Glossary

The glossary collects the 140 terms that can be tapped in the sections written so far. Each definition is the one shown when you tap the term, and each ends with a link to the section where the term is taught.

absolute magnitude: The magnitude a star would have if it were 10 parsecs away. It measures how luminous the star really is, whatever its distance. Chapter 4 treats it in full. It is taught in Section D2, Drawing a graph by hand.
absorption line: A dark line in a spectrum, where a cooler gas in front of a hot source has taken out light of one wavelength. It is taught in Section S4, Physics toolkit.
Airy pattern: The image of a star made by a perfect telescope with a circular aperture, a bright disc surrounded by faint rings. The first dark ring is 1.22λ/D from the centre. It is taught in Section S4, Physics toolkit.
altitude: The angle of an object above the horizon, from 0° on the horizon to 90° at the zenith. An object below the horizon has a negative altitude. It is taught in Section 1.2, Altitude and azimuth.
analemma: The figure of eight traced in the sky by the Sun when it is observed at the same mean time each day through a year. It is taught in Section 2.2, Solar time and the equation of time.
anchor: A bright pattern of stars that is easy to recognise, such as Orion or the Big Dipper, from which other stars are found by star-hopping. It is taught in Section O1.1, How to learn the sky.
angular distance: The angle between the directions to two objects, as seen by the observer. On the celestial sphere it is the arc of the great circle that joins the two points. It is also called the angular separation. It is taught in Section 1.1, The celestial sphere.
angular momentum: The amount of turning motion a body has about a centre, mrv for a body moving at speed v at right angles to the line from the centre. It stays constant when the force on the body points along that line. It is taught in Section S4, Physics toolkit.
angular speed: The angle that a turning body turns through each second, measured in radians per second and written ω. It is taught in Section S4, Physics toolkit.
anomalistic year: The time from one perihelion of Earth to the next, 365.2596 days. It is taught in Section 2.4, Calendars and Julian dates.
apparent solar time: The time shown by a sundial, set by the real Sun. It is 12 hours more than the hour angle of the Sun, so it is 12:00 when the Sun crosses the meridian. It is taught in Section 2.2, Solar time and the equation of time.
arcminute: One sixtieth of a degree, written 1′. It is taught in Section S3, Maths toolkit.
arcsecond: One sixtieth of an arcminute, written 1″. There are 3600 arcseconds in a degree. It is taught in Section S3, Maths toolkit.
asterism: A pattern of stars that is not one of the 88 constellations, such as the Big Dipper, which is part of Ursa Major, or the Summer Triangle, whose stars are in three constellations. It is taught in Section O1.1, How to learn the sky.
astronomical horizon: The great circle 90° from the zenith. Seen from a height, the visible edge of the land or sea lies a little below it. It is taught in Section 1.2, Altitude and azimuth.
astronomical triangle: The spherical triangle whose corners are the celestial pole, the zenith and a star. Its sides are 90° minus the latitude, 90° minus the declination and the zenith distance, and its angles at the pole and the zenith give the hour angle and the azimuth. It is taught in Section 1.5, The astronomical triangle.
astronomical twilight: The part of twilight while the centre of the Sun is between 12° and 18° below the horizon. Once the Sun is 18° below, the sky is fully dark. It is taught in Section 2.6, Sunrise, sunset and twilight.
astronomical unit: The average distance between Earth and the Sun, 1.496 × 10⁸ km, written au. It is taught in Section S3, Maths toolkit.
azimuth: The direction of an object around the horizon, measured from north through east, so that north is 0°, east 90°, south 180° and west 270°. It is taught in Section 1.2, Altitude and azimuth.

Bayer designation: The name of a star made of a Greek letter and the Latin genitive of its constellation, such as α Orionis, written α Ori. The letters go roughly in order of brightness. It is taught in Section O1.1, How to learn the sky.

