Glossary

Astronomy glossary: the terms explained

Ephemerides, coordinates and eclipses have a language of their own, and many of these terms appear throughout Apphemeride. Here are concise explanations of the essentials, with links to guides that explore the subjects in greater depth.

A

Aberration: An apparent shift in an object’s position because Earth moves around the Sun at about 30 kilometres per second and light travels at a finite speed. Annual aberration reaches about 20.5 arcseconds and must be included when calculating precise apparent positions.
Absolute magnitude: The magnitude a star would have at a distance of ten parsecs. It allows the intrinsic brightness of stars to be compared: the Sun would have an absolute visual magnitude of about 4.8. Small solar-system bodies use a different definition based on a distance of one astronomical unit.Guide: Stellar magnitude: brightness, colour and distance
Albedo: A measure of how strongly a body reflects incoming light. The Moon’s geometric albedo is around 0.12, making its surface relatively dark; Venus’s cloud cover is much more reflective. Together with size and distance, albedo helps determine apparent brightness.Guide: Stellar magnitude: brightness, colour and distance
Altitude: An object’s angle above the mathematical horizon, from 0° at the horizon to 90° at the zenith. Negative altitudes place it below the horizon. Altitude and azimuth together form the horizontal coordinate system.Guide: The Sun’s position: azimuth and altitude
Analemma: The figure of eight traced by the Sun when its position is recorded at the same mean solar time each day for a year. Its vertical extent reflects changing solar declination; its sideways displacement follows the equation of time.Guide: The analemma: why the Sun traces a figure of eight
Angular diameter: An object’s apparent diameter expressed as an angle in degrees, arcminutes or arcseconds. The Sun and Moon each span about half a degree; Jupiter’s disk is roughly thirty to fifty arcseconds across depending on distance. Angular diameter depends on physical size and distance.Guide: Supermoons and micromoons: size and distance
Antumbra: The region beyond the tip of an umbral shadow where an intervening body lies centrally across a light source without covering it completely. When the Moon’s umbra falls short of Earth, the antumbra reaches the surface instead. Observers there see an annular solar eclipse, with a bright ring of sunlight around the Moon.Guide: Annular and hybrid solar eclipses explained
Aphelion: The point in an orbit farthest from the Sun. Earth reaches aphelion in early July, at a distance of around 152 million kilometres, during Northern Hemisphere summer. Seasons result from Earth’s axial tilt rather than its distance from the Sun.
Apogee: The farthest point from Earth in the Moon’s orbit, or in another Earth-centred orbit. The Moon is roughly 405,000 kilometres away near apogee. A full moon near apogee is often called a micromoon because its apparent disk is a little smaller.Guide: Supermoons and micromoons: size and distance
Asterism: A recognisable pattern of stars that is not itself one of the official constellations. Examples include the Big Dipper within Ursa Major, the Summer Triangle of Vega, Deneb and Altair, and the Winter Hexagon. An asterism is often easier to recognise than an entire constellation; seasonal names here refer to the Northern Hemisphere.Guide: Finding constellations: orientation in the sky
Astronomical twilight: The interval when the Sun’s centre lies between 12° and 18° below the horizon. Once it is below −18°, the sky is conventionally astronomically dark, with the Sun’s scattered light no longer making a significant contribution to natural sky brightness.Guide: Civil, nautical and astronomical twilight explained
Astronomical unit: A unit of length used for distances within the solar system, defined since 2012 as exactly 149,597,870,700 metres. It is approximately the mean Earth–Sun distance. Its international symbol is au.
Atmospheric extinction: The dimming of starlight as it passes through Earth’s atmosphere, caused by scattering and absorption. It increases with the air mass along the line of sight: under clear conditions it may be around 0.2–0.3 magnitudes at the zenith and several magnitudes near the horizon. Low objects also appear redder.Guide: Sky brightness and darkness for stargazing
Atmospheric refraction: The bending of light in Earth’s atmosphere, which makes objects appear higher than their geometric positions. Near the horizon a standard value is about 34 arcminutes, slightly more than the Sun’s apparent diameter. This allows the Sun to remain visible when its geometric disk is already below the horizon.Guide: Atmospheric refraction and horizon dip explained
Azimuth: An object’s direction measured around the horizon. The convention used here starts at 0° in the north and increases through east at 90°, south at 180° and west at 270°.Guide: The Sun’s position: azimuth and altitude

