## Solar eclipse types

The word “eclipse” is a general astro­nom­ical term that applies to any situ­ation in which the view of one object is blocked by another, caused either by the second object’s shadow falling over the first, or by the second object coming in between the first object and the observer.

In general use the term eclipse usually applies to one of two situ­ations: a solar eclipse, in which the Moon obscures our view of the Sun; or a lunar eclipse in which the Earth’s shadow falls over the Moon. The planes in which the Earth orbits the Sun and which the Moon orbits the Earth are at an angle to each other, which is why there is not a total solar eclipse once every month.

There are three types of solar eclipse:

• A total eclipse, in which the Moon appears the same size as the Sun and blocks light from it completely.
• An annular eclipse, in which the Moon passes between the Sun and the Earth but because of the rel­ative dis­tances of each it appears slightly smaller than the Sun, causing a ring of light to appear.
• A partial eclipse, in which the orbits of the Moon and Sun are such that the Moon only covers only part of the Sun.

A com­par­ison of a total solar eclipse (left) and an annular eclipse (right). Only in the case of a total solar eclipse is the Sun’s corona (the white “cloud”) visible.

A diagram showing how the three dif­ferent types of eclipse are formed.

There are also hybrid eclipses, which appear as total eclipses to some parts of the Earth and an annular eclipse to other parts. These are very rare, with the most recent in April 2005 and visible mainly from the Pacific Ocean and also Costa Rica, Panama, Colombia and Venezuela; and the next to occur in November 2013, visible from Central Africa (Gabon, Congo, Democratic Republic of the Congo, Uganda, Kenya, Ethiopia and Somalia).

## Reaction wheels and pointing satellites

The Kepler tele­scope, a satel­lite tele­scope in orbit around the Sun* designed to look for exo­planets, has come to the end of its ori­ginal mission. The end of the mission was caused by the failure of ‘s reac­tion wheels, which are used to point the telescope.

Reaction wheels are elec­tric motors con­nected to metals discs, usually with masses between . As the speed of the motors are altered the con­ser­va­tion of angular momentum imparts a force on the space­craft, causing it to rotate around its centre of mass.

The aboard the Lunar Reconnaissance Orbiter.

Reaction wheels are usually employed in groups of three, the x-, y– and z-axes, enabling the tele­scope to be pointed accur­ately in . Kepler was fitted with , with all , and each acting as a spare for the other three. After wheels (#2) failed in July 2012 the space­craft was still able to operate nor­mally, but reac­tion wheel #4 began mal­func­tioning in January 2013, and whilst the wheel returned to working order ini­tially, it failed com­pletely on, leaving the space­craft now unable to move and point properly.

wheels aboard Kepler.

Reaction wheels are ofwheels, but the . Momentum wheels are much heavier, and spin at much higher rates, and their role is not to point or steer the space­craft but rather to use the gyro­scopic effect to keep it in a fixed pos­i­tion when sub­jected to per­turbing forces such as solar wind or radi­ation pres­sure.

Satellites in orbit close around Earth can also use a device called a mag­net­orquer to control their pos­i­tion. By altering the flow of current through a set of coils (again usually three, with the space­craft pushes against Earth’s mag­netic field and the reac­tion force against this push causes the satel­lite to rotate.

* Technically an Earth-trailing helio­centric orbit.

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## Squat effect

The MS Oasis of the Seas is the world’s largest cruise ship (holding the title jointly with its sister ship, MS Allure of the Seas) and stands seventy-two metres above the water line. During the delivery process, as it was sailing from the STX Shipyard in Finland where it was built to its first stop, it had to pass under­neath the Great Belt Bridge that con­nects the Danish islands of  Zealand and Sprogø, a bridge that stands only sixty-five metres above the water­line. Oasis of the Seas is equipped with tele­scoping funnels and ballast tanks, but this would only create a tiny margin of safety between the ship and the bridge.

Luckily, the ship was able to rely on some­thing called the squat effect to help it make it under the bridge. The squat effect occurs when a ship moves through shallow water at speed. The water is forced under­neath the ship, increasing its speed, and this causes the water pres­sure to drop (by Bernoulli’s Principle), thereby pulling the ship deeper into the water.

