Thursday, November 7, 2013

Light Pollution

Statistically, most people in the developed world live in cities/suburbs, which are not all that great settings for doing astronomy. In fact, living in such a location probably prevents a great number of people from taking up astronomy in the first place, which need not happen as there are ways to beat the light.
First of all, what does light pollution look like?

Have you ever gone outside on a cloudy night and noticed how light it was? The answer to this question, for anyone living in a city/suburb is going to be “a lot.” This effect can be greatly magnified when there is a snowstorm overnight, too. Naturally, none of this light is natural. In its purest form, a cloudy night should be dark, not have a reddish hue to it. Needless to say, this is a very practical example of light pollution, or stray light beamed away from the ground that serves no other purpose than to brighten up the sky.

Now, this is not to say all light pollution is this bad, as there can be greatly varying levels that can present themselves in different ways.

In 2001, John Bortle devised a scale for determining light pollution. The scale runs from one to nine, with higher numbers indicating more light pollution. Here is a breakdown of the scale, magnitudes listed are for people with exceptionally good vision.

Level 1: Excellent dark sky site, stars as dim as magnitude +8. The brightest areas of the Milky Way cast very visible shadows. Bright planets, Venus, Jupiter, Mars at a close approach, all seem to inhibit proper night vision. This is an observers dream.

Level 2: Typical dark sky sight, stars to magnitude +7.5. The Summer Milky Way is structured to the naked eye, the Zodiacal Light is still bright enough to cast shadows. Many globular clusters are naked eye objects.

Level 3: Rural sky, stars to magnitude +7. Only a hint of light pollution near the horizon, where clouds may appear slightly illuminated. Clouds dark overhead, appear as a starless black void in the sky. Zodiacal Light rises over 60 degrees when standing straight up. Any telescopes are apparent only to about 30 feet.

Level 4: Rural/suburban transition, stars to magnitude +6.5. Light pollution domes over cities are apparent, clouds are illuminated in brighter areas of sky, but still dark overhead. Zodiacal Light extends about 45 degrees up at best. Milky Way still easily visible, but most detail is now gone.

Level 5: Suburban, magnitudes to +5.9. Only hints of Zodiacal Light visible on best nights. Milky Way only a faint haze near zenith and washed out near horizon. Light sources very apparent, any clouds are brighter than the sky.

Level 6: Bright suburban sky, magnitudes to +5.5. Zodiacal Light now invisible and only a hint of the Milky Way is seen near the zenith. Clouds are fairly bright. You will have no trouble seeing eyepieces on a table at a distance. The third of sky nearest to horizon glows a grayish-white color.

Level 7: Suburban/urban transition, magnitudes to +5. Entire sky has a grayish-white hue to it. Milky Way now invisible, clouds appear as glowing.

Level 8: City sky, to magnitude +4.5 at best. Sky begins to take on an orange glow, you can read newspaper headlines easily. Constellations incomplete as dim stars are now invisible.

Level 9: Inner city sky, magnitudes of +4 or less. Sky is lit to zenith, All but the brightest constellations appear incomplete, dim constellations are invisible.
Not only where you observe from can impact what you see, but the characteristics of the air itself can play a huge role. Humid air is an observer’s second worst enemy, only behind light pollution. For visual observers, all of the tiny water droplets in the air reflect light, thus magnifying any already existing light pollution. The drier the air, the better the observing. On the most humid of Summer nights in suburbia, third magnitude stars can be a challenge. However, a few days later on a dry night, the Milky Way might be visible from Zenith down to about 45 degrees. This vast difference in what is visible can be due to humidity alone. 

Now, light pollution understood, how to beat it?

The easiest way (other than driving out into the country) is to simply go to the backyard. Think about it: do people put lights behind their houses? Without all of the walk lights on the fronts of houses, the backyard is a lot darker than the front. In addition, houses are great blockers from street lights, too. So, without all of this artificial lighting, it's an easy claim that you can drop one level of John Bortle's scale just by changing your observation location without leaving one's own property. 

For someone really dedicated and who has a little money to burn, it might be a good idea to build a small, privacy fence-enclosed area in one' s backyard. By doing this, one can block out even more light. By doing so, one can block out all the stray light coming in from the sides, preserving night vision by allowing only a straight up view of the night sky. To add to a fenced in area's effectiveness, paint the inside of the walls flat black so prevent any reflectivity.

If you are really serious about your astronomy and have a lot of money to spend, it may be a good idea to invest in a small observatory. As funny as it may seem to a beginner, many companies sell ready-to-build observatory kits. By building an observatory, one can block out even more light than with a privacy fence and, in addition, have an outdoor storage space for all of one's astro toys. 

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