B

Besselian elements: A set of quantities describing eclipse geometry, including the position and size of the Moon’s shadow on a plane through Earth’s centre, usually expressed as polynomials in time. They provide the basis for calculating eclipse paths and local contact times.Guide: Solar eclipses: paths, contacts and obscuration
Blue hour: The part of twilight when the sky can take on a deep, even blue, often with the Sun several degrees below the horizon. Photographers value the balance between the brightness of the sky and artificial lighting. Definitions vary; The solar-altitude convention used by Apphemeride determines the interval shown in the app.Guide: Golden hour and blue hour explained

C

Catalogue number: An object’s identifier in an astronomical catalogue. Polaris, for example, is HIP 11767 in Hipparcos and HD 8890 in Henry Draper; Tycho-2 stars carry three-part TYC identifiers. A single star can have several numbers because it appears in several catalogues.Guide: Find a star by HIP, HD or TYC catalogue number
Celestial equator: Earth’s equator projected onto the celestial sphere, forming a great circle 90° from both celestial poles. Declination is measured from it. It meets the horizon at east and west; its highest altitude depends on the absolute geographical latitude, so northern and southern locations must be treated symmetrically.Guide: Right ascension and declination explained
Central line: The track of the Moon’s shadow axis across Earth’s surface during a central solar eclipse. Along it, the Sun and Moon are most closely aligned and totality or annularity is generally longest locally across the path. The path limits lie on either side.Guide: Solar eclipses: paths, contacts and obscuration
Circumpolar: An object is circumpolar at a location if its daily path never goes below the horizon. Which stars qualify depends on latitude and hemisphere: Cassiopeia is a familiar northern example, while southern observers have different circumpolar constellations. Beyond a polar circle, even the Sun can remain above the horizon during the local summer.Guide: Midnight sun and polar night explained
Civil twilight: The interval from sunset until the Sun’s centre reaches 6° below the horizon, or the reverse sequence before sunrise. Natural light is often sufficient for ordinary outdoor activities, and the brightest planets and stars become visible.Guide: Civil, nautical and astronomical twilight explained
Colongitude: The selenographic longitude of the Moon’s morning terminator, conventionally around 0° at first quarter, 90° at full moon, 180° at last quarter and 270° at new moon. Lunar observers use it to anticipate when a crater will be lit at a shallow angle.
Colour index: The difference between a star’s magnitudes in two wavelength bands, commonly B−V for blue and visual filters. Small or negative values generally indicate hot, bluish stars; larger values indicate cooler, reddish stars. The Sun’s B−V is around 0.65, making colour index a useful temperature indicator.Guide: Stellar magnitude: brightness, colour and distance
Conjunction: A configuration in which two objects share the same ecliptic longitude or right ascension as seen from Earth. A planet is in conjunction with the Sun when it lies in the same general direction. Mercury and Venus have both inferior and superior conjunctions.Guide: Opposition, conjunction and quadrature explained
Constellation: One of the eighty-eight regions of the sky recognised by the International Astronomical Union, adopted in 1922 with precisely defined boundaries established in 1930. In everyday speech, the word often means just a recognisable star pattern. Astronomically, every point on the sky belongs to one constellation.Guide: Finding constellations: orientation in the sky
Contact times: The instants when apparent disk boundaries touch during an eclipse or transit. Solar-eclipse contacts are labelled C1–C4. Lunar-eclipse contacts use P1, U1–U4 and P4 for encounters with Earth’s penumbra and umbra; not every eclipse includes every contact.Guide: Eclipse contact times: C1–C4 and P1–U4 explained
Culmination: An object’s passage across the local meridian. Upper culmination is its highest daily position; lower culmination is its lowest. The Sun’s upper culmination defines local apparent noon, without implying that it is due south at every latitude.Guide: Equation of time and apparent solar time explained

