Glossary

An asteroid's absolute magnitude, written H, is the brightness it would show if it were placed 1 au from the Sun and 1 au from the observer, fully lit.^[1] Fixing the distances this way lets astronomers compare asteroids by brightness alone, and a lower H means a brighter and usually larger asteroid.^[2]

Very few asteroids have measured sizes, so for most of them H is the main size clue.^[2] Combined with an assumed albedo, it gives a diameter: an asteroid with H of 22 is about 140 m across if its surface reflects 15% of sunlight, but about 240 m across if it reflects only 5%.^[2] H itself is typically uncertain by about half a magnitude, which widens the range further.^[2]

An airburst is the explosion of an incoming asteroid high in the atmosphere. As the object plunges in, the air ahead of it slows and heats it until the pressure difference across it exceeds its strength and it breaks apart; an object that explodes this way is called a bolide.^[1] Small asteroids break up like this regularly, causing little or no damage.^[2]

Larger airbursts can do damage on the ground: the Chelyabinsk asteroid, about 19 m across, broke up over Russia in 2013 and its shock wave injured more than 1,500 people and damaged 7,300 buildings, many people being hurt by flying glass.^[3] The 1908 Tunguska airburst over Siberia knocked down millions of trees across an area of about 830 square miles, roughly 2,150 square km.^[2]

Albedo is the share of incoming sunlight that a surface reflects, on a scale from 0 for pitch black to 1 for a perfect reflector.^[1] The Moon has a very low albedo, about 0.07, while Venus has a high one, about 0.60.^[1]

Albedo matters for asteroids because brightness alone cannot tell a small, shiny rock from a large, dark one. Astronomers combine albedo with absolute magnitude to estimate size, and a wrong guess at the albedo can make an asteroid's estimated diameter wrong by a factor of almost 2.^[2] Dark asteroids reflect little visible light but still glow in infrared light as the Sun warms them, which is why NASA's NEO Surveyor telescope will search in infrared.^[3]

An annular eclipse is a solar eclipse that happens when the Moon is at or near its farthest from Earth. The Moon then looks smaller than the Sun and cannot cover it, leaving a bright ring around its dark disk.^[1]

Because the Sun is never fully covered, there is no moment when an annular eclipse is safe to look at directly; eclipse glasses or a solar viewer are needed throughout.^[2]

The astronomical unit, written au, is the standard unit for distances within the solar system. The International Astronomical Union defines it as exactly 149,597,870,700 m, which is approximately the average distance between Earth and the Sun, about 150 million km.^[1] The IAU specifies the lower-case symbol au, although AU is also widely seen.^[1]

Asteroid orbits and the categories built on them are stated in au. A near-Earth object comes within 1.3 au of the Sun, and a potentially hazardous asteroid can pass within 0.05 au of Earth's orbit, roughly 7.5 million km.^[2] Flybys are usually given in lunar distances instead, since one au is about 389 lunar distances.^[3]

A bolide is a fireball that explodes in a bright flash at the end of its path, often with fragments visible as it breaks apart.^[1] The term is technical: fireballs that explode in the atmosphere are bolides, although the two words are often used interchangeably.^[2]

A bolide that penetrates deep into the atmosphere, below about 50 km, can produce sonic booms heard on the ground a few minutes later.^[1] Larger objects exploding the same way cause an airburst.

The coma is the thin atmosphere of gas and dust that surrounds a comet's nucleus as the comet nears the Sun.^[1] The Sun's heat turns the comet's ices into gas, which lifts fine dust off the surface, and the coma grows as the comet warms.^[1][2] It can extend for hundreds of thousands of kilometers, forming the glowing head, larger than most planets, that is seen from Earth.^[1][3]

A comet's nucleus has too little gravity to hold an atmosphere of its own; even so, most comets that come closer to the Sun than Jupiter's orbit develop a coma.^[2]

