N

nadir -- The point in the sky which is directly underfoot.

nebula -- Latin for 'cloud'. A fuzzy, or cloudlike patch in the sky. Nebulae (plural form) may be dark clouds of interstellar material which obscure the stars behind them, glowing clouds of heated interstellar gas, clouds of dust reflecting the light of nearby stars, or groups of stars (clusters or galaxies) which are simply too far away to see as individual objects.

nebular theory -- The theory that the Solar System was formed from a rotating disk of gas and dust, orbiting the forming Sun. Also called the planetesimal accretion theory. First presented in a form reasonably similar to the present model by Pierre-Simon Laplace. Similar ideas were expressed by Immanuel Kant, somewhat earlier, but it is not clear whether Laplace was aware of Kant's very poorly distributed work. Earlier ideas, such as those of Descartes, involving eddylike motions, were intended as theories of the general nature of the universe, rather than specific explanations of the origin of the Solar System. The Nebular Theory was abandoned, briefly, in the late 1800's, for reasons relating to the rotation of the Sun, but reintroduced, in a reinvigorated way, by the mid-1900's. There is, at the moment, no doubt that some variation on this theory is the correct explanation of the origin of our Sun and the worlds which surround it.

neutrally stratified layer -- A region in an atmosphere in which the normal lapse rate is close to the adiabatic lapse rate, so that the region is usually on the verge of vertical mixing, or convection, but not actually in convection. For most planets, including the Earth, the troposphere is a neutrally stratified layer.

neutrino -- A nearly massless particle created in certain types of nuclear reactions, which, because of its very small mass, travels at nearly (but not quite) the speed of light. The neutrino only interacts with matter through the weak nuclear force, which means that it has to practically pass right through an atomic nucleus in order to react with anything, and even then, usually just goes through without noticing the nucleus. As a result, extremely difficult to detect. It is believed that hundreds of neutrinos pass through every square inch of space every second, but since they go right through the material in that space, under most circumstances, they have no effect on it. Until fairly recently, it was thought that the neutrino was absolutely massless, and traveled at the speed of light, but problems in the flux of neutrinos coming from the Sun (the Solar neutrino problem), and various aspects of the Standard Model of subatomic particles implied that it must have at least some mass, and although that mass is very small (about one billion times less than the mass of the next lightest particle, the electron), it is not quite zero.

neutrino oscillations -- Spontaneous changes of neutrinos of one type, or 'flavor', to neutrinos of a different type. Since a given neutrino telescope observes only neutrinos of one type, changes of neutrinos enroute from the Sun to the Earth can reduce the number of neutrinos which can be observed, creating a deficit in the number of observed neutrinos. This is the cause of the Solar neutrino problem.

neutron -- A subatomic particle, approximately equal in mass to a proton, without any electrical charge. Save for ordinary hydrogen, all atomic nuclei contain both protons and neutrons.

neutron-degenerate gas -- A degenerate gas in which the degeneracy force is caused by neutrons. The basic material of which neutron stars are made.

neutron star -- A dead star comprised primarily of neutron-degenerate gas. Typically, such objects have a mass similar to or slightly larger than that of the Sun, but a diameter of about 10 miles. As in the case of electron-degenerate stars, or white dwarfs, more massive neutron stars are smaller, and when they become smaller than the Schwarschild limit, cannot exist as stable objects. Depending upon the nature of the gas in the core of a neutron star (which may, to a certain extent, involve physics beyond definite current knowledge), the maximum mass of a neutron star is between 2 and 3 Solar masses. The density of such a star is several trillions of times the density of water, the gravity is several billions of times the gravity of the Earth, and the escape velocity is a substantial fraction of the speed of light. Rotating neutron stars may produce pulsars. Neutron stars which have material dumped on them by a companion may produce X-ray bursts, or even more exotic phenomena associated with rapidly spinning accretion disks, surrounding the star.

new Moon -- The time or date when the longitude of the Moon is the same as that of the Sun, so that the two are as nearly lined up as possible. At new Moon, the lit side of the Moon is the same as the farside, and if we could see the Moon, it would be completely dark, as only the unlit side would face us. Hence, we see no Moon at all.