D

Danjon scale: A five-level scale for the brightness and colour of a total lunar eclipse. It ranges from L0, a very dark and almost invisible Moon, to L4, a bright copper-red or orange Moon with a bluish rim. Dust, clouds and other conditions in Earth’s atmosphere strongly affect its appearance.Guide: Lunar eclipses: phases, colour and visibility
Declination: An object’s angular distance north or south of the celestial equator, positive to the north and negative to the south, between +90° and −90°. It is analogous to terrestrial latitude and helps determine how high the object can rise at your observing location.Guide: Right ascension and declination explained
Delta T: ΔT is the difference between uniformly progressing Terrestrial Time and Earth-rotation time UT1. Earth’s rotation is irregular, so this difference changes over time. Accurate eclipse calculations require a value appropriate to the date; predictions for distant dates carry uncertainty.Guide: Julian Date, UT, TT and Delta T explained

E

Earthshine: The faint, ash-grey glow on the Moon’s night side, produced by sunlight reflected from Earth onto the Moon. It is easiest to notice on a thin crescent in twilight a few days before or after new moon.Guide: Earthshine: the glow on the Moon’s dark side
Ecliptic: The Sun’s apparent annual path against the stars, representing the plane of Earth’s orbit projected onto the sky. It is tilted by about 23.4° to the celestial equator. The Moon and planets generally remain near it.Guide: Celestial coordinates: three systems explained
Elongation: The angular separation between a celestial body and the Sun as seen from Earth. Mercury and Venus have limited elongations: Mercury reaches roughly 18–28° and Venus about 45–47°, depending on orbital circumstances.Guide: Greatest elongation of Mercury and Venus explained
Ephemeris: A table or calculation giving an object’s positions and other data at successive times. The term comes from a Greek word meaning “daily” and also inspired the name Apphemeride. The plural is ephemerides.
Epoch: A reference instant associated with coordinates or orbital elements. A star catalogue may specify the epoch of its positions, which change through proper motion, separately from the equinox defining its coordinate axes. J2000 is a widely used standard reference.Guide: J2000, JNow, precession and nutation explained
Equation of time: The difference between apparent and mean solar time. It arises from Earth’s elliptical orbit and the tilt of its rotation axis, varying over the year by roughly −14 to +16 minutes. It explains why a sundial and a uniformly running mean-solar clock do not keep the same time.Guide: Equation of time and apparent solar time explained
Equatorial coordinate system: A celestial coordinate system aligned with the celestial equator. Right ascension measures position along the equator; declination measures angular distance north or south of it. For a specified reference frame, stellar coordinates remain nearly constant over short periods while the sky turns with Earth’s rotation.Guide: Celestial coordinates: three systems explained
Equilux: A date when the interval from sunrise to sunset is actually as long as the night. Refraction and the Sun’s apparent radius lengthen the daylight interval. At many northern latitudes, equilux therefore falls before the March equinox and after the September equinox; the dates depend on latitude.Guide: Solstices and equinoxes: the seasons explained
Equinox: The instant when the Sun’s apparent ecliptic longitude reaches 0° in March or 180° in September, as it crosses the celestial equator. In coordinate terminology, “equinox” also identifies the orientation of a reference frame, as in the equinox J2000.Guide: Solstices and equinoxes: the seasons explained

F

First point of Aries: The intersection of the celestial equator and ecliptic where the Sun crosses northward at the March equinox. It is the origin of right ascension and ecliptic longitude. Precession slowly shifts it westward relative to the stars; the traditional name does not mean it still lies in Aries.Guide: Right ascension and declination explained

G

Galilean moons: Jupiter’s four large moons Io, Europa, Ganymede and Callisto, discovered by Galileo Galilei in 1610. At magnitudes roughly 4.6–5.7 they can be seen with suitable binoculars, although Jupiter’s glare makes observation harder. Their arrangement changes from night to night.Guide: Jupiter’s Galilean moons: positions and events
Gamma of an eclipse: For a solar eclipse, the closest distance of the shadow axis from Earth’s centre, expressed in Earth’s equatorial radii. A value near zero indicates a central passage across Earth; near an absolute value of 0.997 the axis approaches the limit for a central eclipse. For lunar eclipses, gamma similarly describes the Moon’s distance from Earth’s shadow axis.Guide: Solar eclipses: paths, contacts and obscuration
Golden hour: The interval around sunrise or sunset when low sunlight can produce warm tones, soft illumination and long shadows. It is generally defined through solar altitude, often from about 6° above to 4° below the horizon. It is a lighting interval rather than a guaranteed sixty minutes.Guide: Golden hour and blue hour explained
Grazing occultation: A stellar occultation in which a star just grazes the Moon’s limb. Along a narrow geographical boundary, the star can disappear behind lunar mountains and reappear through valleys several times. Timings from observers along that boundary reveal details of the lunar profile.Guide: Lunar occultations: when the Moon hides a star