A comet's tail is material from its coma pushed away from the Sun by the pressure of sunlight and the solar wind, the stream of fast particles from the Sun.^[1] Comets have two tails: a dust tail and an ion tail made of gas.^[1]

Because it is blown outward by the Sun, a tail always stretches away from the Sun, sometimes for millions of miles.^[2]

Two objects are in conjunction when they appear to pass closest together in the sky.^[1] They are only lined up from Earth's point of view: two planets in conjunction remain very far apart in space.^[2]

A planet is in solar conjunction when it passes closest to the Sun as seen from Earth. Mercury and Venus have two kinds: inferior conjunction, passing roughly between Earth and the Sun, and superior conjunction, on the far side of the Sun.^[1] The outer planets can have only superior conjunctions.^[1]

A coronal mass ejection, or CME, is a large eruption of plasma and magnetic field from the Sun's outer atmosphere, the corona, that can carry billions of tons of material.^[1] CMEs travel at speeds from below 250 km per second to nearly 3,000 km per second; the fastest aimed at Earth arrive in as little as 15 to 18 hours, while slower ones take several days.^[1]

The largest geomagnetic storms, and the most widespread aurora, come from CMEs, especially when the cloud's magnetic field points opposite to Earth's.^[2]

A dwarf planet orbits the Sun and is nearly round, but has not been able to clear its orbit of debris, the third test a full planet must pass.^[1] The International Astronomical Union wrote the definition in 2006, when it reclassified Pluto, and so far recognises five: Ceres, Pluto, Haumea, Makemake and Eris, in order of distance from the Sun.^[1] It says perhaps more than a hundred more are waiting to be discovered.^[1] The guide What is a dwarf planet? explains the definition and the candidates.

Elongation is the angle between a planet and the Sun as seen from Earth.^[1] It matters most for Mercury and Venus, whose orbits lie inside Earth's: they always appear close to the Sun, in the morning or evening sky, and their elongation tells how far from the Sun's glare they are.^[1]

When a planet passes closest to the Sun as seen from Earth, it is in solar conjunction.^[1]

An extended mission continues a spacecraft's work after it has completed its original goals. NASA extended the Juno mission at Jupiter, first planned for 2016 to 2017, and in 2021 added 42 orbits running through September 2025.^[1] In its extended mission, Juno's changing orbit brought Jupiter's large moons within reach, so the spacecraft broadened its study to the whole Jovian system.^[1]

A spacecraft can also be sent to a new target: after returning samples from Bennu, OSIRIS-REx became OSIRIS-APEX, a follow-on mission to the asteroid Apophis.^[2]

A flyby is an encounter in which a spacecraft passes its target and observes it on the way, without being captured into orbit around it.^[1] Flyby spacecraft made the first reconnaissance of the solar system; they must point their instruments at targets that sweep past quickly and store data until they can send it to Earth.^[1]

A flyby gives a short look, but one spacecraft can make many. NASA's Lucy is planned to fly past eight Trojan asteroids that share Jupiter's orbit, and New Horizons flew past Pluto in 2015.^[2][3] A flyby of a planet can also change a spacecraft's path; see gravity assist.

The G scale is NOAA's scale for geomagnetic storms, one of three space weather scales alongside R for radio blackouts and S for radiation storms.^[1] It runs from G1, minor, at Kp 5, through G2 moderate, G3 strong and G4 severe, to G5, extreme, at Kp 9.^[1]

Each level lists typical effects and how often it occurs: about 1,700 G1 storms but only about 4 G5 storms in an 11-year solar cycle.^[1] It also gives how far from the poles the aurora has typically been seen, from 55 degrees geomagnetic latitude at G2 to 40 degrees at G5.^[1]

Geomagnetic latitude is latitude measured from Earth's geomagnetic poles rather than its geographic ones. Those poles are where the axis of a simple dipole model of Earth's magnetic field meets the surface; in 2025 the northern one lay at about 80.9°N, 72.8°W, and the axis was tilted about 9° from Earth's rotation axis.^[1]

The aurora forms in ovals closely centered on the geomagnetic poles, which is why NOAA states how far the aurora reaches in geomagnetic latitude.^[1][2] Because the geomagnetic north pole is offset toward North America, places there have higher geomagnetic latitudes than places in Europe or Asia at the same geographic latitude.

