Greek mythology is the body of myths told by the ancient Greeks. These stories concern the origin and the nature of the world, the lives and activities of deities and mythological creatures, the origins and significance of the ancient Greeks' own cult and ritual practices. Modern scholars study the myths in an attempt to shed light on the religious and political institutions of ancient Greece and its civilization, to gain understanding of the nature of myth-making itself; the Greek myths were propagated in an oral-poetic tradition most by Minoan and Mycenaean singers starting in the 18th century BC. Two poems by Homer's near contemporary Hesiod, the Theogony and the Works and Days, contain accounts of the genesis of the world, the succession of divine rulers, the succession of human ages, the origin of human woes, the origin of sacrificial practices. Myths are preserved in the Homeric Hymns, in fragments of epic poems of the Epic Cycle, in lyric poems, in the works of the tragedians and comedians of the fifth century BC, in writings of scholars and poets of the Hellenistic Age, in texts from the time of the Roman Empire by writers such as Plutarch and Pausanias.
Aside from this narrative deposit in ancient Greek literature, pictorial representations of gods and mythic episodes featured prominently in ancient vase-paintings and the decoration of votive gifts and many other artifacts. Geometric designs on pottery of the eighth century BC depict scenes from the Trojan cycle as well as the adventures of Heracles. In the succeeding Archaic and Hellenistic periods and various other mythological scenes appear, supplementing the existing literary evidence. Greek mythology has had an extensive influence on the culture and literature of Western civilization and remains part of Western heritage and language. Poets and artists from ancient times to the present have derived inspiration from Greek mythology and have discovered contemporary significance and relevance in the themes. Greek mythology is known today from Greek literature and representations on visual media dating from the Geometric period from c. 900 BC to c. 800 BC onward. In fact and archaeological sources integrate, sometimes mutually supportive and sometimes in conflict.
Mythical narration plays an important role in nearly every genre of Greek literature. The only general mythographical handbook to survive from Greek antiquity was the Library of Pseudo-Apollodorus; this work attempts to reconcile the contradictory tales of the poets and provides a grand summary of traditional Greek mythology and heroic legends. Apollodorus of Athens wrote on many of these topics, his writings may have formed the basis for the collection. Among the earliest literary sources are the Iliad and the Odyssey. Other poets completed the "epic cycle", but these and lesser poems now are lost entirely. Despite their traditional name, the "Homeric Hymns" have no direct connection with Homer, they are choral hymns from the earlier part of the so-called Lyric age. Hesiod, a possible contemporary with Homer, offers in his Theogony the fullest account of the earliest Greek myths, dealing with the creation of the world. Hesiod's Works and Days, a didactic poem about farming life includes the myths of Prometheus and the Five Ages.
The poet gives advice on the best way to succeed in a dangerous world, rendered yet more dangerous by its gods. Lyrical poets took their subjects from myth, but their treatment became less narrative and more allusive. Greek lyric poets, including Pindar and Simonides, bucolic poets such as Theocritus and Bion, relate individual mythological incidents. Additionally, myth was central to classical Athenian drama; the tragic playwrights Aeschylus and Euripides took most of their plots from myths of the age of heroes and the Trojan War. Many of the great tragic stories took on their classic form in these tragedies; the comic playwright Aristophanes used myths, in The Birds and The Frogs. Historians Herodotus and Diodorus Siculus, geographers Pausanias and Strabo, who traveled throughout the Greek world and noted the stories they heard, supplied numerous local myths and legends giving little-known alternative versions. Herodotus in particular, searched the various traditions presented him and found the historical or mythological roots in the confrontation between Greece and the East.