H

Heliographic coordinates: Latitude and longitude on the Sun, comparable to geographical coordinates on Earth. Longitude is often expressed in the Carrington system, whose sidereal rotation period is about 25.38 days. These coordinates allow sunspot groups to be followed as the Sun rotates.Guide: Sunspots and the solar cycle explained
Horizon: The boundary between sky and ground. The mathematical horizon is perpendicular to the observer’s vertical direction and lies 90° from the zenith. Terrain, elevation and horizon dip make the visible horizon differ from this ideal reference used for altitude and rise or set calculations.Guide: The Sun’s position: azimuth and altitude
Horizon dip: The angle by which the visible horizon falls below the mathematical horizon from an elevated viewpoint. From an unobstructed mountain or coastal viewpoint, this can make sunrise appear earlier and sunset later. The dip increases with observer elevation.Guide: Atmospheric refraction and horizon dip explained
Horizontal coordinate system: A coordinate system based on the observer’s horizon and expressed as azimuth and altitude. It directly describes where to look, but changes continuously with Earth’s rotation and depends on the observing location.Guide: Celestial coordinates: three systems explained
Hour angle: An object’s angular distance westward from the meridian, measured along the celestial equator and often expressed in hours. It is related to local sidereal time and right ascension and is zero at upper meridian transit.Guide: Sidereal time: a clock for the rotating sky
Hybrid eclipse: A rare solar eclipse whose path includes both annular and total phases at different locations. Earth’s curvature can bring the middle part of the path close enough to the Moon for the umbra to reach the surface, while only the antumbra reaches locations towards the ends. The eclipse of 20 April 2023 was an example.Guide: Annular and hybrid solar eclipses explained

J

J2000: The standard epoch 1 January 2000 at 12:00 Terrestrial Time, corresponding to Julian Date 2451545.0. A J2000 reference frame uses the equator and equinox associated with that epoch. Transforming coordinates to the date of observation involves effects including precession and, for a true frame, nutation.Guide: J2000, JNow, precession and nutation explained
Julian Date: A continuous count of days beginning at noon on 1 January 4713 BCE in the Julian calendar. It avoids month boundaries and calendar reforms, making time intervals easy to compare. Decimal fractions represent parts of a day; the time scale used must also be specified for precise work.Guide: Julian Date, UT, TT and Delta T explained

