Space writing is full of words like albedo, opposition and redshift. This glossary explains each one in plain English, with a real example from our solar system and a link to the Dark Cosmic Insights guide that goes deeper. Where a definition has an official wording or an exact number, we quote and link the source.
Jump to: A · B · D · E · F · G · I · K · L · M · N · O · P · R · S · T
A
Albedo
How much light a world reflects. NASA’s fact sheets use the Bond albedo, “the fraction of incident solar radiation reflected back into space without absorption” (NASA fact sheet notes). Earth’s is 0.294 and Saturn’s is 0.342, so Saturn bounces back a slightly larger share of the sunlight that hits it. Go deeper: How Cold Is Saturn?.
Anticyclone
A storm that spins around a center of high pressure. Jupiter’s Great Red Spot is the most famous anticyclone in the solar system. Go deeper: What Color Is Jupiter?.
Aphelion
The point in an orbit farthest from the Sun (NASA fact sheet notes). Earth reaches aphelion in early July, about 152.1 million km from the Sun. Its opposite is perihelion. Go deeper: How Far Is Jupiter?.
Astronomical unit (AU)
The standard distance from the Sun to Earth, 149,597,870.7 km (NASA fact sheet notes). Astronomers use it to describe distances inside the solar system: Jupiter is about 5.2 AU from the Sun and Neptune about 30 AU. Go deeper: Compare the Planets.
Axial tilt (obliquity)
The angle between a planet’s equator and the plane of its orbit (NASA fact sheet notes). Earth’s tilt of 23.4° gives us seasons. Uranus, at 97.8°, rolls around the Sun on its side. Go deeper: Compare the Planets.
B
Bar
A unit of pressure. One bar is about the air pressure at sea level on Earth. Because giant planets have no solid ground, scientists often quote their temperatures and gravity at the “1-bar level.” Go deeper: How Cold Is Saturn?.
Black hole
A region where gravity is so strong that nothing, not even light, can escape once it crosses the boundary called the event horizon. Go deeper: What Is a Black Hole?.
Black-body temperature
The temperature a world would have from sunlight alone if it had no atmosphere but reflected the same share of sunlight (the same albedo), a theoretical reference number rather than a layer you could visit (NASA fact sheet notes). Saturn’s is 81 K. Go deeper: How Cold Is Saturn?.
D
Dark matter
Matter that doesn’t give off, absorb or reflect light, but whose gravity shows up in how galaxies spin and how galaxy clusters bend light. What it’s made of is still unknown. Go deeper: What Is Dark Matter?.
Dwarf planet
A world that orbits the Sun and is massive enough to be pulled into a round shape, but hasn’t cleared other objects out of its orbital neighborhood, and is not a moon. Pluto was moved into this category in 2006. Go deeper: Why Is Pluto Not a Planet?.
E
Eccentricity
A measure of how stretched an orbit is (NASA fact sheet notes). Zero is a perfect circle. Venus’s orbit, at 0.007, is almost circular; Mercury’s, at 0.206, is the most stretched of the planets. Go deeper: Compare the Planets.
Effective temperature
The temperature a perfect black body would need to give off the same total energy a planet actually radiates. It differs from the black-body temperature, which counts sunlight alone, and from the cloud-top temperature, which is measured at one level of the atmosphere. Planets with internal heat, like Saturn, have an effective temperature above their black-body value. Go deeper: How Cold Is Saturn?.
Escape velocity
The speed needed to break free of a world’s gravity without further push (NASA fact sheet notes). It’s 11.2 km/s for Earth and 59.5 km/s for Jupiter. Go deeper: Compare the Planets.
Exoplanet
A planet that orbits a star other than the Sun. Some planets don’t orbit any star at all; those are called rogue planets. Go deeper: Rogue Planets.
F
False color
An image whose colors don’t match what your eyes would see, usually because the camera recorded light we can’t see, such as infrared, and scientists assigned it visible colors. False color isn’t fake; it’s a way of showing real data. Go deeper: Are Space Photos Real?.
G
Galaxy
A huge system of stars, gas, dust and dark matter held together by gravity. Our galaxy, the Milky Way, is a spiral. Go deeper: Types of Galaxies.
Gas giant
A large planet made mostly of hydrogen and helium, with no solid surface to stand on. Jupiter and Saturn are the solar system’s gas giants. Go deeper: What Is Jupiter Made Of?.