Newton, Sir Isaac -- 1643-1727. English physicist, one of the greatest scientists in modern history. Originated the theory of motion as we currently know it, and the calculus (called by him, fluxions), and in so doing, explained Kepler's Laws of Planetary Motion, and discovered the Universal Law of Gravitation. Also did extensive studies of light, discovering the true nature of the spectrum, and inventing the Newtonian telescope. External Link

Newton's Laws of Motion -- The three laws -- the Law of Inertia, the Force Law, and the Law of Action and Reaction -- elucidated by Newton, which govern the motion of objects in inertial reference systems.

Newtonian mechanics -- The laws of motion as elucidated by Newton. Including the Law of Inertia, the Force Law, the Law of Action and Reaction, the Law of Universal Gravitation, and the physical relationships governed by these laws.

Newtonian telescope -- A telescope design originated by Isaac Newton. In such a telescope, light enters the front of a long tube, is reflected by a parabolic mirror, so that it returns to the front of the tube, and then reflects off a flat mirror, tilted at a 45 degree angle, so that it passes through a hole in the side of the tube, where it can be viewed with an eyepiece. Newtonian telescopes have an advantage over refractors, which involve lenses, because mirrors reflect all kinds of light at exactly the same angle, and do not suffer from chromatic aberration. They have a disadvantage over catadioptric telescopes, in which the secondary mirror (the one at the front) is curved, in that they have a relatively long tube, for a given focal length.

node -- One of the two places in an orbit, where it crosses the plane of another orbit. Usually, where the first orbit crosses the plane of the Earth's orbit.

non-inertial reference frame -- A reference frame in which the Law of Inertia does not hold (is not correct). Any reference frame which is accelerated relative to an inertial reference frame. Examples include objects which are speeding up, or moving along curved paths. In such a reference frame, failure to recognize the fact that it is not an inertial reference frame results in oddities of motion and force which appear to be caused by imaginary forces, called fictitious forces, which appear perfectly real to someone in the non-inertial reference frame, but are nonexistent, as far as anyone moving in an inertial reference frame is concerned.

North Celestial Pole -- The point in the sky directly above the North Pole of the Earth. As the Earth turns around its axis of rotation, the sky appears to turn, in the opposite direction, about an axis through the North and South Celestial Poles.

O

oblate spheroid -- A roughly spherical surface which bulges at its 'equator' and is compressed at its 'poles', such as the surface of a rapidly rotating planet. Mathematically, the surface obtained by rotating an ellipse about its minor axis. As seen from 'above' or 'below' the original minor axis, circular in cross-section, but as seen from the 'side', elliptical in cross-section. Contrasted with a prolate spheroid.

oblateness -- The ellipticity of a rapidly rotating planet, expressed in terms of the increase in distance through the equator, compared to the distance through the poles. Thus, if a planet's diameter is 3% more through the equator, than through the poles, it could be said to have an oblateness of 3%. Numerically, identical to the flattening, and only differing from the flattening in terms of the viewpoint. In the case of flattening, the planet is thought of as being squashed through the poles. In the case of oblateness, it is thought of as bulging out at the equator.