A gravity assist uses a close flyby of a planet to change a spacecraft's speed and direction. As the spacecraft passes behind the planet in its orbit around the Sun, it borrows a tiny part of the planet's orbital momentum and leaves faster; passing in front of a planet can slow it down instead.^[1] The planet's loss is too small to measure.^[1]

Gravity assists save propellant. When NASA's Psyche flew past Mars on 15 May 2026, the flyby added about 1,000 miles per hour to its speed and tilted its path by about 1 degree without using propellant.^[2]

A Halley-type comet is a periodic comet that takes between 20 and 200 years to go around the Sun.^[1] The group is named after Halley's Comet, which takes about 76 years.^[2] Halley's orbit is tilted 18 degrees to Earth's and runs in the opposite direction to the planets, and at its farthest it travels beyond the orbit of Neptune.^[2]

Debris shed by Halley's Comet along its orbit produces two meteor showers each year, the Orionids and the Aquariids.^[3]

An impact probability is the chance that an asteroid will hit Earth on a given date, based on the observations available so far. Because no orbit is known exactly, astronomers follow many possible versions of the asteroid that all fit the observations; if 2 out of 100,000 of them strike Earth in a given year, the impact probability for that year is about 1 in 50,000.^[1]

The number describes current knowledge, not a fixed property of the asteroid. Each new observation shrinks the range of possible positions, and for the probability to reach zero, Earth has to fall outside that range.^[2] Most asteroids listed with a possible impact are removed once more observations arrive; NASA notes that the risk was real until the new data showed it was not.^[1]

A Jupiter-family comet is a short-period comet that takes less than 20 years to go around the Sun, in the classical definition used by NASA's Jet Propulsion Laboratory.^[1] JPL also uses a second definition, based on a measure of how an orbit relates to Jupiter's called the Tisserand parameter, under which Jupiter-family comets have values between 2 and 3.^[1]

Comet 67P/Churyumov-Gerasimenko, which ESA's Rosetta spacecraft studied up close, is a Jupiter-family comet that goes around the Sun about every 6.5 years.^[2]

A kinetic impactor is a spacecraft deliberately flown into an asteroid to change its motion: in the simplest terms, one object striking another.^[1] It is one of the methods proposed for deflecting an asteroid on a collision course with Earth.

NASA's DART tested it in 2022, striking the moonlet Dimorphos and shortening its orbit around the asteroid Didymos.^[2] The debris blasted off by the impact roughly doubled the push the spacecraft alone delivered.^[3] The test showed that the method works, given warning time of several years at a minimum and preferably decades.^[1]

The Kp index measures how strongly Earth's magnetic field is disturbed, on a scale from 0 to 9, where 1 is calm and 5 or more means a geomagnetic storm.^[1] It is derived from the largest swings in the field recorded by magnetometers over a three-hour period, averaged from 13 observatories at middle latitudes.^[1] The K comes from the German word Kennziffer, characteristic digit, and the index was introduced by Julius Bartels in 1938.^[1]

NOAA turns Kp into its G scale of geomagnetic storms: Kp 5 is G1 and Kp 9 is G5.^[2] Higher Kp means the aurora can be seen farther from the poles.

The Kuiper belt is a region of icy leftovers from the solar system's early history that begins at the orbit of Neptune, about 30 astronomical units from the Sun, with its main part ending around 50 astronomical units; a second region, the scattered disk, continues outward to nearly 1,000.^[1] It is home to Pluto and most of the known dwarf planets, and to some comets.^[2] Astronomers think it holds millions of small icy objects, including hundreds of thousands larger than 100 km across.^[1] A Kuiper belt object, or KBO, is any body orbiting there; the wider term trans-Neptunian object also takes in the scattered disk. It is named for Gerard Kuiper, who speculated about objects beyond Pluto in 1951, and sometimes for Kenneth Edgeworth as well, who had mentioned them in the 1940s.^[1] The guide The Kuiper belt and beyond describes it.