Gravitational lensing
The bending of light by a massive object, such as a galaxy cluster, which can magnify a more distant object or show it more than once. Go deeper: SN Refsdal.
Gravity assist
A maneuver in which a spacecraft swings past a planet and uses its gravity to change speed and direction without burning fuel. Galileo and Juno both used them to reach Jupiter. Go deeper: How Far Is Jupiter?.
I
Ice giant
The name for Uranus and Neptune. Below their hydrogen and helium atmospheres, much of their mass is made of heavier “ices” such as water, methane and ammonia, in hot, dense fluid form. Go deeper: Why Is Neptune Blue?.
Infrared
Light with wavelengths longer than red light, invisible to our eyes. Warm objects give it off. The James Webb Space Telescope observes mainly in infrared, which lets it see through dust. Go deeper: Hubble vs. Webb.
K
Kelvin (K)
A temperature scale that starts at absolute zero, the coldest possible temperature. 0 K is −273.15 °C, and a change of 1 K equals a change of 1 °C. Go deeper: How Cold Is Space?.
L
Light-travel time
How long light takes to cover a distance. Sunlight takes about 8 minutes 19 seconds to reach Earth (DCI calculation from 1 AU and the speed of light, 299,792.458 km/s), and about 43 minutes to reach Jupiter. Go deeper: How Far Is Jupiter?.
Light-year
A unit of distance, not time: how far light travels in one year, about 9.46 trillion km (IAU Office of Astronomy for Education). Go deeper: How Astronomers Measure Stars.
M
Magnitude
A brightness scale in which lower numbers mean brighter objects, and the brightest have negative numbers. Jupiter averages about magnitude −2.7 at opposition and reaches about −2.9 at its brightest (NASA Jupiter Fact Sheet). Go deeper: How Far Is Jupiter?.
Mass
The amount of matter in an object, measured in kilograms. It doesn’t change simply because you move to another planet. Your weight changes from planet to planet; your mass doesn’t. Go deeper: Compare the Planets.
Metallic hydrogen
Hydrogen squeezed so hard, deep inside Jupiter and Saturn, that it conducts electricity like a metal. Currents in it are thought to power those planets’ magnetic fields. Go deeper: What Is Jupiter Made Of?.
Moon (natural satellite)
A natural object that orbits a planet, dwarf planet or asteroid. Counts change as new moons are confirmed, so DCI always dates them. Go deeper: Which Planet Has the Most Moons?.
N
Natural color (true color)
An image processed to approximate what a human eye would see if it were there. Go deeper: Are Space Photos Real?.
Nebula
A cloud of gas and dust in space. Some are nurseries where new stars form; others are the remains of dying stars. Go deeper: What Is a Nebula?.
O
Opposition
When a planet sits on the opposite side of Earth from the Sun (NASA fact sheet notes). The planet is up all night and near its closest and brightest. Jupiter’s next opposition is February 11, 2027. Go deeper: How Far Is Jupiter?.
Orbital period
The time a world takes to go once around the Sun, measured against the distant stars; a planet’s “year” (NASA fact sheet notes). Mercury’s is 88 days; Neptune’s is about 165 Earth years. Go deeper: Compare the Planets.
P
Parsec
A unit of distance used by professional astronomers, about 3.26 light-years (IAU Office of Astronomy for Education). It comes from parallax, the small shift in a nearby star’s position as Earth orbits the Sun. Go deeper: How Astronomers Measure Stars.
Perihelion
The point in an orbit closest to the Sun (NASA fact sheet notes). Earth reaches perihelion in early January, about 147.1 million km from the Sun. Go deeper: How Far Is Saturn?.
Planet
Under the International Astronomical Union’s 2006 definition, a body that orbits the Sun, is round from its own gravity and has cleared its orbital neighborhood. Eight worlds qualify. Go deeper: Why Is Pluto Not a Planet?.
R
Redshift
The stretching of light toward longer, redder wavelengths, for example as galaxies recede in an expanding universe. Astronomers use it to measure how far and how fast galaxies are moving away. Go deeper: How Old Is the Universe?.
Retrograde rotation
Spinning in the opposite direction to most planets. Venus and Uranus both spin backward, which is why NASA lists their rotation periods as negative numbers. Go deeper: Compare the Planets.
Ring plane crossing
When Earth passes through the plane of Saturn’s rings and we see them edge-on. Because the rings are so thin, they nearly vanish from view for a while. Go deeper: Are Saturn’s Rings Disappearing?.