-oid -- A suffix meaning 'resembling', used in astronomy to mean 'like something, and yet not exactly like it'. In particular, 'planetoid' means a small object which is like a planet, in physical structure, but too small to be called a planet. 'Asteroid' means an object which is so small and so distant that it appears starlike (just a dot in the night sky), but is not really a star (and in fact, is the same as a planetoid). 'Meteoroid' means an interplanetary object which, although not yet a meteor, can cause one, if and when it runs into the Earth's atmosphere at high speed.

optical double -- A pair of stars which appear close together in the sky, but are merely in the same direction from us, and not actually physically connected.

ozone -- A three-atom molecule of oxygen. Oxygen is most stable as a diatomic, or two-atom molecule, and the third atom makes ozone very chemically reactive, and somewhat unstable. Easily destroyed by chemical reactions which can take the extra oxygen atom away, and by the absorption of ultraviolet radiation.

ozone layer -- A region straddling the boundary between the stratosphere and the mesosphere (in other words, near the stratopause -- hence, sometimes, the "ozonopause"), where the absorption of ultraviolet radiation by ozone substantially heats the Earth's lower mesosphere and upper stratosphere. In percentage terms, the greatest concentration of ozone, and therefore the greatest heating of the atmosphere, is near the stratopause. However, since the atmosphere becomes denser as you near the surface of the Earth, the actual number of ozone molecules is greatest near the middle of the stratosphere. Because of this, graphs showing the concentration of ozone molecules in percentage terms, or the heating of the atmosphere by ozone, peak at the stratopause, while graphs based on the number of ozone molecules per unit volume peak near the middle of the stratosphere.

P

parabola -- One of two conic sections for which there is only one closed end. At the other, or open, end, the two sides of the parabola extend into infinity, parallel to each other.

parallactic ellipse -- The elliptical path apparently taken by a star, around its average position, as a result of the Earth's annual motion around the Sun. The semi-major axis of this ellipse is called the parallax of the star, and is a measure of its distance.

parallax -- The apparent change of the position of an object, caused by a lateral, or sideways, change in the position of an observer. Specifically, in astronomy, stellar parallax.

parsec (pc) -- Contraction of parallax second. A unit of distance, equal to the distance at which the parallax of a star is equal to one second of arc. Approximately 3 1/4 light years, or 200000 AUs, or 20 trillion miles, or 30 trillion kilometers.

periastron -- In an orbit around a star, the closest point to the star.

perigee -- In an orbit around the Earth, the closest point to the Earth.

perihelion -- In an orbit around the Sun, the closest point to the Sun.

periselene -- In an orbit around the Moon, the closest point to the Moon.

plage -- A bright region near a sunspot, as observed in a chromospheric image, or spectroheliogram. If the region is visible on photospheric images (usually only possible near the limb of the Sun), it is called a facula.

planet -- Literally, from the Greek, wanderer. One of a number of objects which appear to move among the stars, over a period of time. Nowadays, applied exclusively to large objects which do so because they are in orbit around the Sun, or some other star.

planetesimal -- A small object, as implied by the suffix -esimal, which is fated to become part of a planet. Presumed to be intermediate in size between microscopic bits of dust and ice, and planets themselves.

planetesimal accretion -- A theory of the formation of the Solar System, in which, within a rotating cloud of gas and dust referred to as the Solar Nebula, microscopic bits of dust and ice build up, through random collisions, to larger and larger objects. In an intermediate stage, they are called planetesimals, and at the end, may be called planets, if sufficiently large. However, many of the objects end up as smaller bodies -- asteroids and comets. Also, in the end stages, gravitational attraction may help move the process toward completion, at least for the very largest objects.

plasma -- A gas comprised primarily of charged particles -- usually, negatively charged electrons and positively charged ions.

positive ion -- An atom or molecule which is missing one or more of its normal complement of electrons, giving it a net positive charge.

positron -- An anti-electron. A particle which is, in almost every way, identical to an electron, save for having a positive charge, instead of a negative one. If a positron and electron collide, they both disappear (annihilate), and gamma-radiation with an energy equal to their rest-mass and kinetic energy will appear.

precession -- A change in some direction, due to changes in the orbital or rotational motion of an object. There is often more than one such change going on at the same time. E.g., the perihelion of Mercury's orbit precesses because of the gravitational influences of other planets, and more famously because of the difference between relativistic (Einsteinian) and classical (Newtonian) mechanics. Similarly, as noted below, the Vernal Equinox precesses both because of changes in the rotation of the Earth, and in its orbital motion.