In deep water no squat effect is present.

In shallow water the squat effect causes the ship to sink lower into the water.

Oasis of the Seas approached the bridge at twenty knots (10.3 m/s; 23 mph), close to its twenty-two knot top speed and this was enough to pull it an addi­tional thirty cen­ti­metres into the water, allowing it to pass safely under the bridge.

## FOGBANK

The material is clas­si­fied. Its com­pos­i­tion is clas­si­fied. Its use in the weapon is clas­si­fied, and the process itself is classified.”

FOGBANK is the code­name of a material used in modern nuclear war­heads such as the W76 used in Trident submarine-launched bal­listic mis­siles, the W78 used in silo-launched Minuteman III inter­con­tin­ental bal­listic mis­siles and the W80 used in air-launched cruise mis­siles and the Tomahawk cruise missile.

Exactly what FOGBANK is, and what it does, is unknown. It has been sug­gested that it is an aerogel–like sub­stance that trans­fers energy from the fission stage of a ther­mo­nuc­lear (fission-fusion) device to the fusion stage; pre­venting the fission stage from des­troying the fusion stage before it has time to react.

In 1996, during the refur­bish­ment process for the afore­men­tioned war­heads, it was dis­covered that detailed records of the man­u­fac­turing process for FOGBANK did not exist, and the facility used to man­u­fac­ture it had been moth­balled.  Uncertain whether alternate mater­ials would suffice, the National Nuclear Security Administration spent twenty-three million dollars on research and new facil­ities to recreate FOGBANK.

Unfortunately, the Mark II FOGBANK did not work cor­rectly. Eventually it was dis­covered that this was due to the pres­ence of an impurity, acci­dent­ally incor­por­ated into ori­ginal batches of FOGBANK, that was not included in the second man­u­fac­turing process. This impurity was included in the new formula as an additive and the refur­bish­ment process was successful.

## The Torino Scale

The Torino Scale is a system for cat­egor­ising the risk presented by near earth objects (NEOs) such as aster­oids and comets. On the Torino Scale NEOs are rated on a scale from zero to ten, based on a com­bin­a­tion of the prob­ab­ility of an object striking earth and the kinetic energy of that object.

Because orbits are unstable and can change the scale only applies to poten­tial objects less than one hundred years in the future. The diagram below shows the dif­ferent Torino Scale cat­egories, with a log­ar­ithmic scale on both axes and an approx­imate indic­a­tion of the dia­meter of the asteroid on the kinetic energy axis.

The Torino Scale is sep­ar­ated into five categories:

• White (Torino Scale 0) — No hazard; “the like­li­hood of a col­li­sion is zero, or is so low as to be effect­ively zero”.
• Green (Torino Scale 1) — Normal; “a routine dis­covery in which a pass near the Earth is pre­dicted that poses no unusual level of danger … new tele­scopic obser­va­tions very likely will lead to re-assignment to Level 0″.
• Yellow (Torino Scale 2 – 4) — Meriting atten­tion by astro­nomers; “current cal­cu­la­tions give a 1% or greater chance of col­li­sion capable of loc­al­ised [Level 3] or regional [Level 4] destruction”.
• Orange (Torino Scale 5 – 7) — Threatening; at its most extreme “a very close encounter by a large object, which if occur­ring this century, poses an unpre­ced­ented but still uncer­tain threat of a global catastrophe”.
• Red (Torino Scale 8 – 10) — Certain col­li­sion; “a col­li­sion is certain, capable of causing loc­al­ized destruc­tion [Level 8] … unpre­ced­ented regional dev­ast­a­tion for a land impact or the threat of a major tsunami for an ocean impact [Level 9] … or capable of causing global cli­matic cata­strophe that may threaten the future of civil­isa­tion as we know it, whether impacting land or ocean [Level 10]”.

As new data about an NEO becomes avail­able the Torino Scale rating for an object can jump sud­denly: the Chelyabinsk meteor had a kinetic energy of 0.4 mega­tons TNT equi­valent, giving it a Torino Scale rating of zero, but had it been only a little bit more massive or slightly faster (one megaton) it would have sud­denly jumped to an eight.