Herodotus attempted to reconcile the blending of differing cultural concepts. The poetry of the Hellenistic and Roman ages was composed as a literary rather than cultic exercise, it contains many important details that would otherwise be lost. This category includes the works of: The Roman poets Ovid, Valerius Flaccus and Virgil with Servius's commentary; the Greek poets of the Late Antique period: Nonnus, Antoninus Liberalis, Quintus Smyrnaeus. The Greek poets of the Hellenistic period: Apollonius of Rhodes, Pseudo-Eratosthenes, Parthenius. Prose writers from the same periods who make reference to myths includ
S-type asteroids are asteroids with a spectral type, indicative of a siliceous mineralogical composition, hence the name. 17% of asteroids are of this type, making it the second most common after the carbonaceous C-type. S-types asteroids, with an astronomical albedo of 0.20, are moderately bright and consist of iron- and magnesium-silicates. They are dominant in the inner part of the asteroid belt within 2.2 AU, common in the central belt within about 3 AU, but become rare farther out. The largest is 15 Eunomia, with the next largest members by diameter being 3 Juno, 29 Amphitrite, 532 Herculina and 7 Iris; these largest S-types are visible in 10x50 binoculars at most oppositions. Their spectrum has a moderately steep slope at wavelengths shorter than 0.7 micrometres, has moderate to weak absorption features around 1 µm and 2 µm. The 1 µm absorption is indicative of the presence of silicates. There is a broad but shallow absorption feature centered near 0.63 µm. The composition of these asteroids is similar to a variety of stony meteorites which share similar spectral characteristics.
In the SMASS classification, several "stony" types of asteroids are brought together into a wider S-group which contains the following types: A-type K-type L-type Q-type R-type a "core" S-type for asteroids having the most typical spectra for the S-group Sa, Sk, Sl, Sq, Sr-types containing transition objects between the core S-type and the A, K, L, Q, R-types, respectively. The entire "S"-assemblage of asteroids is spectrally quite distinct from the carbonaceous C-group and the metallic X-group. In the Tholen classification, the S-type is a broad grouping which includes all the types in the SMASS S-group except for the A, Q, R, which have strong "stony" absorption features around 1 μm. Prominent stony asteroid families with their typical albedo are the: Eos family Eunomia family Flora family Koronis family Nysa family Phocaea family Asteroid spectral types X-type asteroid Bus, S. J.. "Phase II of the Small Main-belt Asteroid Spectroscopy Survey: A feature-based taxonomy". Icarus. 158: 146–177.
In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is in a particular frequency interval or band. Light curves can be periodic, as in the case of eclipsing binaries, Cepheid variables, other periodic variables, transiting extrasolar planets, or aperiodic, like the light curve of a nova, a cataclysmic variable star, a supernova or a microlensing event or binary as observed during occultation events; the study of the light curve, together with other observations, can yield considerable information about the physical process that produces it or constrain the physical theories about it. Graphs of the apparent magnitude of a variable star over time are used to visualise and analyse their behaviour. Although the categorisation of variable star types is done from their spectral properties, the amplitudes and regularity of their brightness changes are still important factors; some types such as Cepheids have regular light curves with the same period and shape in each cycle.
Others such as Mira variables have somewhat less regular light curves with large amplitudes of several magnitudes, while the semiregular variables are less regular still and have smaller amplitudes. The shapes of variable star light curves give valuable information about the underlying physical processes producing the brightness changes. For eclipsing variables, the shape of the light curve indicates the degree of totality, the relative sizes of the stars, their relative surface brightnesses, it may show the eccentricity of the orbit and distortions in the shape of the two stars. For pulsating stars, the amplitude or period of the pulsations can be related to the luminosity of the star, the light curve shape can be an indicator of the pulsation mode. Light curves from supernovae can be indicative of the type of supernova. Although supernova types are defined on the basis of their spectra, each has typical light curve shapes. Type I supernovae have light curves with a sharp maximum and decline, while Type II supernovae have less sharp maxima.
Light curves are helpful for classification of faint supernovae and for the determination of sub-types. For example, the type II-P have similar spectra to the type II-L but are distinguished by a light curve where the decline flattens out for several weeks or months before resuming its fade. In planetary science, a light curve can be used to derive the rotation period of a minor planet, moon, or comet nucleus. From the Earth there is no way to resolve a small object in the Solar System in the most powerful of telescopes, since the apparent angular size of the object is smaller than one pixel in the detector. Thus, astronomers measure the amount of light produced by an object as a function of time; the time separation of peaks in the light curve gives an estimate of the rotational period of the object. The difference between the maximum and minimum brightnesses can be due to the shape of the object, or to bright and dark areas on its surface. For example, an asymmetrical asteroid's light curve has more pronounced peaks, while a more spherical object's light curve will be flatter.