cardinal points: The north, east, south and west points of the horizon. The north point is the point of the horizon nearest to the north celestial pole. It is taught in Section 1.2, Altitude and azimuth.
celestial equator: The great circle on the celestial sphere halfway between the two celestial poles. It lies directly above Earth's equator. It is taught in Section 1.1, The celestial sphere.
celestial pole: One of the two points where Earth's axis of rotation, extended into space, meets the celestial sphere. The whole sky appears to turn around them once a day. It is taught in Section 1.1, The celestial sphere.
celestial sphere: An imaginary sphere centred on the observer. Each star, planet or galaxy is drawn where our line of sight to it meets the sphere. Only directions matter, so the radius can be anything we like. It is taught in Section 1.1, The celestial sphere.
centripetal acceleration: The acceleration towards the centre of a body that moves round a circle. It is v²/r for a speed v on a circle of radius r. It is taught in Section S4, Physics toolkit.
Cepheid: A yellow supergiant star that swells and shrinks with a steady period of a few days to a few months, brightening and fading as it does. The longer the period, the more luminous the star, which makes Cepheids a way of measuring the distances of galaxies. It is taught in Section D2, Drawing a graph by hand.
circumpolar: A circumpolar star is one that never sets as seen from a given latitude, because its whole daily circle stays above the horizon. It is taught in Section 1.4, What you can see from where you stand.
civil twilight: The part of twilight while the centre of the Sun is less than 6° below the horizon. There is still enough light to work outdoors. It is taught in Section 2.6, Sunrise, sunset and twilight.
component: The part of a vector that lies along one of a set of axes at right angles to each other. A vector is the sum of its components. It is taught in Section S3, Maths toolkit.
constellation: One of the 88 areas into which the IAU divides the whole sky, each named after the figure that its brighter stars traditionally make. The boundaries run along lines of right ascension and declination of the year 1875. It is taught in Section O1.1, How to learn the sky.
continuous spectrum: A spectrum in which light of all wavelengths is present, as from the hot, dense surface of a star. It is taught in Section S4, Physics toolkit.
cross product: A vector formed from two vectors, at right angles to both. Its size is the area of the parallelogram that the two vectors span, which is the product of their sizes and the sine of the angle between them. It is taught in Section S3, Maths toolkit.

daily circle: The circle a star traces on the sky in one day. It is centred on the celestial pole and parallel to the celestial equator. It is taught in Section 1.1, The celestial sphere.
declination: The angle of a star north or south of the celestial equator, positive to the north. It plays the part of latitude on the sky. It is taught in Section 1.3, Right ascension, declination and hour angle.
deep-sky object: An object outside the solar system that looks like more than a point of light, such as a star cluster, a nebula or a galaxy. It is taught in Section O1.8, Messier and other deep-sky objects.
derivative: The rate at which one quantity changes as another changes, written dy/dx. On a graph of y against x it is the slope of the curve. It is taught in Section S3, Maths toolkit.
diffraction: The spreading of a wave as it passes through an opening or past an edge. It limits how fine a detail any telescope can show. It is taught in Section S4, Physics toolkit.
dip of the horizon: The angle by which the visible horizon lies below the astronomical horizon, for an observer above the ground or the sea. It is taught in Section 1.2, Altitude and azimuth.
diurnal motion: The daily turning of the whole sky from east to west, which is caused by Earth turning from west to east. It is taught in Section 1.1, The celestial sphere.
Doppler effect: The change in the wavelength of light or sound from a source that moves towards or away from the observer. Light from a receding source is shifted to the red, and light from an approaching one to the blue. It is taught in Section S4, Physics toolkit.
dot product: A single number formed from two vectors, the sum of the products of their matching components. It equals the product of their sizes and the cosine of the angle between them, so it is zero for vectors at right angles. It is taught in Section S3, Maths toolkit.

e-folding time: The time in which a quantity that decays exponentially falls by a factor e = 2.718. It is 1.44 times the half-life. It is taught in Section S3, Maths toolkit.
ecliptic: The great circle along which the Sun moves among the stars over a year. It is tilted to the celestial equator by 23.4°. It is taught in Section 1.6, Ecliptic and galactic coordinates.
ecliptic latitude: The angle of an object north or south of the ecliptic, positive towards the north ecliptic pole. It is taught in Section 1.6, Ecliptic and galactic coordinates.
ecliptic longitude: The angle measured eastward along the ecliptic from the March equinox point, from 0 to 360°. It is taught in Section 1.6, Ecliptic and galactic coordinates.
electromagnetic spectrum: The whole range of light, from radio waves through infrared, visible light, ultraviolet and X-rays to gamma rays. It is taught in Section S4, Physics toolkit.
electronvolt: A unit of energy, 1.602 × 10⁻¹⁹ J, the energy an electron gains in crossing a voltage of one volt. It is written eV. It is taught in Section S4, Physics toolkit.
emission line: A bright line in a spectrum, light given out at one wavelength by a hot, thin gas. It is taught in Section S4, Physics toolkit.
energy level: One of the particular energies at which an atom can hold an electron. Jumps between levels give out or absorb photons of fixed wavelengths. It is taught in Section S4, Physics toolkit.
equation of time: The difference between apparent and mean solar time. In this course it is apparent minus mean time, positive when a sundial is ahead of the clock. It ranges from about −14 to +16 minutes through the year. It is taught in Section 2.2, Solar time and the equation of time.
error bar: A short line on a graph through a plotted point, from the value minus its uncertainty to the value plus its uncertainty, with a small crossbar at each end. It is taught in Section D2, Drawing a graph by hand.
escape speed: The smallest speed at which a body can leave a planet or star and never come back, √(2GM/r) at a distance r from a mass M. It is taught in Section S4, Physics toolkit.
eyepiece: The small lens of a telescope through which the eye looks at the image formed by the objective. It is taught in Section S4, Physics toolkit.