Currently NASA’s Jet Propulsion Lab’s Sentry system lists only one NEO with a non-zero Torino Scale rating. 2007 VK 184 has a Torino Scale rating of one, but the earliest pos­sible col­li­sion date is in June 2048, so we don’t have to start wor­rying just yet.

## The Trestle

The Trestle (or more form­ally the Air Force Weapons Lab Transmission-Line Aircraft Simulator) is a unique struc­ture built by the US gov­ern­ment in the Albuquerque desert and which was used to test aircraft’s resi­li­ence against the elec­tro­mag­netic pulses created by nuclear weapons.

The Trestle is three hundred metres long and nearly two hundred metres tall and made entirely from wood and glue. The pres­ence of any metal would distort read­ings from EMP testing and there­fore The Trestle does not even use metal nails or braces. It was built from more than fifteen thou­sand cubic metres of Douglas Fir and Southern Yellow Pine and was strong enough to support the weight of a fully loaded two hundred tonne B-52 Stratofortress stra­tegic bomber.

Source

The Trestle was equipped with a two hundred gigawatt, ten mega­volt Marx gen­er­ator and was used to test bomber, fighter and trans­port air­craft and even long-range mis­siles. The Trestle pro­gramme was shut down in 1991 when com­puter sim­u­la­tions became good enough to sim­u­late the effects of EMPs and the dried-out, creosote-soaked wood now poses a serious fire hazard.

Efforts are being made to have the Trestle site declared a National Historic Landmark, but these efforts are being hampered by the fact that The Trestle is located on Kirtland Air Force Base. Kirtland houses a number of Top Secret units such as the US Air Force Nuclear Weapons Centre, the 498th Nuclear Systems Wing and the Air Force Research Laboratory and there­fore access to the site is highly restricted.

## Leap smear

For reasons I have dis­cussed before it is occa­sion­ally neces­sary to add* a leap second to the time, in order to keep the time on Earth in line with Earth’s incon­sistent rotation.

Many systems require an accurate time to func­tion cor­rectly and the addi­tion of a leap second can cause these systems to mal­func­tion. In June 2012 the addi­tion of a leap second caused a number of major web­sites such as Reddit, FourSquare, Yelp, LinkedIn, Gawker and StumbleUpon to mal­func­tion and crash, but Google came up with a unique work­around — the Leap Smear — that pre­vented this from happening.

Google, like many others, uses the Network Time Protocol (NTP), to syn­chronise time across a network.† In order to cope with the leap second problem they con­figured their NTP servers to gradu­ally add a small frac­tion of a second over a long period of time (in this case one day) so that at the end of this period their NTP servers’ time would have caught up with the adjusted time.

$t \left(\textnormal{Google}\right) = t + gain \left( 1 -cos \left( \pi \left( \frac{t}{window} \right) \right) \right)$

Where t(Google) is the time according to Google’s NTP servers; t is the actual UTC time; gain is the desired amount of gain time (in this case one second); and window is the time over which this gain should happen (in this case twenty-four hours).

The effect of using the cosine func­tion is such that the time offset is small at first (in the first hour only four mil­li­seconds are added) and gradu­ally increases (to sixty-five mil­li­seconds per hour at most) before decreasing again towards the end of the window.

This pre­vented servers and devices con­nected to Google’s NTP servers from “noti­cing” that some­thing was wrong and applying their own corrections.

As they say in their blog post,

The leap smear is talked about intern­ally in the Site Reliability Engineering group as one of our coolest work­arounds, that took a lot of exper­i­ment­a­tion and veri­fic­a­tion, but paid off by ulti­mately saving us massive amounts of time and energy in inspecting and refact­oring code. It meant that we didn’t have to sweep our entire (large) code­base, and Google engin­eers devel­oping code don’t have to worry about leap seconds.

I wouldn’t be at all sur­prised to see others employing Google’s Leap Smear tech­nique in the future.

