波特尔暗空分类法2001

创立过程

光污染现象日益严重，使得足够黑暗的观测点距离城市很远，如今许多业余天文爱好者无法便捷地到达，他们也因此没能积累足够的经验来判断天空亮度。甚至许多人还误以为自己曾在非常暗的环境中进行过观测，却不知其实那只是一般的暗度。有感于此，约翰·波特尔凭借他近50年的观测经验而创立了这套分类法。

量化

因为个人视觉能力、观测能力不同，同时观测的时间长短也有影响，根据肉眼所能看到的最暗星等（肉眼极限星等）来判断天空的暗度是不够精确的。波特尔暗空分类法则综合多方面因素而让观测标准更加统一。

实际应用的模糊性

波特尔暗空分类法的计算并非通过一个精确的公式来完成，而是依据一系列的观测条件和经验判断。这些条件包括可见星星的数量、星等限制、天空背景亮度、对流层散射光和直接光源等。由于波特尔暗空分类法的定义是依据观测者在特定地点的主观感受来描述，所以在实际应用中可能存在一定的模糊性。

不用小数点表示

由于波特尔暗空分类法并非是一个连续的精确数值，而是9个离散的等级，因此不建议使用小数点表示。但在实际观测中，可以根据观测者的经验和对比参考，大致判断天空亮度处于两个等级之间，从而更细致地描述观测条件。

The Bortle dark-sky scale (usually referred to as simply the Bortle scale) is a nine-level numeric scale that measures the night sky's brightness of a particular location. It characterizes the observability of celestial objects, taking into account the interference caused by light pollution. Amateur astronomer John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help skywatchers evaluate and compare the darkness of night-sky observing sites.

Scale

The scale ranges from Class 1, the darkest skies available on Earth, through to Class 9, inner-city skies. The classes are described primarily in terms of the visibility of notable celestial objects and light sources in the sky, but correspond closely with naked-eye limiting magnitude (NELM) and sky quality meter (SQM) measurement of skyglow. At higher classes, light pollution above the horizon is obvious, diffuse light sources such as the Milky Way and Messier objects are invisible to the naked eye, and fewer point light sources such as stars and planets can be seen. At lower classes, light pollution domes are only present in the direction of cities or are absent altogether, the sky is filled with stars, and faint diffuse light sources such as the zodiacal light are contrastful and brilliant.

Reception

The Bortle scale has attained widespread usage among amateur and professional astronomers. It quantifies the link between artificial lighting and reduced night sky quality in an intuitive manner, making it a common tool in scientific communication. A wide database of SQM measurements has been created, allowing the Bortle rating to be estimated for anywhere in the world.

Bortle was motivated to publish the scale in 2001 by increasing light pollution making true dark-sky sites inaccessible to many amateur astronomers. Ongoing urbanization and the global transition to LED lighting have continued this trend. James Brodrick of the US Department of Energy wrote in 2018 that "All manmade lighting is 'unnatural' and thus, has potentially undesirable side effects. Some of these are unavoidable if we're to continue to enjoy the benefits."

The higher light temperature of LED lights has been a particular contributor to increasing light pollution, in ways sometimes not reflected in older studies of light pollution. However, The Washington Post wrote in 2023 that "there is a world where more energy-efficient LED lights exist and don't significantly disrupt nightscapes or our health." DarkSky International calls for responsible outdoor lighting that is carefully targeted, no brighter than necessary, and warm-colored.

2014 research suggests Bortle may have overestimated the visibility of dim objects for the typical observer, even in the darkest skies. Diffuse light sources such as M33 are more difficult to see than point light sources of the same visual magnitude; Bortle's description of M33 (V 5.72) as a naked-eye object is atypical. A naked-eye limiting magnitude (NELM) over 7.1, which Bortle suggests for Class 2, represents a "substantial raising of achievement and expectation". A NELM of 6.0 to 6.5 is a more commonly cited figure. Bortle was aware of NELM varying with the visual acuity of the observer, and he devised the scale as an alternative to raw NELM.