focal length: The distance behind a lens or mirror at which it brings the light from a distant point to a focus. It is taught in Section S4, Physics toolkit.
focal ratio: The focal length of a telescope divided by the diameter of its objective. It is taught in Section S4, Physics toolkit.

galactic equator: The great circle along the middle of the band of the Milky Way, where the plane of our Galaxy meets the celestial sphere. Galactic latitude is measured from it and galactic longitude along it, starting from the direction of the galactic centre. It is taught in Section 1.6, Ecliptic and galactic coordinates.
geometric mean: The square root of the product of two numbers. On a logarithmic scale it lies halfway between them. It is taught in Section S3, Maths toolkit.
globular cluster: A tight ball of a hundred thousand to a few million stars about 12 billion years old, orbiting the centre of our Galaxy far above and below its disc. Most are seen towards Sagittarius, Scorpius and Ophiuchus. It is taught in Section O1.8, Messier and other deep-sky objects.
gravitational potential energy: The energy stored in a body by its position against gravity. At a distance r from a mass M it is −GMm/r, taken as zero very far away. It is taught in Section S4, Physics toolkit.
great circle: A circle on a sphere whose centre is the centre of the sphere, such as Earth's equator or the celestial equator. It cuts the sphere into two equal halves, and its arcs give the shortest routes between points on the sphere. Every other circle on a sphere is a small circle. It is taught in Section S3, Maths toolkit.
Gregorian calendar: The calendar in use today, introduced in 1582. Years ending in 00 are leap years only if they can be divided by 400, which gives an average year of 365.2425 days. It is taught in Section 2.4, Calendars and Julian dates.

horizon: The great circle on the celestial sphere 90° from the zenith. It separates the half of the sky we can see from the half below the ground. It is taught in Section 1.2, Altitude and azimuth.
hour angle: The angle from the meridian to a star, measured westward along the celestial equator and usually given in hours. It tells us how long ago the star crossed the meridian. It is taught in Section 1.3, Right ascension, declination and hour angle.
hour circle: A great circle through the two celestial poles. A star's hour circle is the one through the star, and it meets the celestial equator at right angles. It is taught in Section 1.3, Right ascension, declination and hour angle.
hydrostatic equilibrium: The balance in a gas or liquid at rest between the pressure, which grows with depth, and the weight of the layers above. It is taught in Section S4, Physics toolkit.

ideal gas: A gas whose particles are far enough apart that its pressure, density and temperature obey p = nkT. Most gases in astronomy behave in this way. It is taught in Section S4, Physics toolkit.
in quadrature: Combined by taking the square root of the sum of the squares, the way independent uncertainties are combined. It is taught in Section S3, Maths toolkit.
integral: The sum of a quantity over many small pieces. The integral of f(x) between two values of x is the area under the graph of f between them. It is taught in Section S3, Maths toolkit.
ionisation energy: The energy needed to free an electron from an atom. For hydrogen in its lowest level it is 13.6 eV. It is taught in Section S4, Physics toolkit.