* There are also pro­vi­sions to sub­tract a leap second, but this has never yet happened.

† The NTP does contain a “leap indic­ator” but Google decided to force their NTP servers not to apply this.

## The Roche limit and planetary rings

As a satel­lite orbits around an object (a primary), the grav­it­a­tional force on the side closest to the object is greater than that on the side opposite the object. This dif­fer­ence in grav­it­a­tional attrac­tion gives rise to a tidal force (so-called because it is what causes the tides on Earth). As a satel­lite approaches closer to the body it orbits this tidal force will even­tu­ally become greater than the grav­it­a­tional forces holding the satel­lite together. The point at which this occurs is known as the Roche limit (named for Édouard Roche who first cal­cu­lated it).

The Roche limit d depends on the radius of the primary RM, the density of the primary ρM and the density of the satel­lite ρm.

$d = 2.44\; R_M \sqrt[3]{\frac {\rho_M} {\rho_m}}$

If we take our Earth-Moon system as an example, with the radius of Earth being 6370 kilo­metres, the Earth’s density as 5520 kilo­grams per cubic metre and the Moon’s density as 3350 kg/m3 this gives us a Roche limit of 18 400 km. The Moon’s actual orbit is 385 000 km, so luckily we don’t have to worry about the Moon breaking up any time soon.

The Earth-Moon system, to scale. The area beyond the Roche limit is shaded red.

When an orbiting object passes through the Roche limit it begins to break up, with the material closest to the object moving faster than the material behind it. This even­tu­ally leads to the form­a­tion of rings.

## The composition of nuclear electromagnetic pulses

When a nuclear weapon is det­on­ated at high alti­tude the effects are very dif­ferent to those created by a low-altitude det­on­a­tion. Aside from cre­ating a much larger, much faster-expanding fire­ball, the nuclear det­on­a­tion also creates an elec­tro­mag­netic pulse (EMP) that can damage elec­tro­mag­netic equip­ment on the ground.

A nuclear EMP differs from other EMPs such as those gen­er­ated by light­ning strikes or by con­ven­tional EMP weapons (such as flux com­pres­sion gen­er­ators) in that it is much more powerful* and is com­posed of three dif­ferent pulses called E1, E2 and E3.

## E1

The E1 pulse is the most destructive, occur­ring far too quickly for pro­tective equip­ment to activate. It is the com­ponent that des­troys com­puters and com­mu­nic­a­tions lines by causing the insu­lating com­pon­ents of these devices to become con­ducting and allowing elec­trical current to flow between regions that are not sup­posed to be con­nected, effect­ively short-circuiting all cir­cuits simultaneously.

The E1 pulse is created when the intense gamma radi­ation from the nuclear det­on­a­tion ionises atoms in the upper atmo­sphere (Compton scat­tering); releasing elec­trons which travel down­ward at relativ­istic speeds, about 95% of the speed of light. Any charged particle in a mag­netic field will exper­i­ence a force (the motor effect) and the Earth’s mag­netic field causes the elec­trons lib­er­ated by the gamma radi­ation to follow a spiral path around the mag­netic field lines. As the elec­tron oscil­lates back and forth it creates an elec­tro­mag­netic field and as there are about 1025 elec­trons doing this sim­ul­tan­eously, this creates a very powerful (about 50000 volts per metre, 6.6 mega­watts per square metre) but very short-lived elec­tro­mag­netic pulse. The E1 pulse typ­ic­ally reaches its peak value in about five bil­lionths of a second (five nano­seconds) and ends after about one mil­lionth of a second (one micro­second) as the scattered elec­trons are stopped by col­li­sions with air molecules.

The shape of the region affected by the E1 pulse depends on lat­itude, due to the chan­ging ori­ent­a­tion of the Earth’s mag­netic field. Away from the equator the region is U-shaped, and towards the equator it is more symmetrical.

## E2

The E2 com­ponent is caused, like E1, by Compton scat­tering when scattered gamma rays, gamma rays pro­duced by inter­ac­tion of fission neut­rons with atoms in the air and gamma rays pro­duced by radio­active decay of fission frag­ments ionise air particles.

