of its semi-major axis.

Axial Precession

The slow conical motion of Earth's axis of rotation, tracing a circle on the celestial sphere over ~26,000 years.

Apsidal Precession

The slow shift of the longitude of perihelion (or periapse) in planetary orbits; for Earth, the cycle is ~112,000 years.

Inclination

The angle between a planet's orbital plane and the ecliptic plane.

Ascending Node

The point where a planet's orbit crosses upward through the ecliptic plane.

Periapse

The closest point in an object's orbit to the body it orbits (called perihelion for planets around the Sun, perigee for the Moon around Earth).

Argument of Periapse

The angle between the ascending node and the periapse in an orbit.

True Anomaly

The orbital angle of a planet measured from its periapse to its current position.

Nodal Precession

The gradual shift of the longitude of the ascending node around the celestial sphere; for Earth, the cycle is ~70,000 years (also called regression of nodes).

Precession

General orbital motion where the ellipse slowly rotates due to perturbations, so the orbit is not a closed ellipse.