Julian calendar: The calendar introduced by Julius Caesar in 45 BC, with a leap day every fourth year and an average year of 365.25 days. It is taught in Section 2.4, Calendars and Julian dates.
Julian date: A continuous count of days and fractions of a day from noon UT on 1 January 4713 BC. Noon UT on 1 January 2000 is JD 2 451 545.0. It is taught in Section 2.4, Calendars and Julian dates.

kinetic energy: The energy that a body has because it moves, ½mv² for a body of mass m moving at speed v. It is taught in Section S4, Physics toolkit.

latitude: The angle of a place north or south of Earth's equator, measured at Earth's centre if Earth is treated as a sphere. Northern latitudes are positive and southern ones negative. It is taught in Section S3, Maths toolkit.
light year: The distance light travels in one year, about 9.46 × 10¹² km.
local mean time: Mean solar time at the observer's own meridian. It is later than universal time by 4 minutes for every degree of longitude east of Greenwich. It is taught in Section 2.3, Longitude, time zones and universal time.
logarithm: The power to which 10 must be raised to give a number, written log. Since 10³ = 1000, log 1000 = 3. Multiplying a number by 10 adds 1 to its logarithm. It is taught in Section S3, Maths toolkit.
longitude: The angle of a place east or west of the meridian through Greenwich, measured along the equator. It is taught in Section S3, Maths toolkit.
lower culmination: The moment a star crosses the meridian on the far side of the pole, at its lowest altitude. It is taught in Section 1.4, What you can see from where you stand.

magnification: The factor by which a telescope enlarges the angles between objects, the focal length of the objective divided by that of the eyepiece. It is taught in Section S4, Physics toolkit.
magnitude: A measure of how bright a star looks. Smaller numbers mean brighter stars. The brightest stars are near magnitude 0, and the faintest that can be seen with the naked eye under a dark sky are near magnitude 6. It is taught in Section O1.1, How to learn the sky.
March equinox point: The point where the Sun crosses the celestial equator from south to north each year, around 20 March. Right ascension is measured eastward from it. It is also called the vernal equinox, the spring equinox or the first point of Aries. It is taught in Section 1.3, Right ascension, declination and hour angle.
mean molecular mass: The average mass of the particles of a gas, in units of the mass of a hydrogen atom, written μ. It is about 29 for air and about 0.6 for the ionised gas inside the Sun. It is taught in Section S4, Physics toolkit.
mean solar time: Time measured by the mean Sun, 12 hours more than its hour angle. Clocks keep mean solar time or a time based on it. It is taught in Section 2.2, Solar time and the equation of time.
mean Sun: An imaginary Sun that moves round the celestial equator at a steady rate and goes round once a year. Mean solar time follows it, so every mean solar day lasts exactly 24 hours. It is taught in Section 2.2, Solar time and the equation of time.
median: The middle value of a set of numbers put in order, so that half lie above it and half below it. It is taught in Section S3, Maths toolkit.
meridian: The great circle through the zenith and the celestial poles. It meets the horizon at the north and south points. It is taught in Section 1.2, Altitude and azimuth.
moment of inertia: For a spinning body, the quantity I that turns its angular speed into its angular momentum, L = Iω. It is larger when more of the mass lies far from the axis, and for a sphere of mass M and radius R it is κMR², with κ = 2/5 if the density is uniform. It is taught in Section S4, Physics toolkit.

nadir: The point of the sky directly below the observer, opposite the zenith. It is taught in Section 1.2, Altitude and azimuth.
natural logarithm: A logarithm to the base e = 2.718 instead of 10, written ln. It is related to the ordinary logarithm by ln x = 2.303 log x. It is taught in Section S3, Maths toolkit.
nautical twilight: The part of twilight while the centre of the Sun is between 6° and 12° below the horizon. The horizon at sea can still be seen. It is taught in Section 2.6, Sunrise, sunset and twilight.
nutation: A small nodding of Earth's axis on top of precession, up to 9.2″ in obliquity and 17.2″ in longitude, with a main period of 18.6 years set by the turning of the Moon's orbit. It is taught in Section 2.7, Precession and nutation.

objective: The large lens or mirror of a telescope, which collects the light and forms the image. It is taught in Section S4, Physics toolkit.
obliquity of the ecliptic: The angle between the ecliptic and the celestial equator, equal to the tilt of Earth's axis from the perpendicular to its orbit. It is 23.44°. It is taught in Section 1.6, Ecliptic and galactic coordinates.
open cluster: A loose group of a few dozen to a few thousand young stars that formed together from one cloud of gas, found in the disc of our Galaxy and so along the Milky Way. The Pleiades and the Beehive are examples. It is taught in Section O1.8, Messier and other deep-sky objects.
order of magnitude: The power of ten that a number is nearest to. Two numbers of the same order of magnitude differ by less than a factor of about 10. It is taught in Section S3, Maths toolkit.