precession of the Equinoxes -- The westward movement of the Vernal and Autumnal Equinoxes, relative to the orbit of the Earth, or the stars. The period of this movement is variously given as 21,000 or 26,000 years, leading to great confusion for students. The reason for this confusion is that the Vernal Equinox takes 26,000 years to move once around the sky, relative to the stars; but during that time, the orbit of the Earth is regressing (changing in the opposite direction), so it takes less time (only 21,000 years) for the Vernal Equinox to move once around the sky, relative to our orbit.

Prime Meridian -- The meridian of longitude passing through The Royal Observatory in Greenwich, England.

prolate spheroid -- A roughly spherical surface which bulges at its 'poles' and is compressed at its 'equator', such as an egg with symmetrical ends, or, very approximately, a football. Mathematically, the surface obtained by rotating an ellipse about its major axis. As seen from the apses ('sides') of the original major axis, circular in cross-section, but as seen from the 'top' or 'bottom' (above or below the minor axis), elliptical in cross-section. Contrasted with a oblate spheroid.

proton -- A subatomic particle, approximately equal in mass to a hydrogen atom, with a single positive charge. All atomic nuclei contain at least one proton. The number of protons determines the number of electrons which 'orbit' the nucleus, and in the process, determines the chemical element which corresponds to that nucleus. Protons are approximately two thousand times as massive as electrons, and approximately equal in mass to neutrons, the other particle found inside all atomic nuclei save ordinary hydrogen.

pulsar -- A rapidly 'pulsing' source of radio radiation (and, sometimes, optical and other wavelengths, as well), created by the rotation of a neutron star within a surrounding cloud of ionized gas. As the neutron star rotates, its magnetic field, rotating with it, sweeps through the ionized gas, heating it and accelerating it to speeds close to the speed of light. This causes the gas to give off synchrotron radiation, which is beamed only in a narrow cone, as a result of relativistic effects. As the cone sweeps past us, we see the radiation, and then we don't, in very rapid order.

Q

quarter Moon -- One of two times (first quarter, or third quarter, also known as last quarter) when the longitude of the Sun and Moon differ by exactly 90 degrees. If the Moon is 90 degrees East of the Sun, the Moon is at first quarter, because this is the first such phase occuring after new Moon. If the Moon is 90 degrees West of the Sun, the Moon is at last quarter, because this is the last such phase occurring before new Moon. Last quarter is also known as third quarter, because the Moon is then 3/4 of the way around its cycle of phases, from new Moon. At either quarter Moon, the Moon appears to be exactly half lit -- on the western side, when it is East of the Sun, at first quarter, and on the eastern side, when it is West of the Sun, at last quarter.

R

radial velocity -- The motion of a star towards or away from the Sun, calculated from the Doppler shift in its spectrum, as the rate of increase in the radius vector from the Sun to the star. Positive values indicate an increasing distance (the star is moving away from the Sun), while negative values indicate a decreasing distance (the star is moving toward the Sun).

red giant -- A star which is very large, and relatively cool. Such stars are very bright, and easily visible at large distances. A small minority of them are stars in the process of formation. Most of them are stars in the last stages of their visible life -- in other words, dying stars.

red shift (also redshift) -- An increase of light wavelengths, caused by a motion away from the observer. So called because red light has longer wavelengths than blue light.

reddening -- A change in the color of an object, caused by selective absorption of its light by material lying between it and the observer, in which shorter wavelengths are more affected than longer wavelengths. As in the reddening of the light of the Sun and Moon when they are near the horizon, by our atmosphere, or the reddening of the light of distant objects by clouds of interstellar gas and dust.

retrograde motion -- Motion which is backwards relative to the usual direction. Usually, an apparent westward, or backward, motion of a planet among the stars, caused by its lapping, or being lapped by, another planet.