This allows astronomers to infer information about the spin of asteroids. The Asteroid Lightcurve Database of the Collaborative Asteroid Lightcurve Link uses a numeric code to assess the quality of a period solution for minor planet light curves, its quality code parameter "U" ranges from 0 to 3: U = 0 → Result proven incorrect U = 1 → Result based on fragmentary light curve, may be wrong. U = 2 → Result based on less than full coverage. Period may be wrong by ambiguous. U = 3 → Secure result within the precision given. No ambiguity. U = n.a. → Not available. Incomplete or inconclusive result. A trailing plus sign or minus sign is used to indicate a better or worse quality than the unsigned value; the occultation light curve is characterised as binary, where the light from the star is terminated instantaneously, remains constant for the duration, is reinstated instantaneously. The duration is equivalent to the length of a chord across the occulting body. Circumstances where the transitions are not instantaneous are.
When the occulted body is large, e.g. a star like Antares the transitions are gradual. When the occulting body has an atmosphere, e.g. the moon TitanThe observations are recorded using video equipment and the disappearance and reappearance timed using a GPS disciplined Video Time Inserter. Occultation light curves are archived at the VizieR service. Light curve inversion is a mathematical technique used to model the surfaces of rotating objects from their brightness variations; this can be used to image starspots or asteroid surface albedos. Microlensing is a process where small and low-mass astronomical objects cause a brief small increase in the brightness of a more distant object; this is caused by the small relativistic effect as larger gravitational lenses, but allows the detection and analysis of otherwise-invisible stellar and planetary mass objects. The properties of these objects can be inferred from the shape of the lensing light curve. For example, PA-99-N2 is a microlensing event that may have been due to a star in the Andromeda galaxy that has an exoplanet.
The AAVSO online light curve generator can plot light curves for thousands of variable stars The Open Astronomy
The term apsis refers to an extreme point in the orbit of an object. It denotes either the respective distance of the bodies; the word comes via Latin from Greek, there denoting a whole orbit, is cognate with apse. Except for the theoretical possibility of one common circular orbit for two bodies of equal mass at diametral positions, there are two apsides for any elliptic orbit, named with the prefixes peri- and ap-/apo-, added in reference to the body being orbited. All periodic orbits are, according to Newton's Laws of motion, ellipses: either the two individual ellipses of both bodies, with the center of mass of this two-body system at the one common focus of the ellipses, or the orbital ellipses, with one body taken as fixed at one focus, the other body orbiting this focus. All these ellipses share a straight line, the line of apsides, that contains their major axes, the foci, the vertices, thus the periapsis and the apoapsis; the major axis of the orbital ellipse is the distance of the apsides, when taken as points on the orbit, or their sum, when taken as distances.
The major axes of the individual ellipses around the barycenter the contributions to the major axis of the orbital ellipses are inverse proportional to the masses of the bodies, i.e. a bigger mass implies a smaller axis/contribution. Only when one mass is sufficiently larger than the other, the individual ellipse of the smaller body around the barycenter comprises the individual ellipse of the larger body as shown in the second figure. For remarkable asymmetry, the barycenter of the two bodies may lie well within the bigger body, e.g. the Earth–Moon barycenter is about 75% of the way from Earth's center to its surface. If the smaller mass is negligible compared to the larger the orbital parameters are independent of the smaller mass. For general orbits, the terms periapsis and apoapsis are used. Pericenter and apocenter are equivalent alternatives, referring explicitly to the respective points on the orbits, whereas periapsis and apoapsis may refer to the smallest and largest distances of the orbiter and its host.
For a body orbiting the Sun, the point of least distance is the perihelion, the point of greatest distance is the aphelion. The terms become apastron when discussing orbits around other stars. For any satellite of Earth, including the Moon, the point of least distance is the perigee and greatest distance the apogee, from Ancient Greek Γῆ, "land" or "earth". For objects in lunar orbit, the point of least distance is sometimes called the pericynthion and the greatest distance the apocynthion. Perilune and apolune are used. In orbital mechanics, the apsides technically refer to the distance measured between the barycenters of the central body and orbiting body. However, in the case of a spacecraft, the terms are used to refer to the orbital altitude of the spacecraft above the surface of the central body; these formulae characterize the pericenter and apocenter of an orbit: Pericenter Maximum speed, v per = μ a, at minimum distance, r per = a. Apocenter Minimum speed, v ap = μ a, at maximum distance, r ap = a.
