Monthly Archives: March 2013

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.

random-weighted-averageAn example of a truly randomly distributed map.

weighted-weighted-averageAn 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.



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.


A radome protecting a height-finding radar in Hawaii.

* 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.


National Radio Quiet Zone

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.