retrograde rotation -- Rotation in a direction opposite to the orbital motion of a body.

retrograde revolution -- Orbital motion opposite to the usual direction of orbital motion. No planet has retrograde orbital motion. For moons, orbital motion opposite to the direction of their planet's rotation.

right ascension -- In the Equatorial coordinate system, a number representing the angle measured eastward, along the Celestial Equator, from the Vernal Equinox to the hour circle through a star. Somewhat analogous to longitude, in the Terrestrial coordinate system, but measured only to the east, and in time units, instead of degrees.

S

Saros, the -- A period of approximately 18 years, during which eclipses tend to repeat their characteristics, as a result of a nearly perfect (coincidental) equality between 223 synodic months, 242 draconic months, and 239 anomalistic months. Known as far back as Chaldean times, and used by the Chaldeans to predict lunar eclipses.

Schwarschild radius -- The radius of a black hole. Also, the radius of the event horizon of a black hole. The distance from the singularity which lies at the center of the black hole, at which the curvature of space-time (the gravity of the black hole) becomes so large that nothing, including light, can escape it.

seasons -- Annual changes in the temperature and/or weather on a planet, caused by the tilt, or inclination of its axis of rotation, to its orbit.

Selene -- The Greek name for the Moon. Also used as a prefix or suffix to represent a quantity involving the Moon.

semi-major axis -- In an ellipse, half the length of the major axis. In an orbit around the Sun, the average of the perihelion and aphelion distances.

sidereal -- Relative to the stars.

(mean) sidereal day -- The rotation period of the Earth around its axis, relative to the Vernal Equinox. Slightly shorter than the actual rotation period, because of the precession of the Equinoxes.

sidereal month -- The time required for the Moon to move around the Earth, relative to the fixed stars.

sidereal period -- The time required for an object to move once around the sky, relative to the stars.

sidereal period of revolution -- The time required for an object to move once around another object, as measured by its position among the stars. The orbital period of that object.

sidereal period of rotation -- The time required for an object to rotate once around its own axis, as measured by the apparent motion of the stars produced by its rotation. The rotation period for that object.

sidereal time -- Time measured by the motion of the stars.

sidereal year -- The time required for the Earth to move around its orbit, relative to the fixed stars. Equal to the time required for the Sun to move around the Ecliptic, relative to the fixed stars.

singularity -- In mathematics, a place where a continuous function becomes infinitely discontinuous. By extension, in astronomy, a place where a mass has collapsed to a point of zero size, and infinite density, temperature, gravity and tidal force. Every black hole has a singularity at its center.

skyline -- The visible horizon, including trees, buildings, mountains and the like, dividing the sky into an upper portion, which is visible, and a lower portion, which is not. Generally at least somewhat similar to, but not identical to, the astronomical horizon.

small circle -- Any circle on a sphere, which is not a great circle, such as, on the Earth, a parallel of latitude other than the Equator, or on the Celestial Sphere, a declination circle other than the Celestial Equator.

solar -- Relating to the Sun.

solar day -- The time required for the Sun to move from one meridian transit to the next, as a result of the Earth's rotation. Longer than the sidereal day, because of the apparent motion of the Sun, caused by our orbital motion. The apparent solar day is the actual time required for this motion on any given day, and varies throughout the year, because of the tilt of our axis and the eccentricity of our orbit. The mean solar day is the average length of the apparent solar day, over the course of one year.

Solar Nebula -- A cloud of gas and dust, rotating around the Sun, nearly in its rotational plane, during the time that the Sun and planets were forming. The planets built up through collisions of bits of dust and ice which were in this cloud, and in the process, acquired orbital motions which are very nearly in the same plane as the original rotation. Also see nebular theory, or planetesimal accretion theory.