The E2 com­ponent lasts from about one micro­second after det­on­a­tion to one second after det­on­a­tion and is very similar to the pulses created by light­ning strikes. The E2 com­ponent is easy to protect against using con­ven­tional light­ning pro­tec­tion equip­ment, but this pro­tective equip­ment is likely to have been damaged by the E1 com­ponent and will there­fore not func­tion cor­rectly, allowing the E2 com­ponent to cause further wide­spread damage.

## E3

The E3 com­ponent is very dif­ferent to the E1 and E2 com­pon­ents. It lasts tens to hun­dreds of seconds and is caused when the Earth’s mag­netic field “snaps” back into place after being pushed out of the way by the ionised plasma created by the expanding nuclear fire­ball. This induces elec­trical cur­rents in con­ductors on the ground such as pipelines, power lines and transformers.

The E3 com­ponent is similar to the pulse created by a geo­mag­netic storm, when a severe (X-class) solar flare pushes on the Earth’s mag­neto­sphere and is some­times referred to as “Solar EMP”. In 1989 a solar EMP-type event caused the col­lapse of the Hydro-Québec power grid when a huge coronal mass ejec­tion fol­lowed a X15-class solar flare.

* During a Soviet nuclear EMP test called K-3 the E3 com­ponent of the pulse fused the entire length of a 570-kilometre over­head tele­phone line (with cur­rents reaching up to 3400 amperes) and the E1 com­ponent caused all of the attached over­voltage pro­tectors to fire. The test also started a fire that caused the Karaganda power station to burn down and pen­et­rated nearly a metre into the Earth to burn out 1000 km of buried power cables.

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## Hypnic jerk

Have you ever been lying in bed, trying to get to sleep, and sud­denly felt like you were falling? If you have, then you’ve exper­i­enced a hypnic jerk.

A hypnic jerk is a pos­itive myoclonic twitch that occurs during hyp­n­agogia, the state between being awake and being asleep. Myoclonus is a sudden, invol­un­tary muscle twitch and a pos­itive myoclonic twitch is one that causes a muscle, or a group of muscles, to sud­denly con­tract.* A hiccup is a myoclonic twitch affecting the diaphragm.

The exact cause of hypnic jerks is unknown, though I have heard it sug­gested that it is linked to human beings’ origins as tree-dwelling prim­ates, or that it is a defence mech­anism designed to jerk you back into con­scious­ness if the body thinks it is “shut­ting down” too quickly as you fall asleep.

* A neg­ative myoclonic twitch causes a muscle or muscle group to relax.

## Critical mass and fizzles

In nuclear weapon design a crit­ical mass is the minimum amount of nuclear material required to sustain the chain reac­tion neces­sary for a nuclear explosion.

There are a number of factors that affect the crit­ical mass.

• Material, enrich­ment and purity: Different nuclear mater­ials have dif­ferent prop­er­ties (most import­antly fission cross-section). To make a nuclear weapon far less plutonium-239 is required than uranium-235, because Pu-239 has a much larger fission cross-section.
• Shape: As neut­rons escape through the mass’s surface, and collide throughout the mass’s volume, it is desir­able to keep the surface area to volume ratio as low as pos­sible: a sphere is there­fore the appro­priate choice.
• Density: If the density of a mass is increased then the nuclei are forced closer together and the crit­ical mass required decreases. Modern nuclear weapons use explosive lenses to com­press a sub-critical mass of plutonium into a super-critical state.
• Temperature: The like­li­hood of fission depends on the rel­ative velo­city of the fission neut­rons and the nuclei of the nuclear fuel. If the tem­per­ature of the mass is decreased, fission becomes more likely.
• Reflectors and tampers: A neutron reflector placed around the crit­ical mass pre­vents neut­rons from escaping and reflects them back into the mass, decreasing the crit­ical mass required. A tamper pre­vents the crit­ical mass from expanding and keeps it con­cen­trated in a small volume, also decreasing the crit­ical mass. In many bomb designs the reflector and tamper are the same structure.