L

Leap second: A one-second adjustment historically used to keep atomic-clock-based UTC close to Earth-rotation time UT1. Twenty-seven positive leap seconds were added between 1972 and the end of 2016. CGPM Resolution 5 (2026) sets the transition to continuous UTC for 20 May 2027; this does not make Earth’s rotation uniform or remove the need for UT1 in astronomy.Guide: Julian Date, UT, TT and Delta T explained
Libration: The Moon’s apparent rocking, which allows about 59% of its surface to be seen from Earth over time rather than just one half. Its elliptical orbit, axial tilt and the observer’s changing viewpoint all contribute.Guide: Lunar libration: the Moon’s apparent rocking
Light travel time: The time light takes to reach Earth from a celestial body: a little over one second from the Moon and just over eight minutes from the Sun. We therefore see an object as it was, and where it was, when that light left it.
Light-year: The distance light travels in a vacuum during one Julian year, about 9.46 trillion kilometres. It measures distance rather than time. Proxima Centauri, the nearest star after the Sun, is about 4.2 light-years away.
Limiting magnitude: The magnitude of the faintest star just visible under given conditions. With the unaided eye under a dark sky, it is often around magnitude 6–6.5. Twilight, moonlight and light pollution reduce this limit, making faint stars harder to detect.Guide: Stellar magnitude: brightness, colour and distance
Local apparent solar time: Time based on the Sun’s actual position at your location. The Sun culminates at 12:00 local apparent solar time. This differs from civil time because of geographical longitude, the equation of time and the chosen time zone, including daylight-saving arrangements where applicable.Guide: Equation of time and apparent solar time explained
Lunar age: The time in days since the last new moon. First quarter is around an age of 7.4 days and full moon around 14.8 days, but actual timings vary because the Moon’s orbital speed changes.Guide: Moon phases: from new moon to full moon
Lunar nodes: The two points where the Moon’s orbit, tilted by about 5.1°, crosses the ecliptic: the ascending and descending nodes. Eclipses are possible only when new or full moon occurs close to a node. The line joining the nodes regresses around the ecliptic in about 18.6 years.Guide: Solar eclipses: paths, contacts and obscuration
Lunar occultation: An event in which the Moon passes in front of a star or planet. Stars can disappear abruptly because the Moon has essentially no atmosphere. The Moon’s large parallax makes the timing and visibility strongly dependent on the observer’s location.Guide: Lunar occultations: when the Moon hides a star
Lunar phase: The visible shape of the Moon’s illuminated portion, determined by the relative geometry of the Sun, Moon and Earth. The principal phases are new moon, first quarter, full moon and last quarter, with waxing and waning crescents and gibbous phases between them. A complete cycle takes one synodic month.Guide: Moon phases: from new moon to full moon
Lunation: A complete cycle of lunar phases from one new moon to the next. Its mean duration is 29.53 days, one synodic month. Individual lunations vary by several hours because the orbits of Earth and the Moon are elliptical.Guide: Moon phases: from new moon to full moon

M

Magnitude: A logarithmic measure of apparent brightness, expressed in magnitudes or mag. Smaller values indicate brighter objects. A difference of five magnitudes represents a brightness factor of exactly one hundred; one magnitude is a factor of about 2.5.Guide: Stellar magnitude: brightness, colour and distance
March and September equinoxes: The instants when the Sun crosses the celestial equator, around 20 March and 22 or 23 September. Day and night are nearly equal over much of Earth, although refraction and the Sun’s finite disk make the sunrise-to-sunset interval longer than twelve hours. March marks northern spring and southern autumn; September reverses them.Guide: Solstices and equinoxes: the seasons explained
Meridian: The great circle passing through the north and south points of the horizon and the zenith. An object culminates when it crosses this circle. At middle northern latitudes, the Sun and planets reach their highest positions towards the south; elsewhere the culmination direction depends on latitude and declination.
Meteor: The brief streak of light produced when a small particle enters Earth’s atmosphere and excites the surrounding air. It is commonly called a shooting star, although it is not a star. A particle in space is a meteoroid; material that reaches the ground is a meteorite.Guide: Meteor showers: radiants, ZHR and observed rates
Meteor shower: An increased number of meteors recurring at a similar time each year when Earth crosses debris left by a comet or asteroid. The meteors appear to radiate from a common point, the radiant. Showers are named after the constellation containing that radiant.Guide: Meteor showers: radiants, ZHR and observed rates

N

Nadir: The point directly below the observer, opposite the zenith. It lies 90° below the horizon and cannot be seen in the sky, but serves as a reference in the horizontal coordinate system.
Nautical twilight: The interval when the Sun’s centre is between 6° and 12° below the horizon. The term comes from navigation at sea: a horizon may still be distinguishable while bright stars are available for taking sights.Guide: Civil, nautical and astronomical twilight explained
Nutation: Shorter-period oscillations superimposed on the slow precession of Earth’s axis, driven mainly by the Moon. The largest term has a period of about 18.6 years and an amplitude of around 17 arcseconds in ecliptic longitude.Guide: J2000, JNow, precession and nutation explained

O

Observer elevation: The height of an observing location above sea level or its surroundings. An elevated viewpoint can alter rise and set times through horizon dip; pressure changes also influence refraction. For the nearby Moon, elevation has a small additional effect on the topocentric position.Guide: Atmospheric refraction and horizon dip explained
Opposition: The configuration in which an outer planet lies opposite the Sun as seen from Earth. It rises roughly around sunset and culminates near local apparent midnight. The planet is relatively close to Earth and opposition is usually a favourable period for observation, subject to altitude and local conditions.Guide: Opposition, conjunction and quadrature explained