Solar neutrino problem -- A problem involving a lower than expected flux of neutrinos from the Sun. The Standard model of Solar structure suggests that a certain number of neutrinos should be observed by neutrino telescopes on Earth, but the observed number has always been less than the predicted number. The answer to this problem, as it turns out, is that some of the neutrinos change their character, on their way to the Earth, and are not detected by the neutrino telescopes.

solar wind -- An extremely rarefied gas which is streaming away from the Sun at very high speeds. Typical speeds for solar wind particles range from a couple of hundred, to several hundred, miles per second, depending upon the current state of solar 'activity'. The density of the solar wind ranges from a few dozens to a few thousands of particles per cubic inch, again depending upon the activity of the Sun. In general, the solar wind is faster and denser when the Sun is 'active', and slower and less dense when the Sun is 'quiet'.

solstice -- Literally, Sun-stand-still. One of two places on the Ecliptic, the Summer Solstice and the Winter Solstice, where the declination of the Sun is at its greatest northern or southern value, and is neither increasing toward the north, nor toward the south. Also, one of the days when it is at one of those positions.

Solstitial Colure -- A great circle extending from the North Celestial Pole, through the Summer Solstice, to the South Celestial Pole, then through the Winter Solstice, and back to the North Celestial Pole. Consists of the hour circles of 6 and 18 hours right ascension.

Sosigenes of Alexandria -- Egyptian astronomer and mathematician (flourished first century BCE) consulted by Julius Caesar to reform the Roman calendar. Otherwise relatively unknown, virtually all of his works having been lost in the fire which destroyed the Library of Alexandria, but is thought to have believed that Mercury orbited the Sun, rather than the Earth.

South Celestial Pole -- The point in the sky directly above the South Pole of the Earth. As the Earth turns around its axis of rotation, the sky appears to turn, in the opposite direction, about an axis through the North and South Celestial Poles.

space velocity -- The motion of a star through space, relative to the Sun. Calculated from the radial velocity, toward or away from the Sun, and the tangential velocity, in the plane of the sky, using the Pythagorean Theorem.

spectral -- Referring to a spectrum.

spectral analysis -- Study of a spectrum, so as to learn the detailed physical and chemical characteristics of the object which produced the light, and the gases through which the light passed, after leaving the object.

spectral lines -- Absorption or emission lines.

spectroheliogram -- Literally, a spectral picture of the Sun. A picture of the Sun taken by using light of only one specific wavelength. This can reveal conditions within layers of the solar atmosphere which are lost within the glare of the photosphere when using light of all colors.

spectroscopic binary -- A binary star in which the two stars are too close to be seen as separate stars, but can be detected through a careful examination of the spectrum. Usually detected through changes in the Doppler shift of spectral lines.

spectrum -- A band of light of various colors, similar to a rainbow, produced by passing light through a prism or other optically dispersive instrument.

stellar -- Of, or pertaining to, stars.

stellar aberration -- A change in the apparent direction of a star, caused by a motion of the Earth towards, or away from, the star.

stellar parallax -- The change in a star's position, during a year, due to the Earth's motion around the Sun. More specifically, the semi-major axis of the parallactic ellipse produced by this motion.

stone -- A meteorite which is made of rock.

stony-iron -- A meteorite which is made of rock, but contains nickel-iron inclusions.

stratopause -- The boundary between the top of the stratosphere and the bottom of the mesosphere, as defined by the altitude at which temperature stops increasing with height.

stratosphere -- An inversion layer in the Earth's atmosphere, lying between the troposphere and the mesosphere. In the stratosphere, temperature increases with height, making vertical mixing, or convection, impossible. As a result, different parts of the stratosphere have slightly different compositions, or are "stratified". The term is also applied to a similar layer in the atmosphere of Titan.

