What astronomical observation is most severely hampered by increasing levels of urban light pollution?
- (a)Sunspots
- (b)Planetary transit
- (c)Deep-sky objects
- (d)Lunar phases
Answer
Why
Correct — C. Urban light pollution brightens the whole night sky into a background glow called skyglow, and faint, diffuse targets vanish into it first.
Deep-sky objects, such as galaxies, nebulae and star clusters beyond the solar system, are exactly those targets. Under an inner-city sky on the Bortle scale, the Moon, the planets, bright satellites and a few of the brightest star clusters are about all that remain. That is option (c).
Why the others are wrong
- (a)Sunspots — Sunspots are observed on the Sun in daytime, through proper solar filters. Artificial light at night has no bearing on them.
- (b)Planetary transit — A transit of Mercury or Venus is seen as a small dark disc crossing the Sun, in daylight. City lights at night do not affect it.
- (d)Lunar phases — The Moon is the brightest object in the night sky and stays visible from the most light-polluted city. Its phases can be followed from any street.
Concept
Light pollution is artificial light scattered by the atmosphere, raising the brightness of the night sky. What it hides depends on surface brightness, not total brightness.
A planet or the Moon concentrates its light in a small, bright disc that stands out even against a glowing sky. A galaxy or nebula spreads its faint light over a wide patch, so its contrast with the sky is lost first.
The Bortle scale, published by John E. Bortle in Sky & Telescope in February 2001, grades night skies in nine classes, from class 1, an excellent dark-sky site, to class 9, an inner-city sky.
Key facts
- The Bortle scale rates night-sky darkness in nine classes, from class 1 (excellent dark-sky site) to class 9 (inner-city sky).
- John E. Bortle published the scale in Sky & Telescope in February 2001.
- Under an inner-city sky, the Moon, the planets, bright satellites and a few of the brightest star clusters remain observable.
- Deep-sky objects lie beyond the solar system: galaxies, nebulae and star clusters.
Study next
Common traps
- Picking planetary transit because it sounds like a hard observation, when transits of Mercury and Venus are seen against the Sun in daytime.
- Assuming light pollution hides everything equally, when bright, compact objects such as the planets survive it.
The stem pairs one cause, urban light pollution, with four observations, and the test is spotting the one made at night against a faint background.
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