P

Parallax: The apparent shift of an object when viewed from different locations. A star’s annual parallax gives its distance. For the nearby Moon, changing the viewpoint from Earth’s centre to an observing site can shift the apparent position by almost one degree.
Parsec: The distance at which one astronomical unit subtends an angle of one arcsecond. A parsec is about 3.26 light-years or 30.9 trillion kilometres. The word combines “parallax” and “arcsecond”.
Penumbra: A shadow region where the light source is only partly obscured. During a lunar eclipse, the Moon first enters Earth’s penumbra and dims subtly. Locations within the Moon’s penumbra on Earth experience a partial solar eclipse.Guide: Lunar eclipses: phases, colour and visibility
Perigee: The nearest point to Earth in the Moon’s orbit or another Earth-centred orbit. Lunar perigee distances are roughly 356,000–370,000 kilometres. A full moon near perigee is commonly called a supermoon.Guide: Supermoons and micromoons: size and distance
Perihelion: The point in an orbit nearest the Sun. Earth reaches perihelion in early January, around 147 million kilometres from the Sun. Mercury’s perihelion distance is about 46 million kilometres.
Phase angle: The angle between the directions to the Sun and Earth as seen from a celestial body. At 0° the visible disk is fully illuminated, at 90° it is half illuminated and at 180° its night side faces us. Outer planets have comparatively small phase angles; Mars reaches at most about 47°.
Photosphere: The Sun’s visible surface, a gaseous layer only a few hundred kilometres thick with a temperature around 5,800 kelvin, from which most visible sunlight emerges. Sunspots, faculae and granulation appear there. The disk becomes darker towards its edge, an effect called limb darkening.Guide: Sunspots and the solar cycle explained
Polar circle: Either of the latitude circles at about 66.6° north and south, offset from the poles by Earth’s axial tilt. Beyond them the geometry allows at least one day with midnight sun and, neglecting refraction, at least one day of polar night. Their positions change slowly with the axial tilt.Guide: Midnight sun and polar night explained
Polar night: The period at high latitudes when the Sun does not rise above the horizon even at its daily maximum. Refraction can shorten or eliminate it close to a polar circle; near the poles it lasts almost half a year. Twilight may still brighten the sky around noon.Guide: Midnight sun and polar night explained
Polaris: The brightest star in Ursa Minor, of about magnitude 2 and less than a degree from the north celestial pole. It appears almost stationary in the northern sky, at an altitude approximately equal to northern geographical latitude. It is not a useful guide from most southern locations, and precession transfers the pole-star role over millennia.Guide: Finding constellations: orientation in the sky
Population index: The factor by which a meteor shower’s counts increase from one magnitude class to the next fainter class, usually denoted r. Typical values are around 2–3.5; a smaller index indicates a larger share of bright meteors. It is used when converting observed counts to a zenithal hourly rate.Guide: Meteor showers: radiants, ZHR and observed rates
Position angle: The direction of one point relative to another on the sky, measured from north through east. On a north-up, east-left sky view it increases anticlockwise. It can describe where a star disappears behind the lunar limb, where an eclipse contact occurs or how a double star is oriented.Guide: Eclipse contact times: C1–C4 and P1–U4 explained
Precession: The slow change in the direction of Earth’s axis, caused by the gravitational influence of the Moon and Sun on Earth’s equatorial bulge. A full cycle takes about 25,800 years. The equinox shifts by roughly fifty arcseconds a year along the ecliptic.Guide: J2000, JNow, precession and nutation explained
Proper motion: A star’s gradual change of position across the sky due to its actual motion through space, measured in arcseconds or milliarcseconds per year. Most stars shift by only fractions of an arcsecond annually; Barnard’s Star is the familiar high-proper-motion example, moving by more than ten arcseconds a year.Guide: Find a star by HIP, HD or TYC catalogue number

Q

Quadrature: A configuration in which an object is 90° east or west of the Sun as seen from Earth. An outer planet at eastern quadrature culminates roughly around sunset; at western quadrature, around sunrise. The Moon is close to quadrature at first and last quarter.Guide: Opposition, conjunction and quadrature explained