Summer Solstice -- The place, on the Ecliptic, where the Sun is at its greatest northern declination. Also, the day when it is at that point. Also, the first day of summer in the northern hemisphere.

sunspot -- A region of the solar surface which is darker than the rest of the Sun, as a result of cooling by intense magnetic fields. Sunspots usually have temperatures of 4500 to 5000 Kelvins, compared to 6000 Kelvins for the normal surface of the Sun.

supercluster -- A region of space where galaxies and clusters of galaxies are far more numerous than elsewhere. In a sense, a cluster of clusters of galaxies.

supergiant -- A very large, very bright star. Typically, 100 to 1000 times the size of the Sun, and several 100's to several 100000's of times brighter than the Sun.

synchronous rotation -- Rotating at exactly the same rate as the orbital period around some other body. E.g., the Moon rotates synchronously, relative to the Earth, so that its orbital period and rotational period are exactly the same. As a result, it always keeps one side (the near side) towards the Earth, and the Earth is always in very nearly the same place in the lunar sky (the position depending upon where you are, on the Moon). However, it does still rotate relative to the Sun and stars, at a rate equal to its rotation period, relative to the stars, and equal to its synodic period of rotation, relative to the Sun.

synodic -- Relative to the Sun.

synodic month -- The time required for the Moon to move around its orbit, from new moon to new moon.

synodic period -- The time required for an object to move once around the sky, relative to the Sun.

synodic period of revolution -- The time required for an object to move around the sky, relative to the Sun. For an inferior planet, the time required to move from inferior conjunction to inferior conjunction. For a superior planet, or the Moon, the time required to move from conjunction to conjunction.

synodic period of rotation -- The time required for an object to rotate enough that the Sun moves completely around its sky. The length of the day for that object.

T

tachocline -- A region at the base of the convective zone in the Sun, where the velocity of the gases rapidly changes. (from the Greek tachos or tachy, meaning swift)

tachyon -- A particle which always moves faster than the speed of light. Such particles are hypothetically possible, but none has ever been observed, and their actual physical existence is now considered doubtful. Also see luxon and tardyon.

tangential velocity<,/big> -- The motion of a star, relative to the Sun, perpendicular to the line of sight between the star and the Sun. Over time, the tangential velocity causes a proper motion, or change in position of the star relative to the background stars.

tardyon -- A particle which always moves slower than the speed of light -- in other words, anything other than light itself. A jocular name used in contrast with tachyons, hypothetical particles which always travel faster than light.

temperature -- The heat of an object, measured by a thermometer for solids, liquids or dense gases, or by the average kinetic energy of the gas molecules, for rarefied gases. Some materials, such as degenerate gases, cannot radiate heat because of their physical state. In such cases, temperature is defined by the radiable heat, and may be as low as absolute zero, even though the energy of the particles may correspond to extremely high temperatures.

temperature gradient -- The rate at which temperature increases or decreases with altitude, in an atmosphere, or with depth, inside an object. In the lower atmosphere of a planet, the same as the lapse rate.

Terrestrial coordinate system -- The spherical coordinate system of latitude and longitude used on the Earth, based on the rotation of the Earth around its axis.

thermosphere -- The part of the Earth's atmosphere lying between the mesosphere and the exosphere. In the thermosphere, temperature rapidly rises with height, because of the absorption of high-energy solar radiation (photons and solar wind particles). Also, the similar region in the outer atmosphere of other planets.

third quarter -- See quarter Moon.

triplet -- A lens or eyepiece made of three pieces of glass (or other materials) selected so as to reduce the chromatic aberration of the lens.

tropical year -- The time required for the Earth to move around its orbit, relative to the Vernal Equinox. Differs from the sidereal year because of the precession of the Equinoxes.

tropics -- The region near the Equator, where the Sun can pass directly overhead at least once during the year.

tropopause -- The boundary between the top of the troposphere and the bottom of the stratosphere, as defined by the altitude at which temperature no longer decreases with height. The term is also applied to the top of the troposphere for other planets; but since they do not have stratospheres, their tropopauses are the boundary between the troposphere and the mesosphere.

