Comprehensive Astronomy and Physics: Units, Celestial Mechanics, and Earth's Movements

Science

A systematic method for studying the natural world using observation, experiment, and reasoning.

scientific hypothesis

A testable, falsifiable proposed explanation for an observation.

astronomy

The scientific study of celestial objects, space, and the universe.

meter

The SI unit of length (distance light travels in vacuum in 1/299,792,458 of a second).

second

The SI unit of time (based on the cesium atomic clock definition).

kilogram

The SI unit of mass (defined via fundamental constants).

astronomical unit (AU)

The average distance from Earth to the Sun (about 149.6 million kilometers).

light-year

The distance light travels in one year (about 9.46 trillion kilometers).

parsec

A distance unit ≈ 3.26 light-years; defined by a parallax of 1 arcsecond.

parallax

The apparent shift in an object's position when seen from different viewpoints; used to measure distance.

constellation

A recognized pattern or region of stars on the sky.

celestial sphere

An imaginary sphere onto which celestial objects are projected for positional reference.

celestial pole

Points where Earth's rotation axis intersects the celestial sphere (north and south).

celestial equator

Projection of Earth's equator onto the celestial sphere.

ecliptic

The Sun's apparent path on the celestial sphere; plane of Earth's orbit.

horizon

The boundary between sky and Earth as seen by an observer.

zenith

The point on the sky directly overhead an observer.

circumpolar

Stars or constellations that never set below the horizon for a given latitude, circling the pole.

latitude

Angular distance north or south of Earth's equator.

longitude

Angular distance east or west of a defined prime meridian.

solar day

Time between successive solar noons (about 24 hours).

sidereal day

Time for Earth to rotate once relative to distant stars (about 23h 56m).

summer solstice

Moment when a hemisphere is maximally tilted toward the Sun; longest day of the year there.

winter solstice

Moment when a hemisphere is maximally tilted away from the Sun; shortest day of the year there.

vernal equinox

When the Sun crosses the celestial equator northward; start of spring in the northern hemisphere.

autumnal equinox

When the Sun crosses the celestial equator southward; start of fall in the northern hemisphere.

precession

The slow, conical wobble of Earth's rotation axis that changes the orientation of the celestial poles over ~26,000 years.

waxing

Phase when the Moon's illuminated portion visible from Earth is increasing.

waning

Phase when the Moon's illuminated portion visible from Earth is decreasing.

crescent

A Moon phase with less than half the disk illuminated, appearing as a thin arc.

gibbous

A Moon phase with more than half the disk illuminated but not full.

solar eclipse

When the Moon passes between Earth and Sun, blocking some or all sunlight.

lunar eclipse

When Earth passes between Sun and Moon, casting Earth's shadow on the Moon.

umbra

The darkest central part of a shadow where the light source is fully blocked.

penumbra

The outer, partially shaded part of a shadow where the light source is partially blocked.

retrograde motion

Apparent backward motion of a planet against the background stars due to relative orbital motion.

position

Location of an object in space relative to a reference frame, typically given by coordinates.

velocity

Rate of change of position with time, including both speed and direction.

speed

The magnitude of velocity; how fast an object is moving regardless of direction.

acceleration

Rate of change of velocity with time (change in speed or direction).

force

An interaction that changes an object's motion (quantified by mass × acceleration).

Newton's Laws

Three laws describing motion: inertia, F = ma, and action-reaction pairs.

ellipse

A closed oval curve defined as the set of points where the sum of distances to two foci is constant.

focus

Either of the two fixed points used to define an ellipse; in orbital dynamics, the central mass lies at one focus.

semimajor axis

Half the longest diameter of an ellipse; in orbits, it represents the average orbital distance.

Kepler's Laws

Three laws describing planetary motion: (1) orbits are ellipses with the Sun at a focus, (2) equal areas in equal times, (3) P^2 ∝ a^3 relating period and semimajor axis.