While, in accordance with Kepler's laws of planetary motion and the conservation of energy, these two quantities are constant for a given orbit: Specific relative angular momentum h = μ a Specific orbital energy ε = − μ 2 a where: a is the semi-major axis: a = r per + r ap 2 μ is the standard gravitational parameter e is the eccentricity, defined as e = r ap − r per r ap + r per = 1 − 2 r ap r per + 1 Note t
The Hilda asteroids are a dynamical group of more than 4000 asteroids located beyond the asteroid belt in a 3:2 orbital resonance with Jupiter. The namesake is the asteroid 153 Hilda. Hildas move in their elliptical orbits so that their aphelia put them opposite Jupiter, or 60° ahead of or behind Jupiter at the L4 and L5 Lagrangian points. Over three successive orbits each Hilda asteroid approaches all of these three points in sequence. A Hilda's orbit has a semi-major axis between 3.7 and 4.2 AU, an eccentricity less than 0.3, an inclination less than 20°. Two collisional families exist within the Hilda group: the Schubart family; the namesake for the latter family is 1911 Schubart. Hildas' surface colors correspond to the low-albedo D-type and P-type. D-type and P-type asteroids have surface colors, thus surface mineralogies, similar to those of cometary nuclei; this implies. The asteroids of the Hilda group are in 3:2 mean-motion resonance with Jupiter; that is, their orbital periods are 2/3 that of Jupiter.
They move along the orbits with a semimajor axis near 4.0 AU and moderate values of eccentricity and inclination. Unlike the Jupiter trojans they may have any difference in longitude with Jupiter avoiding dangerous approaches to the planet; the Hildas taken together constitute a dynamic triangular figure with convex sides and trimmed apices in the triangular libration points of Jupiter—the "Hildas Triangle". The "asteroidal stream" within the sides of the triangle is about 1 AU wide, in the apexes this value is 20-40% greater. Figure 1 shows the positions of the Hildas against a background of all known asteroids up to Jupiter's orbit at January 1, 2005; each of the Hilda objects moves along its own elliptic orbit. However, at any moment the Hildas together constitute this triangular configuration, all the orbits together form a predictable ring. Figure 2 illustrates this with the Hildas positions against a background of their orbits. For the majority of these asteroids, their position in orbit may be arbitrary, except for the external parts of the apexes and the middles of the sides.
The Hildas Triangle has proven to be dynamically stable over a long time span. The typical Hilda object has a retrograde perihelion motion. On average, the velocity of perihelion motion is greater when the orbital eccentricity is lesser, while the nodes move more slowly. All typical objects in aphelion would approach to Jupiter, which should be destabilising for them—but the variation of the orbital elements over time prevents this, conjunctions with Jupiter occur only near the perihelion of Hilda asteroids. Moreover, the apsidal line oscillates near the line of conjunction with different amplitude and a period of 2.5 to 3.0 centuries. In addition to the fact that the Hildas triangle revolves in sync with Jupiter, the density of asteroids in the stream exhibits quasi-periodical waves. At any time, the density of objects in the triangle's apexes is more than twice the density within the sides; the Hildas "rest" at their aphelia in the apexes for an average of 5.0-5.5 years, whereas they move along the sides more averaging 2.5 to 3.0 years.
The orbital periods of these asteroids are 7.9 years, or two thirds that of Jupiter. Although the triangle is nearly equilateral, some asymmetry exists. Due to the eccentricity of Jupiter's orbit, the side L4-L5 differs from the two other sides; when Jupiter is in aphelion, the mean velocity of the objects moving along this side is somewhat smaller than that of the objects moving along the other two sides. When Jupiter is in perihelion, the reverse is true. At the apexes of the triangle corresponding to the points L4 and L5 of Jupiter's orbit, the Hildas approach the Trojans. At the mid-sides of the triangle, they are close to the asteroids of the external part of the asteroid belt; the velocity dispersion of Hildas is more evident than that of Trojans in the regions where they intersect. It should be noted that the dispersion of Trojans in inclination is twice that of the Hildas. Due to this, as much as one quarter of the Trojans cannot intersect with the Hildas, at all times many Trojans are located outside Jupiter's orbit.
Therefore, the regions of intersection are limited. This is illustrated by the adjacent figure that shows the Hildas and the Trojans along the ecliptic plane. One can see the spherical form of the Trojan swarms; when moving along each side of the triangle, the Hildas travel more than the Trojans, but encounter a denser neighborhood of outer-asteroid-belt asteroids. Here, the velocity dispersion is much smaller; the observed peculiarities in the Hildas' motion are based on data for a few hundred objects known to date and generate still more questions. Further observations are needed to expand on the list of Hildas; such observations are most favorable when Earth is near conjunction with the mid-sides of the Hildas Triangle. These moments occur each 1/3 months. In these circumstances the brilliance of objects of similar size could run up to 2.5 magnitudes as compared to the apexes. The Hildas traverse regions of the Solar system from 2 AU up to Jupiter's orbit; this entails the neighborhood of various groups of asteroids.