Changing any one of these factors could allow a material to go from sub-critical to crit­ical. For example, a rod-shaped mass sud­denly com­pressed into a sphere could become crit­ical, as could a warm mass that is sud­denly cooled.

The graph below shows the import­ance of choosing a nuclear material. Many iso­topes require a smaller crit­ical mass than uranium-235 or plutonium-239 but these are unsuit­able for various reasons con­nected to the factors listed above. For example, iso­topes of cali­fornium are too radio­active, neptunium-236 is too dif­fi­cult to sep­arate from its parent iso­topes and plutonium-238 releases too much heat due to alpha-decay.

A fizzle occurs when a bomb creates a crit­ical mass too slowly and nuclear pre­det­on­a­tion occurs, des­troying the bomb before the fission chain reac­tion can propagate throughout it. The first North Korean nuclear test in 2006 was a fizzle, with a yield of less than one kiloton, twenty times smaller than any other country’s initial test.

In the diagram above two sub-critical masses are brought together too slowly, causing a pre­det­on­a­tion that throws the two masses away from each other, causing a fizzle and pre­venting a nuclear explosion.

## Turboencabulator

Scientists at General Electric are now close to per­fecting a machine that would not only supply inverse reactive current for use in uni­lat­eral phase detractors, but would also be capable of auto­mat­ic­ally syn­chron­ising car­dinal gram­meters. This machine has come to be known as a “Turboencabulator” but is also some­times referred to as a “retro-turboencabulator”, depending on the con­fig­ur­a­tion of the phase detractors.

Extract of ori­ginal GE tur­boen­cab­u­lator patent filing

The pro­to­type of the machine has a base-plate of pre­fab­u­lated amulite, sur­mounted by a mal­le­able log­ar­ithmic casing in such a way that the two spurving bear­ings are in a direct line with the pen­ta­metric fan. The main winding is of the normal lotus-δ type, which is placed in pan­en­dermic semi– or full-boloid slots in the stator, with every seventh con­ductor being con­nected by a non­re­vers­ible treme pipe to the dif­fer­en­tial on the ‘up’ end of the grammeters.

GE’s ori­ginal pro­to­type turboencabulator

Twenty-one or forty-two man­estic­ally spaced grouting brushes are arranged to feed into the rotor slip­stream a mixture of high S-value phenyl­hy­droben­zamine and 5% rem­in­ative tetryliodohex­amine. Both of these liquids have spe­cific pericos­ities given by P = 2.5 C.n where n is the dia­thet­ical evolute of ret­ro­grade tem­per­ature phase dis­pos­i­tion and C is the annular grillage coef­fi­cient. Initially, n was meas­ured with the aid of a meta­polar refractive pil­fro­meter, but recent advant­ages have used hopper dado­scopes. The tur­boen­cab­u­lator has already been suc­cess­fully used for oper­ating nofer trun­nions. In addi­tion, whenever a bares­cent skor motion is required, the tur­boen­cab­u­lator may be employed in con­junc­tion with a in-drawn recip­roc­ating arm to reduce sinus­oidal depleneration.

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## The mean centre of world lighting

The amount of light emitted by a civil­isa­tion is closely cor­rel­ated with the level of devel­op­ment of that civil­isa­tion. If one com­pares North and South Korea at night this becomes very obvious — almost the only city lit up in the DPRK is the capital Pyongyang.

A new paper* looks at the “mean centre of world lighting”, a weighted average of all the lighting on Earth. In the example maps below the mean centre of lighting is indic­ated in red.

An example of a truly ran­domly dis­trib­uted map.

An example map in which there are more and brighter lights in the “south west” quad­rant of the map.

The researchers behind the paper find that the mean centre of world lighting is moving east­wards at about sixty kilo­metres per year.

The move­ment of the mean centre of world lighting is primarily due to increased devel­op­ment in coun­tries like India and China and the move­ment of rural pop­u­la­tions to urban centres. As an example, the image below (taken from the paper) shows increased devel­op­ment in the Nile Delta region between 1992 and 2009.

* Nicola Pestalozzi, Peter Cauwels and Didier Sornette, “Dynamics and Spatial Distribution of Global Nighttime Lights”, arXiv: 1303.2901.