A Lagrange point is a position in space where the gravity of two large bodies, such as the Sun and Earth, balances so that a small object can move along with them; spacecraft placed there need little fuel to stay in position.^[1] Each pair of bodies has five such points: L1, L2 and L3 lie on the line through the two bodies and are unstable, while L4 and L5 are stable.^[1]

Asteroids gather at the stable points; those at Jupiter's L4 and L5 are called Trojans.^[1] NASA's ESCAPADE spacecraft waited in an orbit around the Sun-Earth L2 point, about a million miles from Earth, before heading to Mars.^[2]

A lander is a spacecraft designed to reach the surface of a planet, moon or small body and survive long enough to send data back to Earth.^[1] Landers have worked on Venus, Mars and the Moon, carrying out analyses of rock and soil and returning images.^[1]

NASA's NEAR Shoemaker touched down on Eros in 2001 although it was never designed to land, and Japan's Hayabusa touched down twice on Itokawa to collect dust.^[2][3]

A long-period comet takes more than 200 years to go around the Sun, the dividing line from short-period comets.^[1] Many come from the Oort Cloud, a vast region about 100,000 astronomical units from the Sun, and the longest of their orbits take as long as 30 million years.^[1] Their arrivals are much less predictable than those of short-period comets.^[1]

Comet C/2023 A3 Tsuchinshan-ATLAS, which passed Earth in October 2024, came from the Oort Cloud on its first documented pass through the inner solar system.^[2]

A lunar distance, written LD, is the average distance between Earth and the Moon.^[1] NASA's Center for Near-Earth Object Studies uses 384,400 km for one LD, about 0.00257 au.^[1]

Lunar distances make asteroid flybys easy to picture: a pass at 2 LD is twice as far away as the Moon, and a pass under 1 LD is closer than the Moon.^[1] When the asteroid Apophis passes Earth in April 2029 at about 32,000 km from the surface, it will be closer than many satellites in geosynchronous orbit.^[2] That is less than a tenth of a lunar distance.^[3]

Magnitude, or apparent magnitude, measures how bright a star, planet, comet or meteor looks from Earth. The scale runs backward: a lower number means a brighter object, and a star of magnitude 1 looks 100 times brighter than one of magnitude 6.^[1] The brightest objects have negative magnitudes: the star Sirius is -1.46, and Venus can be brighter than -4.^[1]

Under perfect skies the eye can see stars down to about magnitude 6.5; the faintest star visible from a given place is its limiting magnitude.^[1] A meteor of magnitude -4 or brighter is called a fireball.^[2] Magnitude in the sky is different from an asteroid's absolute magnitude, which is measured at a fixed standard distance.

A meteor is the flash of light produced when a meteoroid enters Earth's atmosphere at high speed and burns up, the event popularly called a shooting star.^[1] Most are made by particles between the size of a grain of sand and a pea.^[1] Several meteors an hour can usually be seen on any clear night, and many more during a meteor shower.^[1]

A meteor brighter than about magnitude -4 is called a fireball.^[2] If part of the object survives to reach the ground, that piece is a meteorite.^[1]

A meteorite is a meteoroid that survives its trip through the atmosphere and lands on the ground.^[1] Most are between the size of a pebble and a fist.^[1] There are three main kinds, stony, iron and stony-iron, and a fresh meteorite usually has a dark, sometimes shiny fusion crust, melted during its fall.^[1]

About 99.8% of meteorites come from asteroids, and the rest from Mars and the Moon.^[1] Some are as old as 4.6 billion years, and studying them helps scientists learn about the history of the solar system.^[1]

