What is light pollution?
o Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light. It obscures the stars in the night sky for city dwellers, interferes with astronomical observatories, and, like any other form of pollution, disrupts ecosystems and has adverse health effects.
o Light pollution can be divided into two main types: 1) annoying light that intrudes on an otherwise natural or low-light setting and 2) excessive light (generally indoors) that leads to discomfort and adverse health effects. 
o Since the early 1980s, a global dark-sky movement has emerged, with concerned people campaigning to reduce the amount of light pollution.
Sources of light pollution
Light pollution is a side effect of industrial civilization. Its sources include building exterior and interior lighting, advertising, commercial properties, offices, factories, streetlights, and illuminated sporting venues.
It is most severe in highly industrialized, densely populated areas of North America, Europe, and Japan and in major cities in the Middle East and North Africa like Cairo, but even relatively small amounts of light can be noticed and create problems. Like other forms of pollution (such as air, water, and noise pollution) light pollution causes damage to the environment.
Types of light pollution
1. Sky glow
Skyglow refers to the "glow" effect that can be seen over populated areas. It is the combination of all light reflected from what it has illuminated escaping up into the sky and from all of the badly directed light in that area that also escapes into the sky, being scattered (redirected) by the atmosphere back toward the ground.
Sky glow is of particular irritation to astronomers, because it reduces contrast in the night sky to the extent where it may even become impossible to see any but the brightest stars.
2. Light Clutter
Light clutter refers to excessive groupings of lights. Groupings of lights may generate confusion, distract from obstacles (including those that they may be intended to illuminate), and potentially cause accidents. Clutter is particularly noticeable on roads where the street lights are badly designed, or where brightly lit advertising surrounds the roadways.
Clutter may also present a hazard in the aviation environment if aviation safety lighting must compete for pilot attention with non-relevant lighting.
3. Over illumination
Over-illumination is the excessive use of light.
Specifically within the United States, over-illumination is responsible for approximately two million barrels of oil per day in energy wasted. This is based upon U.S. consumption of equivalent of 50 million barrels per day (7,900,000 m³/d) of petroleum .
4. Glare
Glare can be categorized into different types
Blinding glare: describes effects such as that caused by staring into the Sun.
Disability glare describes effects such as being blinded by oncoming car lights, or light scattering in fog or in the eye, reducing contrast, as well as reflections from print and other dark areas that render them bright, with significant reduction in sight capabilities.
Discomfort glare does not typically cause a dangerous situation in itself, though it is annoying and irritating at best. It can potentially cause fatigue if experienced over extended periods.
Glare from bad lighting is a public-health hazard—especially the older you become. Glare light scattering in the eye causes loss of contrast and leads to unsafe driving conditions, much like the glare on a dirty windshield from low-angle sunlight or the high beams from an oncoming car. In essence bright and/or badly shielded lights around roads may partially blind drivers or pedestrians and contribute to accidents.
5. Light trespass
Light trespass occurs when unwanted light enters one's property, for instance, by shining over a neighbor's fence. A common light trespass problem occurs when a strong light enters the window of one's home from the outside, causing problems such as sleep deprivation or the blocking of an evening view.
A number of cities in the U.S. have developed standards for outdoor lighting to protect the rights of their citizens against light trespass. To assist them, the International Dark-Sky Association has developed a set of model lighting ordinances
Consequences of light pollution
1. Energy waste
Lighting is responsible for one-fourth of all energy consumption worldwide, and case studies have shown that several forms of over-illumination constitute energy wastage, including non-beneficial upward direction of night-time lighting.
In Australia, public lighting is the single largest source of local government's greenhouse gas emissions, typically accounting for 30 to 50% of their emissions. There are 1.94 million public lights—one for every 10 Australians—that annually cost A$210 million, use 1,035 GWh of electricity and are responsible for 1.15 million tonnes of CO2 emissions. Current public lighting in Australia, particularly for minor roads and streets, uses large amounts of energy and financial resources, while often failing to provide high quality lighting.