parallax: The change in the direction to an object when the observer moves. The nearer the object, the larger the change. The parallax of a star is the angle that 1 au, the radius of Earth's orbit, covers as seen from the star. It is half the largest change in the star's direction over six months. It is taught in Section 1.1, The celestial sphere.
parsec: The distance from which 1 au covers an angle of 1″. It is 206 265 au, or 3.26 light years, and a star with a parallax of p arcseconds is 1/p parsecs away. It is taught in Section S3, Maths toolkit.
photon: A packet of light. A photon of frequency ν carries the energy hν, where h is Planck's constant. It is taught in Section S4, Physics toolkit.
planetary nebula: A shell of gas thrown off by a dying star like the Sun and lit by the hot core left in the middle. It has nothing to do with planets, and the Ring Nebula, M57, is an example. It is taught in Section O1.8, Messier and other deep-sky objects.
polar distance: The angle from the north celestial pole to a star, equal to 90° minus its declination. It is taught in Section 1.3, Right ascension, declination and hour angle.
position angle: A direction on the sky, measured from north through east. A position angle of 90° points due east. It is taught in Section S3, Maths toolkit.
power law: A relation of the form y = kxⁿ, in which y is proportional to a power of x. On logarithmic axes it is a straight line whose slope is the power n. It is taught in Section S3, Maths toolkit.
precession: The slow turning of Earth's axis round the pole of the ecliptic, once in about 25 800 years, caused by the pull of the Sun and the Moon on Earth's equatorial bulge. It moves the celestial poles and makes the equinoxes slide westward along the ecliptic by 50.29″ a year. It is taught in Section 2.7, Precession and nutation.
pressure: Force per unit area, measured in pascals. One pascal is one newton per square metre. It is taught in Section S4, Physics toolkit.
prime vertical: The vertical circle through the east and west points of the horizon, at right angles to the meridian. It is taught in Section 1.2, Altitude and azimuth.
proper motion: The slow drift of a star across the sky, caused by its velocity across the line of sight. It is usually given in arcseconds or milliarcseconds per year. It is taught in Section S3, Maths toolkit.

radial velocity: The part of a star's velocity along the line of sight, measured from the Doppler shift of its spectrum. It is positive for a star moving away from us and negative for one coming towards us. It is taught in Section S3, Maths toolkit.
radian: The angle at the centre of a circle that cuts off an arc as long as the radius. One radian is 57.30°, and a full turn is 2π radians. It is taught in Section S3, Maths toolkit.
Rayleigh criterion: The rule that two equal stars can just be told apart when the centre of the image of one falls on the first dark ring of the other, 1.22λ/D away. It is taught in Section S4, Physics toolkit.
redshift: The change in the wavelength of a line divided by its rest wavelength, written z. It is positive for a source moving away and negative for one coming towards us. It is taught in Section S4, Physics toolkit.
relative uncertainty: An uncertainty divided by the value it belongs to, often given as a percentage. It is taught in Section S3, Maths toolkit.
rest wavelength: The wavelength of a spectral line as it is given out by a source at rest, as measured in a laboratory. It is taught in Section S4, Physics toolkit.
right ascension: The angle of a star eastward along the celestial equator from the March equinox, usually given in hours, with 24 h making a full circle. It plays the part of longitude on the sky. It is taught in Section 1.3, Right ascension, declination and hour angle.