A meteoroid is a piece of rock or metal traveling through space, ranging from a grain of dust to a small asteroid; the term applies only while it is still in space.^[1] Most meteoroids are fragments broken or blasted off larger bodies: comets, asteroids, and even the Moon and other planets.^[1]

Meteoroids released by one comet or asteroid travel on similar orbits as a stream, and when Earth passes through such a stream there is a meteor shower.^[2][1] When a meteoroid enters the atmosphere, its streak of light is a meteor.^[1]

A minimoon, or mini-moon, is a natural object that Earth's gravity holds temporarily. The definition has two parts and no minimum stay: the object's energy relative to Earth is negative, so it is bound, and it lies within about three Hill radii of Earth, roughly three times the distance within which Earth's pull dominates the Sun's.^[1] A temporarily captured orbiter completes at least one revolution around Earth before it leaves; a temporarily captured flyby is bound but gone before finishing one loop.^[1]

They are small, about a meter or a few meters across, and models predict one or two of that size in orbit at any time, staying about nine months on average.^[1] The first found, 2006 RH120, was captured in mid-2006 and left in mid-2007; the second, 2020 CD3, was discovered in February 2020 after orbiting Earth for more than a year.^[2] The guide Minimoons describes each known one. A quasi-satellite is different: it orbits the Sun, not Earth, and is never bound to us.

A near-Earth object, or NEO, is an asteroid or comet whose closest point to the Sun lies within 1.3 astronomical units, that is, within 1.3 times Earth's average distance from the Sun.^[1] In practice this means its orbit comes within about 48 million km of Earth's orbit.^[2] About 99% of NEOs are asteroids, known as near-Earth asteroids; a comet counts only if it also takes less than 200 years to go around the Sun.^[1][3]

The name describes an orbit, not a danger: most near-Earth asteroids never come especially close to Earth.^[4] Whether an asteroid could come close enough to matter is what potentially hazardous asteroid status describes.^[3] Whether it could actually hit Earth within the next century is what Sentry checks.^[5]

A planet is at opposition when it and Earth are on the same side of the Sun, with all three in a line, so that the planet appears opposite the Sun in the sky.^[1] It then rises around sunset and stays up for much of the night.^[2] Around opposition, a planet is at its closest to Earth for the year and looks its brightest.^[3]

Only the outer planets, from Mars outward, can reach opposition; Mercury and Venus never can.^[1] The full Moon is also in opposition to the Sun.^[1]

An orbiter is a spacecraft that goes into orbit around a planet, moon or asteroid. It must carry enough propulsion to slow down at the right moment to be captured, and it has to cope with passing into the target's shadow, cut off from sunlight and from contact with Earth.^[1] Orbiters follow the first reconnaissance of flybys with in-depth study.^[1]

NASA's NEAR Shoemaker became the first spacecraft to orbit an asteroid when it reached Eros in 2000, and Psyche is planned to spend about two years orbiting its target.^[2][3]

The Palermo scale is the specialists' measure of impact risk. It compares the chance of one predicted impact with the background risk: the average chance that some asteroid of the same size or larger hits Earth in the years before that date.^[1]

The scale is logarithmic. A value of 0 means the predicted impact is as threatening as the background, a value of -2 means it is only 1% as likely, and a value of +2 means it is 100 times more likely.^[1] Values below -2 reflect events with no likely consequences, values between -2 and 0 merit careful monitoring, and positive values generally merit some concern.^[1] Events above zero are very rare.^[2] Unlike the Torino scale, the Palermo scale takes into account how many years remain before the possible impact.^[1]

A shower's parent body is the comet or asteroid that released the stream of particles Earth passes through.^[1] Each Perseid meteor is a tiny piece of comet Swift-Tuttle, the Orionids and Aquariids come from Halley's Comet, and the Leonids from comet Tempel-Tuttle.^[2]