In 2007, Terna, the company responsible for managing electricity flow in Italy, reported a saving of 645.2 million kWh in electricity consumption during the daylight saving period from April to October. It attributes this saving to the delayed need for artificial lighting during the evenings
2. Effects on human health and psychology
Medical research on the effects of excessive light on the human body suggests that a variety of adverse health effects may be caused by light pollution or excessive light exposure, and some lighting design textbooks use human health as an explicit criterion for proper interior lighting.
Health effects of over-illumination or improper spectral composition of light may include: increased headache incidence, worker fatigue, medically defined stress, decrease in sexual function and increase in anxiety
3. Disruption of ecosystems
• Light pollution poses a serious threat to wildlife, having negative impacts on plant and animal physiology. Light pollution can confuse animal navigation, alter competitive interactions, change predator-prey relations, and cause physiological harm. The rhythm of life is orchestrated by the natural diurnal patterns of light and dark, so disruption to these patterns impacts the ecological dynamics.
• Light pollution kills billions of insects every night.
• Lit tall buildings disturb bird migration.
• Lit shores lure sea turtles inland to their death.
• Lightings of buildings also endangers bats.
• Inappropriate street lighting and other stronger light sources can kill the predaceous water beetles.
4. Effect on astronomy
Skyglow reduces the contrast between stars and galaxies in the sky and the sky itself, making it more difficult to detect fainter objects.
Light trespass can impact observations when stray light enters the tube of the telescope from off-axis, and is reflected from surfaces other than the telescope's mirrors (if any) so that it eventually reaches the eyepiece, causing a glow across the field of view since it has not been focused.
Summery:
Light pollution causes many unfavorable activities:
o Light pollution is destroying natural heritage.
o Light pollution is destroying biodiversity.
o New power-saving lights (LED) pose an even greater threat to biodiversity.
o Night light is also dangerous to human health.
o Light pollution can confuse animal navigation, alter competitive interactions, change predator-prey relations, and cause physiological harm.
o Migrating birds can be disoriented by lights on tall structures.
o Light pollution disturb astronomical activities.
o Light pollution causes energy waste.
o Increase of greenhouse gas emission.
Why should we preserve natural dark sky in Nepal ?
o To continue ecosystem.
o To save energy waste.
o To be safe from many adverse health effects which could attack if we do not preserve dark sky.
o To make Nepal a dark sky destination.
o To promote astrotourism.
Reduction of light pollution
What can be done?
Reducing light pollution implies many things, such as reducing sky glow, reducing glare, reducing light trespass, and reducing clutter. The method for best reducing light pollution, therefore, depends on exactly what the problem is in any given instance.
o Improving lighting fixtures
o Adjusting types of light sources
o Re-designing lighting plans
o Utilizing light sources of minimum intensity necessary to accomplish the light's purpose.
o Turning lights off using a timer or occupancy sensor or manually when not needed.
o Improving lighting fixtures, so that they direct their light more accurately towards where it is needed, and with less side effects.
o Adjusting the type of lights used, so that the light waves emitted are those that are less likely to cause severe light pollution problems.
o Evaluating existing lighting plans, and re-designing some or all of the plans depending on whether existing light is actually needed.
Solutions
o To ensure a natural sky at least in nature parks, fully shielded lighting is vital in both rural and cities.
o Most lamps (at least 90%) are not fully shielded. We need global agreement on fully shielded lamps, using environ-friendly types.
o Controls on illumination of surfaces, especially advertisements, are required as is a limit on unnecessary night lighting.
o Experience shows that light pollution can be countered only by effective laws (e.g. Italy and Slovenia).
o Light pollution will not be reduced by voluntary work, so we recommend the formation ot a professional international association of dark parks, with the purpose of establishing professional networks in all countries. At least one professional per million residents would be needed for effective results.
Conclusion
o Light pollution is going to be a major problem in the world very soon.
o Nepal government should address this issue equally as other pollution issues.
o Nepal could be a dark sky destination if we preserve our natural dark sky.
o Nepal government should start lightning plans before it becomes too late.
