Astronomy Terms and Definitions

Hypothesis in astronomy

A reasonable description of nature that is tested through experiments and observations.

Light year

The distance that light travels in one year.

Star distance observation

A star is 230 light years away. The light we see tonight from that star left it 230 years ago.

Star providing energy for life

The Sun.

Earth's location in the Milky Way

A little less than 30,000 LY from the center.

Visibility of stars in the Milky Way

Dust in the space between stars builds up over large distances and blocks the light of stars behind the dustier parts of the Milky Way.

Smallest piece of an element

An atom.

Universe definition

Everything that we can observe.

Earth's location in cosmic structures

You can't fool me, we are located in all of the above.

Astronomers and detectives analogy

Both astronomers and detectives must test their hypotheses against any evidence that they gather.

Laws of nature

More than one of the above.

Speed of light

Light travels 3 x 10^5 meters every second, which is 300,000 meters.

Closest natural object to Earth

The Moon.

Astronomical Unit (AU)

The average distance between the Earth and the Sun.

Large body in space that emits light

A star.

Stars visible to the unaided eye

In the Milky Way Galaxy.

Molecules composition

Two or more atoms.

Element definition

The number of protons in its nucleus.

Largest cosmic structure

The Universe.

Closest celestial body

The Moon.

Celestial point directly above

Zenith.

Celestial sphere rotation

The planet on which we live is rotating.

Celestial poles location

The Earth's axis.

Halfway point on the celestial sphere

Celestial equator.

Star not turning with the sky

At the north celestial pole.

Familiar stars at the equator

All stars rise and set (no star remains in the sky all night long).

Celestial Pole

Point in the sky directly above Earth's axis.

Celestial Equator

Imaginary line equidistant from celestial poles.

North Star

Star closest to the North Celestial Pole.

Circumpolar Zone

Region around celestial poles where stars never set.

Zenith

Point in the sky directly above the observer.

Ecliptic

Sun's apparent path across the celestial sphere.

Zodiac

Band of the sky where Sun and planets move.

Constellations

88 sectors dividing the celestial sphere.

Asterism

Recognizable star pattern within a constellation.

Retrograde Motion

Temporary westward movement of a planet in the sky.

Precession

Slow tipping of Earth's axis over 26,000 years.

Eratosthenes' Method

Measured Sun's height in two cities to find Earth's size.

Hipparchus' Contributions

Cataloged stars and developed magnitude system.

Ptolemy's System

Planets orbit Earth, explaining retrograde with epicycles.

Altitude of North Star

Equal to observer's latitude in Northern Hemisphere.

Angular Distance of Sky

Sky spans 180 degrees from horizon to horizon.

Circumpolar Stars

Stars that remain visible all year in certain latitudes.

Celestial Sphere

Imaginary sphere where celestial objects appear.

Celestial Navigation

Using stars for navigation and positioning on Earth.

Astrology

Study of celestial positions affecting human affairs.

Polaris

Current North Star, located near celestial pole.

Big Dipper

Famous asterism in the Ursa Major constellation.

Southern Circumpolar Zone

Area where southern stars never rise in Boston.

Tilt of Earth's Axis

Earth's axis tilted about 23 degrees from vertical.

Celestial Hot-Zone

Region with high concentrations of celestial bodies.

Photographic Evidence

Images of Earth from space show its round shape.

Astrological Predictions

Claims about celestial influence on personality and fate.

Scientific Test

Method to evaluate the validity of a hypothesis.

Geocentric Model

Earth-centered model of the solar system.

Heliocentric Model

Sun-centered model of the solar system.

Ptolemy

Ancient astronomer who influenced astrology's development.

Copernicus

Proposed heliocentric model of the solar system.

Galileo

First to test heliocentric model experimentally.

Tycho Brahe

Astronomer known for precise astronomical observations.

Johannes Kepler

Formulated three laws of planetary motion.

Elliptical Orbit

Shape of a stable planet's orbit around a star.

Eccentricity

Measure of how much an orbit deviates from a circle.

Kepler's Third Law

Relationship between a planet's orbital period and distance from the Sun.

Inertia

Tendency of objects to maintain their state of motion.

Force

Required to change an object's motion or direction.

Milky Way

Galaxy containing our solar system.

Phases of Venus

Demonstrated that Venus orbits the Sun, not Earth.

Astrological Signs

Zodiac signs based on constellations' positions.

Newton's Laws

Laws governing motion and forces in the universe.

Planetary Motion

Movement of planets in their orbits around the Sun.

Telescope

Instrument used to observe distant celestial objects.

Renaissance Astronomy

Period of significant advancements in astronomical knowledge.

Retrograde Motion Explanation

Galileo's heliocentric hypothesis clarified planetary movements.

Astronomical Unit

Average distance from Earth to the Sun.

Orbital Speed

Speed of a planet in its orbit around the Sun.

Momentum

The product of mass and velocity of an object.

Acceleration

Change in velocity per unit time due to force.

Equal and Opposite Forces

For every action, there is an equal reaction.

Density

Mass per unit volume of a substance.

Aphelion

Furthest point in an orbit from the Sun.

Gravity

Attractive force between two masses.

Newton's 3rd Law

Forces occur in action-reaction pairs.

Weight in Space

Weight decreases with distance from Earth's center.

Newton's Law of Gravity

Force of gravity varies with mass and distance.

Calculus

Mathematical technique developed by Newton for gravity.

Orbital Period

Time taken to complete one orbit around a body.

Escape Velocity

Minimum speed to break free from a gravitational field.

Circular Satellite Velocity

Speed to maintain a stable orbit around Earth.

Comet's Perihelion

Closest point in a comet's orbit to the Sun.

Asteroid Belt

Region between Mars and Jupiter with rocky bodies.

First Artificial Satellite

Sputnik, launched by the Soviet Union in 1957.

Kepler's 2nd Law

Planets move faster when closer to the Sun.

Friction in Orbits

Satellites lose speed due to atmospheric drag.

Mass and Gravity

More mass results in stronger gravitational pull.

Free Fall

Motion of an object under the influence of gravity.

Newton's Universal Law

Gravitational force acts universally, everywhere in space.

Eccentric Orbits

Orbits that are elongated rather than circular.

Gravity's Strength

Decreases with the square of the distance.

Space Shuttle Floatation

Astronauts float due to free fall with the Shuttle.