On further observation some theories on the Hildas may have to be revised
The kilometre, or kilometer is a unit of length in the metric system, equal to one thousand metres. It is now the measurement unit used for expressing distances between geographical places on land in most of the world. K is used in some English-speaking countries as an alternative for the word kilometre in colloquial writing and speech. A slang term for the kilometre in the US and UK military is klick. There are two common pronunciations for the word; the former follows a pattern in English whereby metric units are pronounced with the stress on the first syllable and the pronunciation of the actual base unit does not change irrespective of the prefix. It is preferred by the British Broadcasting Corporation and the Australian Broadcasting Corporation. Many scientists and other users in countries where the metric system is not used, use the pronunciation with stress on the second syllable; the latter pronunciation follows the stress pattern used for the names of measuring instruments. The problem with this reasoning, however, is that the word meter in those usages refers to a measuring device, not a unit of length.
The contrast is more obvious in countries using the British rather than American spelling of the word metre. When Australia introduced the metric system in 1975, the first pronunciation was declared official by the government's Metric Conversion Board. However, the Australian prime minister at the time, Gough Whitlam, insisted that the second pronunciation was the correct one because of the Greek origins of the two parts of the word. By the 8 May 1790 decree, the Constituent assembly ordered the French Academy of Sciences to develop a new measurement system. In August 1793, the French National Convention decreed the metre as the sole length measurement system in the French Republic; the first name of the kilometre was "Millaire". Although the metre was formally defined in 1799, the myriametre was preferred to the "kilometre" for everyday use; the term "myriamètre" appeared a number of times in the text of Develey's book Physique d'Emile: ou, Principes de la science de la nature, while the term kilometre only appeared in an appendix.
French maps published in 1835 had scales showing myriametres and "lieues de Poste". The Dutch gave it the local name of the mijl, it was only in 1867 that the term "kilometer" became the only official unit of measure in the Netherlands to represent 1000 metres. Two German textbooks dated 1842 and 1848 give a snapshot of the use of the kilometre across Europe - the kilometre was in use in the Netherlands and in Italy and the myriametre was in use in France. In 1935, the International Committee for Weights and Measures abolished the prefix "myria-" and with it the "myriametre", leaving the kilometre as the recognised unit of length for measurements of that magnitude. In the United Kingdom, road signs show distances in miles and location marker posts that are used for reference purposes by road engineers and emergency services show distance references in unspecified units which are kilometre-based; the advent of the mobile phone has been instrumental in the British Department for Transport authorising the use of driver location signs to convey the distance reference information of location marker posts to road users should they need to contact the emergency services.
In the US, the National Highway System Designation Act of 1995 prohibits the use of federal-aid highway funds to convert existing signs or purchase new signs with metric units. The Executive Director of the US Federal Highway Administration, Jeffrey Paniati, wrote in a 2008 memo: "Section 205 of the National Highway System Designation Act of 1995 prohibited us from requiring any State DOT to use the metric system during project development activities. Although the State DOT's had the option of using metric measurements or dual units, all of them abandoned metric measurements and reverted to sole use of inch-pound values." The Manual on Uniform Traffic Control Devices since 2000 is published in both metric and American Customary Units. Some sporting disciplines feature 1000 m races in major events, but in other disciplines though world records are catalogued, the one kilometre event remains a minority event; the world records for various sporting disciplines are: Conversion of units, for comparison with other units of length Cubic metre Metric prefix Mileage Odometer Orders of magnitude Square kilometre Media related to Distance indicators at Wikimedia Commons
Orders of magnitude (length)
The following are examples of orders of magnitude for different lengths. To help compare different orders of magnitude, the following list describes various lengths between 1.6 × 10 − 35 metres and 10 10 10 122 metres. To help compare different orders of magnitude, this section lists lengths shorter than 10−23 m. 1.6 × 10−11 yoctometres – the Planck length. 1 ym – 1 yoctometre, the smallest named subdivision of the metre in the SI base unit of length, one septillionth of a metre 1 ym – length of a neutrino. 2 ym – the effective cross-section radius of 1 MeV neutrinos as measured by Clyde Cowan and Frederick Reines To help compare different orders of magnitude, this section lists lengths between 10−23 metres and 10−22 metres. To help compare different orders of magnitude, this section lists lengths between 10−22 m and 10−21 m. 100 ym – length of a top quark, one of the smallest known quarks To help compare different orders of magnitude, this section lists lengths between 10−21 m and 10−20 m. 2 zm – length of a preon, hypothetical particles proposed as subcomponents of quarks and leptons.