Most parent bodies are comets. The Geminids are an exception: they come from 3200 Phaethon, a rocky asteroid; see Phaethon.^[3]

Perihelion is the point in an orbit that is closest to the Sun.^[1] For a comet, the approach to perihelion is when the Sun's heat turns its ices to gas and its coma grows.^[2] A close perihelion can also destroy a comet: comet ISON broke apart at perihelion in 2013.^[3] The interstellar comet 3I/ATLAS reached perihelion around 30 October 2025, about 1.4 au from the Sun.^[4]

Perihelion distance also defines a near-Earth object: an asteroid or comet whose perihelion lies within 1.3 au of the Sun.^[5]

A periodic comet is one that takes less than 200 years to go around the Sun, or that has been observed at more than one close pass by the Sun.^[1] Its designation begins with P/, and once it has been seen to return it also receives a number, so Halley's Comet is 1P.^[1] A comet that does not meet the definition carries the prefix C/, and a periodic comet that has disappeared carries D/.^[1]

Most short-period comets come from the Kuiper Belt beyond Neptune, and because they have passed before, their returns can often be predicted.^[2] Halley's Comet returns about every 76 years and is next due in 2061.^[3]

A phase is the portion of a body's sunlit side that can be seen from Earth. The Sun always lights half of the Moon, and as the Moon orbits Earth, different amounts of that half can be seen.^[1] The Moon's eight phases run from new Moon through first quarter and full Moon to third quarter, and the cycle repeats every 29.5 days.^[1]

Mercury and Venus also show phases, from a thin crescent to nearly full, because their orbits carry them between Earth and the Sun.^[2]

A potentially hazardous asteroid, or PHA, is one whose orbit can bring it within 0.05 au of Earth's orbit and whose absolute magnitude is 22.0 or brighter.^[1] In everyday terms, that means an asteroid about 140 m across or larger, longer than a football field, that can pass within 7.5 million km of Earth's orbit.^[2]

The label marks asteroids worth watching closely, not asteroids on their way. Being a PHA does not mean an asteroid will hit Earth; it only means such a threat is possible, which is why astronomers keep refining these orbits as new observations arrive.^[3]

A quasi-satellite, or quasi-moon, is an asteroid that orbits the Sun in almost exactly the time Earth does, so that from Earth it appears to circle our planet slowly at a great distance.^[1] It is never gravitationally bound to Earth: it is in a one-to-one resonance with Earth's orbit, not in orbit around Earth.^[2] NASA describes the best-known one, Kamoʻoalewa, as running ahead of Earth for half of each year and falling behind for the other half, in a leapfrog pattern that lasts for centuries.^[1]

Seven quasi-satellites of Earth are known, the latest found in August 2025.^[2] Kamoʻoalewa, found in 2016, never comes closer than about 38 times the Moon's distance.^[1] That distance is what separates a quasi-satellite from a minimoon, which is temporarily bound to Earth and passes within a few lunar distances. Both are described in Minimoons.

The radiant is the point in the sky from which the meteors of a shower appear to come.^[1] The meteors of one shower travel on nearly parallel paths, which makes them appear to spread out from that single point.^[1]

A shower is named after the constellation where its radiant lies: the Perseids appear to come from Perseus.^[2] The constellation only helps tell showers apart and is not the source of the meteors, which can appear anywhere in the sky.^[2]

A rendezvous mission matches its target's path around the Sun so it can stay alongside, instead of passing by. NASA's Near Earth Asteroid Rendezvous mission, NEAR, went into orbit around Eros in 2000 and studied it for nearly a year.^[1]

ESA's Ramses is to rendezvous with the asteroid Apophis and accompany it through its close pass by Earth in 2029.^[2] ESA's Hera is the first probe to rendezvous with a binary asteroid, the Didymos system.^[3]