Eccentricity

A number between 0 and 1 that measures how stretched an ellipse is (0 = circle).

displacement

Change in position vector from start to end point.

mass

Amount of matter in an object; resistance to acceleration.

weight

Force of gravity on an object (mass × local gravitational acceleration).

Universal Law of Gravitation

Every mass attracts every other with force proportional to the product of their masses and inversely proportional to the square of their separation.

inverse square law

A quantity that spreads out so its strength falls off proportional to 1/r^2 with distance r.

tidal force

Differential gravitational force across an extended object that stretches it (source of tides).

spring tide

Especially large tides that occur when the Sun, Earth, and Moon are aligned (new or full Moon).

neap tide

Weaker tides that occur when the Sun and Moon are at right angles relative to Earth (quarter moons).

tidal friction

Energy dissipation from tidal deformation, altering rotation and orbits over time.

synchronous rotation

When an object's rotation period equals its orbital period, so the same face always points toward its partner.

momentum

Mass times velocity; a conserved quantity in absence of external forces.

kinetic energy

Energy of motion; classically 1/2 mv^2.

potential energy

Stored energy due to position or configuration (e.g., gravitational potential energy).

radiative energy

Energy carried by electromagnetic radiation (light).

temperature

Measure of the average kinetic energy of particles in a substance.

Kelvin scale

Absolute temperature scale with zero at absolute zero; degree size equals Celsius degree.

photon

Quantum (particle) of light carrying energy E = hf.

electromagnetic spectrum

Range of all wavelengths/frequencies of electromagnetic radiation.

wavelength

Distance between successive peaks of a wave.

frequency

Number of wave cycles per second.

proton

Positively charged particle in an atomic nucleus.

neutron

Neutral particle in an atomic nucleus.

electron

Negatively charged particle that orbits an atomic nucleus.

nucleus

Central part of an atom containing protons and neutrons.

ion

Atom or molecule with a net electric charge due to loss or gain of electrons.

isotope

Versions of an element with the same number of protons but different numbers of neutrons.

atomic number

Number of protons in an atom's nucleus (defines the element).

atomic mass number

Total number of protons plus neutrons in the nucleus.

solid

State of matter with fixed shape and volume.

liquid

State of matter with fixed volume but variable shape.

gas

State of matter with variable shape and volume, particles widely separated.

plasma

Ionized gas of charged particles (electrons and ions).

electron energy levels

Discrete energy states electrons can occupy in an atom.

ground state

Lowest energy state of an atom or electron.

excited state

Any energy state higher than the ground state.

emission spectrum

Bright lines at specific wavelengths produced when atoms emit photons.

absorption spectrum

Dark lines where light at specific wavelengths is absorbed by material.

continuous spectrum

Smooth range of wavelengths without discrete spectral lines.

Doppler effect

Change in wavelength/frequency of waves due to relative motion between source and observer.

redshift

Increase in observed wavelength when an object moves away (Doppler redshift).

blueshift

Decrease in observed wavelength when an object moves toward the observer.

refraction

Bending of light as it passes between media with different propagation speeds.

reflection

Bouncing of light off a surface.

spherical aberration

Optical blur from spherical lenses/mirrors not focusing all rays to a single point.

chromatic aberration

Color blurring caused by different wavelengths focusing at different points.

sagging

Deformation of telescope components under gravity that causes image distortion.

inhomogeneities

Nonuniformities (in materials or atmosphere) that degrade imaging or propagation.

angular resolution

Smallest angular separation a telescope can distinguish.

twinkling

Rapid scintillation of starlight caused by atmospheric turbulence.

light pollution

Artificial sky brightness that reduces visibility of celestial objects.

adaptive optics

Technology that corrects atmospheric blurring in real time for telescopes.

interferometry

Technique combining signals from multiple telescopes to improve resolution.

terrestrial planet

A small, rocky planet like Mercury, Venus, Earth, or Mars.