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Radomes at the Misawa Air Base in Japan.

I have always loved radomes. From my Godmother’s farm it’s pos­sible to see the radomes at a local RAF base and I always used to refer to them as “golf balls”.

Radomes at Menwith Hill RAF base.

The true purpose of radomes is very simple: they exist to protect radio install­a­tions from weather and to conceal (in the case of dir­ec­tional antennae) the dir­ec­tion in which the antenna is pointed.* The material from which they are made must be trans­parent to the microwave radio signals being trans­mitted and received; I suspect fiber­glass or Tyvek would be com­monly used.

* For example, the antennae at the Menwith Hill RAF base are believed to reg­u­larly inter­cept satel­lite trans­mis­sions and cov­ering the antennae pre­vents the satel­lites being tar­geted from being identified.

The US National Radio Quiet Zone (NRQZ) is a rect­angle of land, approx­im­ately thirty-four thou­sand square kilo­metres in area, that crosses into Virginia, West Virginia and Maryland and con­tains the National Radio Astronomy Observatory at Green Bank and the Sugar Grove Research Facility at Sugar Grove (part of the US Navy’s Information Operations Command and said to be an important part of the NSA’s ECHELON system).

The Green Bank Telescope, the world’s largest steer­able radio telescope.

Within the NRQZ radio emis­sions are highly restricted; con­ven­tional tele­vi­sion and radio  trans­mit­ters do not oper­ates and people who (incor­rectly) believe that they are sens­itive to elec­tro­mag­netic emis­sions have flocked there in order to deal with their “problem”. Electric fences, elec­tric blankets, car elec­tronics and even radio-tagged animals have all caused prob­lems in the NRQZ and all on-site vehicles must have diesel engines rather than petrol engines, as diesel engines use the heat gen­er­ated by com­pressing petrol vapour rather than spark plugs to ignite their fuel.

## Depth perception in jumping spiders

The vast majority of animals judge dis­tance by using bin­ocular (two-eyed) ste­reo­scopic vision; other animals use accom­mod­a­tion (how much the animal has to adjust focus) and motion par­allax (how much the image moves across the retina). But a new paper* shows that at least one animal, a jumping spider known as Hasarius adan­soni, per­ceives depth by using image defocus, com­paring a defo­cused image from one sensor with a focused image from another sensor.

A hori­zontal cross-section of the spider retina, showing the dif­ferent photore­ceptive layers at dif­ferent depths within the retina.

The researchers invest­ig­ating the spider’s eye showed that their hypo­thesis about depth per­cep­tion was correct with a very neat exper­i­ment. They used dif­ferent colours of light, which are refracted by the spider’s eye by dif­ferent amounts and there­fore have dif­ferent focal lengths and took video of the spider’s jumping attack. Under green light, which is detected by the L1 and L2 layers indic­ated above the spiders were able to accur­ately judge the dis­tance to their prey, but under red light, which is not detected by the L1/L2 layers the spiders jumped short (as red light has a shorter wavelength than green). This showed that the spider was judging dis­tance by com­paring a defo­cused image on L2 with a focused image on L1; under red light the spider receives incor­rect inform­a­tion about the amount of defo­cusing required and there­fore doesn’t attack correctly.

* Takashi Nagata et al, “Depth Perception from Image Defocus in a Jumping Spider”, Science 335 (2012): 469 – 471. doi: 10.1126/science.1211667.

## Siderolites and pallasites

Meteorites can be sep­ar­ated into four main groups:

Chondrites are grainy stony met­eor­ites com­posed mainly of silicate min­erals like olivine and pyroxene. They make up about 80% of the met­eor­ites found on Earth.

Achrondrites are similar to chon­drites, but at some point in their life­time they have been melted (like magma) and there­fore they do not demon­strate the same graini­ness that chon­drites do.

Iron met­eor­ites are com­posed mainly of met­eoric iron. Despite its name met­eoric iron is actu­ally an iron-nickel alloy, and most iron met­eor­ites are com­posed of either kama­cite or taenite.