troposphere -- The lowest portion of the Earth's atmosphere, containing 75% of its mass, and virtually all of its weather. In the troposphere, or "sphere of mixing", temperature rapidly decreases with height, making vertical mixing (convection) sufficiently common to thoroughly mix the atmosphere. The term is also applied to the lowest portion of other planets' atmospheres, if they have similar mixing, and to the lowest portion of Titan's atmosphere.

turnoff point -- On the Main Sequence, the place where stars seem to be 'turning off' toward the red giant region of the Hertzsprung-Russell Diagram. An indicator of the age of the cluster (or group) of stars which is plotted in that particular HR Diagram. Stars which would lie above the turnoff point have lifetimes shorter than the age of the cluster, and are already dead or well on their way to dying. Stars which lie below the turnoff point have lifetimes longer than the age of the cluster, and are still on the Main Sequence. Stars which are at the turnoff point are nearing the end of their lives, and changes occurring in their cores are gradually changing them from Main Sequence stars to red giants, causing the points representing their characteristics to turn away from the Main Sequence.

U

Universal Time (UT) -- Standard time at the Prime Meridian passing through Greenwich, England. Also called Coordinated Universal Time, because it is an average of the time kept by atomic clocks in various countries.

V

vernal -- Of, or pertaining to, spring.

Vernal Equinox -- The point on the Ecliptic where the Sun crosses the Celestial Equator, heading north. Also, the day when it does so. Also, the first day of spring in the northern hemisphere.

vertical circle -- A circle in the sky, running from the zenith, straight down, and perpendicular to the horizon -- and therefore, vertical with respect to the observer.

visual binary -- A binary star whose components are far enough apart that they can be seen as separate objects through direct telescopic observation.

W

waning -- Referring to the time when the Moon, moving from full Moon to new Moon, is closer and closer to the Sun each day, and lit up less and less.

waxing -- Referring to the time when the Moon, moving from new Moon to full Moon, is further and further from the Sun each day, and lit up more and more.

white dwarf -- Originally, a very small, very hot star. Now, an electron degenerate star which is hot enough to give off visible light. The last stage of life for most stars.

white hole -- A singularity in space which, opposite the situation in a black hole, emits large amounts of matter and energy. Theoretically possible in the pure mathematics of black holes, but extremely unlikely to have any relationship to actual physical reality.

Winter Solstice -- The place, on the Ecliptic, where the Sun is at its greatest southern declination. Also, the day when it is at that point. Also, the first day of winter in the northern hemisphere.

worm hole -- A region of space-time with a black hole at one end, and a white hole at the other end. In the pure mathematical theory of black holes, a way of moving from one part of space-time to another, completely different part of space-time, by cutting through a region of nearly infinite space-time curvature (the black hole). Extremely unlikely to have any relationship to actual physical reality, but a popular method of travel in space soap operas, as requiring an interstellar flight to take decades or centuries, as it would in the real world, would somewhat reduce the drama of the situation.

X

Y

Z

Zeeman effect -- A situation in which a strong magnetic field can change the energy of subatomic energy states which are normally indistinguishable (degenerate energy states) so that their energy values become different. A splitting of the absorption lines which result from absorptions beginning or ending at such states, in the presence of a strong magnetic field.

zenith -- The point in the sky which is directly overhead.

zenith distance -- The angle between the zenith, and a celestial object. Equal to 90 degrees, minus the altitude of the object.

Zodiac -- The region, centered on the Ecliptic, which encompasses the motions of the Sun, Moon and naked eye planets.

Zodiacal Light -- A luminous glow seen after sunset, or before sunrise, extending along the Ecliptic, caused by reflection of sunlight from meteoric matter in interplanetary space. It is brightest near the Sun, because of the greater brightness of sunlight in that region, and in the direction opposite the Sun (see counterglow), because we see the particles 'fully' lit.