scale height: The height over which a quantity that falls off exponentially with height, such as the density of an atmosphere, drops by a factor e = 2.718. It is taught in Section S3, Maths toolkit.
sidereal day: The time Earth takes to turn once relative to the stars, which is 23 h 56 min 4 s. The stars return to the same places in the sky after one sidereal day. It is taught in Section 2.1, Sidereal and solar days.
sidereal time: The hour angle of the March equinox point, which equals the right ascension of a star at upper culmination. Its full name is local sidereal time, and it is later by one hour for every 15° of longitude further east. It is taught in Section 1.3, Right ascension, declination and hour angle.
sidereal year: The time Earth takes to go once round the Sun relative to the stars, 365.2564 days, about 20 minutes longer than the tropical year. It is taught in Section 2.4, Calendars and Julian dates.
sign of the zodiac: One of twelve equal stretches of the ecliptic, 30° each, counted eastward from the March equinox point and named after the constellations they roughly matched more than two thousand years ago. Precession has since moved each sign mostly into the constellation west of its namesake. It is taught in Section O1.7, The zodiac.
small circle: A circle on a sphere whose centre is not the centre of the sphere, such as a parallel of latitude other than the equator. The daily path of most stars is a small circle. It is taught in Section S3, Maths toolkit.
small-angle approximation: The use of an angle θ itself, in radians, in place of sin θ and tan θ, and of 1 − θ²/2 in place of cos θ, when θ is small. For angles below about 10° each is correct to better than 1%. It is taught in Section S3, Maths toolkit.
solar day: The time from one crossing of the meridian by the Sun to the next, from noon to noon. Its average over a year is the mean solar day of 24 hours, which is about 3 min 56 s longer than the sidereal day. It is taught in Section 2.1, Sidereal and solar days.
solid angle: The area that a region covers on a sphere centred on the observer, divided by the square of the radius. It is measured in steradians, and the whole sky is 4π steradians. It is taught in Section S3, Maths toolkit.
solstice: Either of the two points of the ecliptic furthest from the celestial equator, or the moment the Sun passes it. The Sun reaches the June solstice, at declination +23.44°, around 21 June and the December solstice, at −23.44°, around 21 December. It is taught in Section 1.6, Ecliptic and galactic coordinates.
spherical triangle: A triangle drawn on a sphere whose three sides are arcs of great circles. Its sides are measured by the angles they make at the centre of the sphere, and its angles add up to more than 180°. It is taught in Section 1.5, The astronomical triangle.
standard deviation: The typical scatter of a set of measurements about their mean, found as the square root of the mean of the squared differences from the mean. It is taught in Section S3, Maths toolkit.
star-hopping: Finding a star by following a line through two known stars and going on along it by a distance measured with the gap between them or with a hand held at arm's length. It is taught in Section O1.1, How to learn the sky.
steradian: The unit of solid angle. A region covers one steradian if its area on a sphere is the square of the radius. One steradian is 3283 square degrees. It is taught in Section S3, Maths toolkit.
surface brightness: The brightness of an extended object per unit area of sky, usually given as the magnitude of the light from one square arcsecond. An object of total magnitude m spread evenly over A square arcseconds has a mean surface brightness of m + 2.5 log A. It is taught in Section O1.8, Messier and other deep-sky objects.

tangential velocity: The part of a star's velocity at right angles to the line of sight. It shows as the star's proper motion across the sky. It is taught in Section S3, Maths toolkit.
tropical year: The time from one March equinox to the next, 365.2422 days. It is the year of the seasons, which calendars try to follow. It is taught in Section 2.4, Calendars and Julian dates.

uncertainty: An estimate of how far a measured or calculated value may be from the true value, written after the value with ±. It is taught in Section S3, Maths toolkit.
unit vector: A vector of size 1, used to mark a direction. It is taught in Section S3, Maths toolkit.
universal time: The local mean time at Greenwich, at longitude 0°, written UT. Astronomers give the times of events in UT. It is taught in Section 2.3, Longitude, time zones and universal time.
upper culmination: The moment a star crosses the meridian at its greatest altitude. It is also called transit. It is taught in Section 1.4, What you can see from where you stand.

vector: A quantity with both a size and a direction, such as a velocity or a force. It is written in bold, and its size in ordinary type. It is taught in Section S3, Maths toolkit.
vertical circle: A great circle through the zenith and the nadir. It meets the horizon at right angles. It is taught in Section 1.2, Altitude and azimuth.

zenith: The point of the sky directly overhead. It is taught in Section 1.2, Altitude and azimuth.
zenith distance: The angle from the zenith to an object, equal to 90° minus its altitude. It is also called the zenith angle. It is taught in Section 1.2, Altitude and azimuth.
zodiac: The band of sky about 9° either side of the ecliptic, in which the Sun, the Moon and the bright planets are always found. The ecliptic crosses 13 constellations, the twelve that give their names to the signs and Ophiuchus. It is taught in Section O1.7, The zodiac.
zone time: The time on the clocks of a country or region, set to the mean time of a chosen meridian, usually a whole number of hours from universal time. It is taught in Section 2.3, Longitude, time zones and universal time.