A sample return mission collects material from another world and brings it back to Earth for study in laboratories. Japan's Hayabusa returned the first asteroid sample, from Itokawa, in 2010.^[1] Hayabusa2 brought back 5.4 g from Ryugu in 2020, and NASA's OSIRIS-REx returned 121.6 g from Bennu in 2023, the largest asteroid sample collected in space.^[2][3]

Comet material has been returned too: NASA's Stardust collected particles from comet Wild 2 and brought them to Earth in 2006.^[4]

Sentry is the impact-monitoring system run by NASA's Center for Near-Earth Object Studies at the Jet Propulsion Laboratory. It is highly automated and scans the catalog of known asteroids for possible Earth impacts over the next 100 years.^[1] Every day it receives new observations from the Minor Planet Center and recomputes orbits as fresh measurements arrive.^[2]

Each possible impact Sentry finds is listed on its risk table with an impact probability and a rating on the Torino and Palermo scales.^[2] Several newly found asteroids each month are listed and then removed shortly afterwards as their orbits improve, which is the expected outcome.^[2] The current version, Sentry-II, has been in use since July 2021.^[2]

A solar flare is a sudden eruption of radiation from the Sun that lasts from minutes to hours.^[1] Flares usually erupt from active regions with strong magnetic fields, typically around groups of sunspots.^[1] They are classed by their peak X-ray brightness as A, B, C, M or X, each class ten times stronger than the one before.^[1]

The radiation reaches Earth at the speed of light and affects only the sunlit side, where strong flares can disrupt high-frequency radio. NOAA rates these radio blackouts from R1, for an M1 flare, to R5, for an X20 flare.^[1][2]

The Torino scale rates a possible impact on Earth from 0 to 10, combining how likely the impact is with how much damage it could do.^[1] It was adopted by the International Astronomical Union in 1999, mainly to help astronomers explain impact predictions to the public.^[1]

Level 0 (white) means no hazard, and level 1 (green) a routine pass that will very likely drop to 0 with new observations.^[1] Levels 2 to 4 (yellow) merit attention from astronomers, and levels 5 to 7 (orange) describe serious but uncertain threats.^[1] Levels 8 to 10 (red) are reserved for collisions that are certain.^[1] In January 2025, asteroid 2024 YR4 reached level 3.^[2] Within a month, new observations had lowered its chance of hitting Earth to 0.004%.^[3] The full list of levels is in the impact risk guide.

Totality is the part of a total eclipse when the eclipsed body is completely covered. In a total solar eclipse, the Moon blocks the whole face of the Sun for people in the center of its shadow; the sky darkens as at dawn or dusk, and the Sun's faint outer atmosphere, the corona, becomes visible.^[1]

Totality is the only time it is safe to look at a solar eclipse without eclipse glasses, and only while no part of the bright Sun shows.^[2] In a total lunar eclipse, totality is when the whole Moon is inside Earth's inner shadow and turns a coppery red.^[3]

A trans-Neptunian object, or TNO, is a body that orbits the Sun beyond Neptune, whose orbit lies about 30 astronomical units out.^[1] The term covers everything in the Kuiper belt and the scattered disk beyond it, including the dwarf planets Pluto, Eris, Haumea and Makemake, and NASA's page counts more than 2,000 catalogued so far.^[1] JPL's Small-Body Database listed 8,337 on 5 November 2026, a number that is rising quickly as the Rubin Observatory's survey reports its finds.^[2] The guide The Kuiper belt and beyond describes the families of orbits they fall into.

The zenithal hourly rate, or ZHR, is the number of meteors from a shower that one observer would see in an hour if the shower's radiant were directly overhead and the sky were dark enough to show stars of magnitude 6.5.^[1] Visual observers can monitor showers with a ZHR as low as 2.^[1] For the Perseids, one of the year's most popular showers, NASA expects 50 to 100 meteors an hour at the peak when conditions are right.^[2]

Because the ZHR assumes ideal conditions, the number of meteors actually seen is usually lower: a radiant low in the sky, haze, light pollution or moonlight all reduce it.^[1]