R

Radiant: The point on the sky from which a meteor shower’s tracks appear to diverge. The particles travel on nearly parallel paths, and perspective makes them seem to emerge from one point. A higher radiant generally means more of the shower is available to observe.Guide: Meteor showers: radiants, ZHR and observed rates
Rayleigh scattering: Scattering by particles much smaller than a light wave’s wavelength, such as air molecules. Short wavelengths are scattered much more strongly than long ones. This helps explain both the blue daytime sky and the reddish colour of the low Sun.Guide: Civil, nautical and astronomical twilight explained
Retrograde motion: A planet’s apparent westward motion against the stars, opposite to its usual eastward progress. It results from relative orbital motion, such as Earth overtaking an outer planet or being overtaken by Mercury or Venus. The planet does not reverse its actual orbital direction.Guide: Retrograde motion: why planets seem to turn back
Right ascension: The coordinate measured eastward along the celestial equator from the equinox, usually expressed in hours, minutes and seconds from 0 to 24 hours. It is analogous to longitude on Earth. Together with local sidereal time it determines when an object crosses the meridian.Guide: Right ascension and declination explained
Ring opening angle: The angle at which we see Saturn’s ring plane. Across a Saturnian year of nearly 29.5 Earth years, the rings range from edge-on, at about 0°, to widely open at around 27°. The apparent opening determines how prominent the rings look.Guide: Saturn’s rings: tilt, opening and visibility

S

Saros: An eclipse cycle of 223 synodic months, approximately eighteen years, eleven days and eight hours. The Sun, Moon and lunar nodes return to similar relative positions, producing a related eclipse. The extra eight hours shift a corresponding solar-eclipse path roughly 120° westward.Guide: Solar eclipses: paths, contacts and obscuration
Sidereal day: The time for one rotation of Earth relative to the equinox, about 23 hours, 56 minutes and 4 seconds of mean solar time. It is about three minutes and fifty-six seconds shorter than a mean solar day, so the stellar sky returns to a similar position roughly four minutes earlier each night.Guide: Sidereal time: a clock for the rotating sky
Sidereal time: The hour angle of the equinox: a measure of Earth’s rotation relative to the celestial reference frame. A sidereal day is about three minutes and fifty-six seconds shorter than a mean solar day. Local sidereal time indicates which right ascension is crossing the local meridian.Guide: Sidereal time: a clock for the rotating sky
Solar cycle: The roughly eleven-year rise and fall of solar activity, visible in sunspot numbers, flares and auroras. The Sun’s magnetic field reverses in each cycle, so a full magnetic cycle takes about twenty-two years. Solar Cycle 25 began in late 2019.Guide: Sunspots and the solar cycle explained
Solstice: The instant when the Sun reaches its greatest northerly or southerly declination, about 23.4°, around June and December. In the Northern Hemisphere the June solstice brings the longest day and the December solstice the shortest; the Southern Hemisphere has the reverse seasons.Guide: Solstices and equinoxes: the seasons explained
Spectral class: A classification of a star by features in its spectrum, primarily associated with surface temperature. The main sequence of classes O, B, A, F, G, K and M runs from hot bluish stars to cooler reddish ones. The Sun is class G2.
Sunrise: The instant when the Sun’s upper limb appears at the horizon; sunset is when it disappears. Under the standard convention the Sun’s centre is about fifty arcminutes below the mathematical horizon, allowing roughly sixteen arcminutes for the solar radius and thirty-four for refraction.Guide: Sunrise and sunset explained
Sunspot number: A measure of solar spot activity also called the Wolf number, after Rudolf Wolf. It combines ten times the number of sunspot groups with the count of individual spots, adjusted for the observer and instrument. The historical series extends back into the eighteenth century.Guide: Sunspots and the solar cycle explained
Supermoon: A full moon near perigee. Compared with a full moon near apogee, often called a micromoon, it can appear up to about 14% larger in diameter and around 30% brighter. Without a direct comparison, the size difference is difficult to judge with the unaided eye.Guide: Supermoons and micromoons: size and distance
Synchronous rotation: Rotation in which a body turns once on its axis in the same time that it completes an orbit around its parent. Tidal forces brought the Moon into this state, keeping roughly the same face towards Earth. Libration nevertheless lets us see about 59% of its surface over time.Guide: Lunar libration: the Moon’s apparent rocking
Synodic month: The interval from one new moon to the next, averaging about 29.53 days. It is longer than the sidereal month of about 27.32 days because Earth has moved along its orbit, so the Moon must travel farther to regain the same alignment with the Sun.Guide: Moon phases: from new moon to full moon
Syzygy: A nearly straight-line arrangement of the Sun, Earth and Moon, or other celestial bodies. New and full moons are syzygies; when the Moon is also near an orbital node, an eclipse can occur. Stronger spring tides also occur around these alignments.Guide: Moon phases: from new moon to full moon