Ring system
Bands of dust, rock or ice orbiting a planet. All four giant planets have rings; Saturn’s are made mostly of water ice, while Jupiter’s are faint, dark dust. Go deeper: Does Jupiter Have Rings?.
Rogue planet
A planet that drifts through space without orbiting any star. Go deeper: Rogue Planets.
S
Sidereal rotation
One spin of a world measured against the distant stars (NASA fact sheet notes). It can differ a lot from the solar day: Mercury spins once every 1,407.6 hours, but its noon-to-noon day lasts 4,222.6 hours. Go deeper: Compare the Planets.
Solar day
The time for the Sun to return to the same spot in the sky, noon to noon (NASA fact sheet notes). On Earth it’s 24 hours; on Jupiter, about 9.9. Go deeper: What Is Jupiter Made Of?.
Spectroscopy
Splitting light into its colors to read the fingerprints of chemical elements. It tells astronomers what stars and planets are made of, how hot they are and how fast they’re moving. Go deeper: How Astronomers Measure Stars.
Supernova
The explosion of a star, either a massive star collapsing at the end of its life or a white dwarf that blows apart. For a short time it can outshine its whole galaxy. Go deeper: Life Cycle of a Star.
Synodic period
The time between repeats of the same arrangement of a planet, Earth and the Sun, such as one opposition to the next (NASA fact sheet notes). For Jupiter it’s about 398.9 days, which is why Jupiter is best seen about once every 13 months. Go deeper: How Far Is Jupiter?.
T
Tidal locking
When gravity has slowed a moon’s spin until one rotation matches one orbit, so it always shows the same face to its planet. Our Moon is tidally locked to Earth. Go deeper: What Is Tidal Locking?.
Commonly Confused Pairs
These terms often get swapped, even in news stories. Here’s how to tell each pair apart.
AU vs. light-year
Both measure distance, at different scales. The AU (149,597,870.7 km, the Sun–Earth distance) suits the solar system: Neptune is about 30 AU out. The light-year (about 9.46 trillion km) suits the stars. One light-year is about 63,000 AU (DCI calculation). Neither one is a unit of time.
Solar day vs. rotation
Rotation (the sidereal day) is one spin measured against the stars; the solar day is noon to noon. Because a planet also moves along its orbit, the two differ. On Earth the gap is about four minutes; on Mercury, a single solar day lasts three spins (NASA fact sheet notes; DCI calculation from 4,222.6 h and 1,407.6 h).
Mass vs. weight
Mass is how much matter you contain, in kilograms, and it doesn’t change simply because you move to another planet. Weight is the pull of gravity on that mass. A 70 kg person on Mars still has 70 kg of mass but feels about 38% as heavy, because Mars’s surface gravity is 3.7 m/s² against Earth’s 9.8 (DCI calculation from the NASA Planetary Fact Sheet).
Cloud-top vs. black-body vs. effective temperature
Giant planets have no surface, so their “temperature” depends on which one you mean. The cloud-top temperature is measured at a set pressure level, such as 134 K at Saturn’s 1-bar level. The black-body temperature (81 K for Saturn) is a theoretical value from sunlight alone. The effective temperature reflects all the energy the planet actually gives off, including its own internal heat. Read more in How Cold Is Saturn?.
Opposition vs. closest approach
Opposition is when a planet sits opposite the Sun in our sky. Closest approach is when it’s nearest to Earth. Because orbits are stretched, not perfect circles, the two can fall days apart, most noticeably for Mars.
Moon count vs. moon discovery
A moon can be spotted long before it’s confirmed and counted. Totals change as new moons get official orbits, which is why DCI always dates its moon counts. See Which Planet Has the Most Moons?.
Keep Exploring
- All eight planets side by side: Compare the Planets
- Every planet, one stop at a time: Solar System Explorer
- Everything on the ringed giant: Saturn hub
- Everything on the biggest planet: Jupiter hub
- How a claim earns its label: How We Know
How This Glossary Was Checked
The pair explanations and definitions with official wording or exact values are quoted from, or checked against, the NASA fact sheet notes and the IAU Office of Astronomy for Education. Planet numbers come from the NASA Planetary Fact Sheet. The 8-minute-19-second light-travel time is a DCI calculation. Every other definition is written in plain English and links to a Dark Cosmic Insights guide that cites its own primary sources. Checked November 2026; new terms are added as new guides go live.