The fourth and final group are the sider­ol­ites, stony-iron met­eor­ites, and they are beau­tiful. I am par­tic­u­larly keen on pal­la­s­ites, which contain centimetre-sized grains of peridot olivine embedded within a matrix of met­eoric iron.

The origin of pal­la­s­ites is uncer­tain,  but it’s thought pos­sible that they may be formed when two aster­oids collide, or when a met­eorite col­lides with the surface of a planet.

## Nuclear mines in the Bay of Naples

During the Cold War the USSR con­sidered the Bay of Naples, off the south-west coast of Italy, to be an important stra­tegic loc­a­tion. They con­sidered it so important that K-8, a November-class sub­marine, was tasked with mining the area with nuclear torpedo mines.

The Bay of Naples

Project 627A November-class submarine

A torpedo mine is laid anchored to the sea floor and waits patiently to be activ­ated. Once activ­ated, the mine uses passive sonar to listen for an approaching sub­marine. Once an enemy sub­marine is detected and within range, the mine’s torpedo is auto­mat­ic­ally launched. It then tracks and des­troys its target as a normal torpedo would.

K-8, the sub­marine said to have carried out the oper­a­tion on 10th January 1970, ori­gin­ally carried a com­ple­ment of twenty-four mines. Three months later, during a large-scale naval exer­cise two fires occurred sim­ul­tan­eously in two dif­ferent com­part­ments aboard K-8, causing all hands to abandon the boat. As it was being towed for repairs through the Bay of Biscay it sank in rough seas — allegedly with only four mines still aboard — killing fifty-two members of the crew.

It is alleged that the nuclear mines are still in place, but after more than forty years they are very unlikely to still be func­tioning. Still, they may pose a serious con­tam­in­a­tion risk as they rust and degrade.

Disclaimer: It’s worth noting that this inform­a­tion comes from Mario Scaramella, who has been the subject of some con­tro­versy. The IAEA lists the event as “not con­firmed” in a report entitled Inventory of Accidents and Losses at Sea Involving Radioactive Material.

## Bhangmeter

A Crooke’s radiometer. The higher the flux of infrared radi­ation the faster it spins.

Bhangmeters are placed on recon­nais­sance satel­lites* in order to detect nuclear weapon det­on­a­tions and to measure their yield. Bhangmeters are designed to look for the char­ac­ter­istic “double flash” created when nuclear weapons det­onate: the first initial bright flash being caused by the actual det­on­a­tion of the weapon and the second being caused when the ionised gas shock wave cools enough to allow light from the fire­ball to escape.

The name “bhang­meter” was created by Frederick Reines (who later won the Nobel Prize for Physics for his work on detecting neut­rinos), who sug­gested that one would have to be on bhang (an Indian drink made from marijuana) to believe that the detector would work.

* The US Department of Defense’s GPS satel­lites also carry bhangmeters.

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## Halley VI Research Station

The Brunt Ice Shelf, with the Brunt Ice Falls behind it.

One of the prob­lems with building a research station located* on the Brunt Ice Shelf in Antarctica is that the Brunt Ice Shelf is floating on the Weddell Sea. Not only do you have to keep jacking-up the base of the station to prevent it from being buried by accu­mu­lating snow, but you also run the risk of the entire ice shelf calving away as an iceberg.

The UK’s Halley VI Research Station, which was offi­cially opened on 5th February, has been designed to combat both of these issues. The base sits on legs that can be hydraul­ic­ally raised to prevent it from being sub­merged by snow, and at the end of each leg are skis that allow the entire base to be moved to a new loc­a­tion if and when it becomes necessary.

Close-up view of Halley VI’s legs.

The station is designed to be modular, making con­struc­tion and main­ten­ance easier. The blue modules are science modules, and the central red module is the main space for eating, drinking and recreation.

Halley’s primary purpose is atmo­spheric research. It was at Halley in 1985 that the hole in the ozone layer was discovered.

* The Brunt Ice Shelf is in the north-west quad­rant of Antarctica, in an area claimed by the UK but also by Argentina and Chile.