T

Terminator: The boundary between a body’s illuminated and unilluminated sides. Near the Moon’s terminator, craters and mountains cast long shadows. This makes it an especially rewarding region for seeing surface detail through a telescope.Guide: Moon phases: from new moon to full moon
Terrestrial Time (TT): A uniform time scale used for ephemerides referred to observers on Earth. Its conventional relation to International Atomic Time is an offset of 32.184 seconds. The difference between TT and Earth-rotation time UT1 is called ΔT.Guide: Julian Date, UT, TT and Delta T explained
Tides: Periodic deformations of Earth and its oceans caused by differences in the Moon’s and Sun’s gravitational pull across Earth. Many coasts experience two high tides a day, although local patterns vary. Near new and full moon the effects reinforce one another as spring tides; around quarter moons they combine into weaker neap tides.Guide: Moon phases: from new moon to full moon
Topocentric: Referred to an observer’s location on Earth’s surface rather than Earth’s centre, the geocentric viewpoint. The difference is particularly important for the nearby Moon because Earth’s radius is significant compared with the lunar distance.
Transit: Two common astronomical meanings are an object’s passage across the local meridian, or culmination, and a smaller body crossing a larger apparent disk. Mercury and Venus passing across the Sun are examples of the second meaning. The context determines which is intended.Guide: Mercury and Venus transits across the Sun
Twilight: The transition before sunrise and after sunset, when the atmosphere scatters sunlight into the sky. Its stages are defined by the Sun’s depth below the horizon: civil twilight to −6°, nautical twilight to −12° and astronomical twilight to −18°.Guide: Civil, nautical and astronomical twilight explained
Tycho-2: A star catalogue of around 2.5 million stars, combining measurements from the Hipparcos satellite with older positional catalogues. It provides positions, proper motions and brightness measurements in two wavelength bands. It is nearly complete to about magnitude 11.Guide: Find a star by HIP, HD or TYC catalogue number

U

Umbra: The region behind an obstructing body where no direct light from the source arrives. A total lunar eclipse occurs when the Moon lies completely within Earth’s umbra. A total solar eclipse is seen where the Moon’s umbra reaches Earth’s surface.Guide: Lunar eclipses: phases, colour and visibility
Umbral magnitude: For a lunar eclipse, the fraction of the Moon’s diameter immersed in Earth’s umbra at greatest eclipse. Values between zero and one indicate a partial umbral eclipse; values of one or greater indicate totality. A purely penumbral eclipse has a negative umbral magnitude.Guide: Lunar eclipses: phases, colour and visibility
Universal Time (UT): A time scale tied to Earth’s rotation, broadly related to mean solar time at Greenwich. Its main precise form, UT1, is not uniform because Earth’s rotation is irregular. UTC is the atomic time scale used for civil time; leap-second rules link their history, but UT1 and UTC are distinct quantities.Guide: Julian Date, UT, TT and Delta T explained

Z

Zenith: The point directly overhead, 90° above the horizon. An object can pass through the zenith only when its declination matches the observer’s geographical latitude. For the Sun, this can happen only within the tropics.Guide: The Sun’s position: azimuth and altitude
ZHR: Zenithal hourly rate: the number of shower meteors an observer would see per hour under standard ideal conditions, with the radiant at the zenith and a limiting magnitude of 6.5. The rate actually observed is usually lower because of radiant altitude, sky brightness and other conditions.Guide: Meteor showers: radiants, ZHR and observed rates
Zodiac: The belt of constellations along the ecliptic through which the Sun, Moon and planets appear to move. The Sun crosses thirteen astronomical constellations, including Ophiuchus. These are different from the twelve equal astrological signs, which have shifted relative to the constellations through precession.Guide: Finding constellations: orientation in the sky

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