2 zm – radius of effective cross section for a 20 GeV neutrino scattering off a nucleon 7 zm – radius of effective cross section for a 250 GeV neutrino scattering off a nucleon To help compare different orders of magnitude, this section lists lengths between 10−20 m and 10−19 m. 15 zm – length of a high energy neutrino 30 zm – length of a bottom quark To help compare different orders of magnitude, this section lists lengths between 10−19 m and 10−18 m. 177 zm – de Broglie wavelength of protons at the Large Hadron Collider To help compare different orders of magnitude, this section lists lengths between 10−18 m and 10−17 m. 1 am – sensitivity of the LIGO detector for gravitational waves 1 am – upper limit for the size of quarks and electrons 1 am – upper bound of the typical size range for "fundamental strings" 1 am – length of an electron 1 am – length of an up quark 1 am – length of a down quark To help compare different orders of magnitude, this section lists lengths between 10−17 m and 10−16 m. 10 am – range of the weak force To help compare different orders of magnitude, this section lists lengths between 10−16 m and 10−15 m. 100 am – all lengths shorter than this distance are not confirmed in terms of size 850 am – approximate proton radius The femtometre is a unit of length in the metric system, equal to 10−15 metres.
In particle physics, this unit is more called a fermi with abbreviation "fm". To help compare different orders of magnitude, this section lists lengths between 10−15 metres and 10−14 metres. 1 fm – length of a neutron 1.5 fm – diameter of the scattering cross section of an 11 MeV proton with a target proton 1.75 fm – the effective charge diameter of a proton 2.81794 fm – classical electron radius 7 fm – the radius of the effective scattering cross section for a gold nucleus scattering a 6 MeV alpha particle over 140 degrees To help compare different orders of magnitude, this section lists lengths between 10−14 m and 10−13 m. 1.75 to 15 fm – Diameter range of the atomic nucleus To help compare different orders of magnitude, this section lists lengths between 10−13 m and 10−12 m. 570 fm – typical distance from the atomic nucleus of the two innermost electrons in the uranium atom, the heaviest naturally-occurring atom To help compare different orders of magnitude this section lists lengths between 10−12 and 10−11 m. 1 pm – distance between atomic nuclei in a white dwarf 2.4 pm – The Compton wavelength of the electron 5 pm – shorter X-ray wavelengths To help compare different orders of magnitude this section lists lengths between 10−11 and 10−10 m. 25 pm – approximate radius of a helium atom, the smallest neutral atom 50 pm – radius of a hydrogen atom 50 pm – bohr radius: approximate radius of a hydrogen atom ~50 pm – best resolution of a high-resolution transmission electron microscope 60 pm – radius of a carbon atom 93 pm – length of a diatomic carbon molecule To help compare different orders of magnitude this section lists lengths between 10−10 and 10−9 m. 100 pm – 1 ångström 100 pm – covalent radius of sulfur atom 120 pm – van der Waals radius of a neutral hydrogen atom 120 pm – radius of a gold atom 126 pm – covalent radius of ruthenium atom 135 pm – covalent radius of technetium atom 150 pm – Length of a typical covalent bond 153 pm – covalent radius of silver atom 155 pm – covalent radius of zirconium atom 175 pm – covalent radius of thulium atom 200 pm – highest resolution of a typical electron microscope 225 pm – covalent radius of caesium atom 280 pm – Average size of the water molecule 298 pm – radius of a caesium atom, calculated to be the largest atomic radius 340 pm – thickness of single layer graphene 356.68 pm – width of diamond unit cell 403 pm – width of lithium fluoride unit cell 500 pm – Width of protein α helix 543 pm – silicon lattice spacing 560 pm – width of sodium chloride unit cell 700 pm – width of glucose molecule 780 pm – mean width of quartz unit cell 820 pm – mean width of ice unit cell 900 pm – mean width of coesite unit cell To help compare different orders