When observed from space at night, most cities look very similar.
But Tokyo looks very different.
Unlike most major cities, Tokyo still uses mercury-vapour lamps (which were invented in 1901) rather than sodium-vapour lamps (which were invented in 1920) for its street lighting. The spectra of light emitted by mercury- and sodium-vapour lamps are very different:
Above: the sodium spectrum; Below: the mercury spectrum.
The overall colour of light produced by a sodium-vapour lamp is a bright yellow,* whereas the colour of light produced by a mercury-vapour lamp is a bright turquoise-white.
In the photographs above, Helsinki (top) is using sodium-vapour bulbs for its street lighting (though it still has some mercury-vapour lamps it is replacing those), and Tokyo (bottom) is using mercury-vapour bulbs. In Berlin, the division between the old East German and West German parts is still visible from space due to the different types of bulbs used in their streetlamps.
West Germany (on the left of the image) uses mercury-vapour bulbs, and East Germany (on the right) uses sodium-vapour bulbs.
* Light from a sodium-vapour lamp is almost monochromatic, at 589.3?nm. Optical telescope users prefer sodium-vapour light pollution because it is easier to filter out.
Radioluminesence is the emission of light due to bombardment by ionising radiation, the most common example of which is tritium illumination. Tritium is an isotope of hydrogen, made up of one proton and two neutrons (i.e. it is hydrogen-3). It has a half-life of 12.3 years and decays by emitting beta particles (high speed electrons) to form helium-3.
In a tritium illumination light source the tritium gas is trapped in a glass tube that has been coated on the inside with a phosphor. When the gas decays, the electrons produced strike the phosphor and their kinetic energy is transferred into light energy. By choosing different phosphors, different colours of light are produced.
Novelty keychains containing tritium light sources.
Because tritium illumination requires no power source and lasts for a long time, it is commonly used in situations where long-term but low-power lighting is required. For example, tritium illumination is used on watchfaces, compasses, instrument dials and gunsights. The encapsulation of the tritium source prevents any radiation risk, and if a tritium light source is broken open then simply leaving the area and allowing the gas to disperse will mitigate any health risks.
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.