保育星空‧流星共享
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關於流星和流星雨 About meteor and meteor shower

太陽系充斥著細小的石質或金屬流星體。流星體以每秒數十公里的速度進入地球大氣層,與空氣摩擦而產生高熱,令流星體及四周空氣發光,形成一條劃破天際的光跡,天文學上稱之為流星。
A meteoroid is a small body made of rock or metal and an enormous number of them exist in our Solar System. When a meteoroid enters the Earth’s atmosphere with a speed of tens of kilometers per second, the resulting excessive heat from the air friction will cause these meteoroids and the surrounding atmosphere to glow, creating a bright streak of light in the sky called a meteor.

部分流星體是彗星或小行星的殘骸,當地球運行到這些流星體集中的區域,流星體以同一方向進入大氣層,流星像從天空中某一點(稱為「輻射點」)向外散射而出,形成流星雨。流星雨以輻射點所在的星座命名。
Some meteoroids originated from remnants of comets or asteroids. When the orbiting Earth sweeps past regions where the densely populated with meteoroids, multiple meteors will appear in the night sky as if they are radiating from a single point in the sky (known as a “radiant”), forming a meteor shower. Meteor showers are named by the constellation where the radiant is located.

由於地球每年在相若的時間穿越特定的流星體區域,因此個別流星雨每年會在相近時期出現。一年中有數個主要流星雨,例如8月出現的英仙座流星雨、10月出現的獵戶座流星雨、11月出現的獅子座流星雨和12月出現的雙子座流星雨等。每個流星雨的每年可見流星數目都不同,觀測條件視乎每年流星雨的活躍程度、高峰期出現時間和月相等因素而定。
The yearly orbital motion of the Earth implies that it will sweep past certain group of meteoroids around the same time every year, thus explaining why meteor showers reappear roughly at the same time each year. There are several major annual meteor showers, including the Perseids in August, the Orionids in October, the Leonids in November and the Geminids in December. The numbers of observable meteors of different meteor showers are different each year, depending on various factors such on the activity of the shower in that particular year, timing of the peak, and moon phase, etc.

有關雙子座流星雨 About Geminid Meteor Shower
雙子座流星雨是主要的流星雨之一,輻射點位於雙子座,流星體源自3200號小行星法厄同(Phaethon)。雙子座流星雨的高峰期在12月14日出現,「每小時天頂流星數」為120顆。
The Geminid Meteor Shower is one of the major meteor showers. Its radiant is located at the constellation Gemini. Its meteoroids are related to the asteroid named 3200 Phaethon. The peak of Geminids occurs on December 14 with a Zenithal Hourly Rate of 120.

2014年雙子座流星雨流星 A meteor of the Geminis in 2014
拍攝 photo by: 資深天文導師朱子亮先生 Mew Chu

有關「每小時天頂流星數」About “Zenithal Hourly Rate” (ZHR)
「每小時天頂流星數」,簡稱ZHR,是流星雨流星數目的指標。ZHR是指在理想的環境下,一位眼睛完全適應黑暗環境的觀測者,當流星雨的輻射點在天頂,每小時該流星雨的流星數目。ZHR所指的理想環境,包括要求觀測地點的天空毫無遮擋,及沒有受月光及光污染影響。由於ZHR是一個理想化的數字,因此實際可見的流星數目一定比ZHR的數值少。加上由於預測2017年雙子座流星雨高峰期為香港時間12月14日下午2時半左右的日間,晚間可見流星的數目更會進一步下跌。
“Zenithal Hourly Rate”, or ZHR in short, is the hourly number of meteors of a particular meteor shower observable. To achieve this number, an observer with totally dark-adapted eyes under idealized observing conditions are required. The conditions include that the radiant has to be directly up the zenith, and the observing site has an unobstructed field-of-view open without moonlight and light pollution. Thus the actual number observable will always be smaller than this idealized number of ZHR. Since the peak activity of the Geminids this year is expected to occur at daytime on December 14 (~2:30 pm) in Hong Kong, the actual number of observable meteors at night would be further reduced.

實際可見的流星數目取決於觀測地點的光污染情況和地理位置、流星雨的活躍程度、高峰期出現時間、輻射點的仰角高度、觀測者視力和經驗、月相、雲量和視野等因素。根據本港的情況及「香港夜空光度監測網絡」的數據,港大天文學家計算出一般天氣情況下,今年雙子座流星雨每小時可見流星數目如下:
The actual number of observable meteors depends on a number of factors, including the light pollution condition and the geographic location of the observing site, activity of the shower, timing of the peak, altitude of the radiant, observer’s eyesight and experience, moon phase, cloud amount, and the field-of-view, etc. By incorporating the light pollution data from the Hong Kong Night Sky Brightness Monitoring Network, HKU astronomers calculated the number of observable meteors per hour in this year’s Geminid Meteor Shower under general weather conditions as follows:

參考資料 Reference
International Meteor Organization
https://www.imo.net/

Meteor stream activity I. The annual streams
Author: Jenniskens, P.
Publication: Astronomy & Astrophysics (1994)
Bibliographic Code: 1994A&A…287..990J
http://adsabs.harvard.edu/full/1994A%26A…287..990J

Meteor Showers and their Parent Comets
Editor: Jenniskens, P.
Publisher: Cambridge University Press (2008)
ISBN: 1978-0521076357

Fluxtimator
https://leonid.arc.nasa.gov/estimator.html

關於光污染 About light pollution
「光污染」(又稱光害)是一種因過度使用人為室外照明,而對自然環境和生態等引起的負面影響。人為室外照明包括街燈、霓虹招牌、廣告版照明等。光污染不單令我們失去美麗的星空,更破壞大自然及人類的生活環境,亦浪費能源。
“Light Pollution” is a form of environmental degradation in which excessive artificial outdoor lighting, such as street lamps, neon signs, and illuminated signboards, affect the natural environment and the ecosystem. Light pollution not only rids us of the starry night sky, it also indicates a waste of electric energy and damages the natural and our living environment.

星空保育 Dark sky conservation
過量或設計不良的室外照明光線,會從地面直射或反射上天空,再經大氣中的雲、霧和污染物(如懸浮粒子等)散射至大範圍,導致一大片的夜空被照亮,夜空中可見星星的數目因而減少。
The wasteful light emitted directly upwards or reflected upwards from excessive and poorly-designed artificial light sources can be scattered to a wide area by the cloud, fog, and pollutants like suspended particulates in the atmosphere. A wide patch of the night sky is thus brightened, leading to a reduced number of stars visible due to a reduction of the light contrast in the sky.

星空是各個民族的共同文化遺產,也是生活的重要部份。我們正面臨星空持續地消失的威脅,這是地球需要面對的環境挑戰之一。星空保育是指保護黑夜環境,讓人類和動植物享有不受污染的黑夜。
The night sky is not only a common cultural heritage of all mankind but also an important part of our life. We are under a severe threat of the disappearing starry night, which is just another component of the environmental challenge the Earth faces. Dark sky conservation is the protection of our starry night sky and the preservation of our naturally dark sky, for the benefits of all humans, fauna and flora.

香港大學光污染研究 HKU light pollution research
香港大學物理系於2003年開始進行光污染研究。研究團隊建立了「香港夜空光度監測網絡」,自2010年5月起測量各區光污染情況,共收集了超過二千萬個夜空光度數據,數據量冠絕全球。研究團隊分析所收集的夜空光度數據,發現市區在晚上8時半至11時的夜空光度比國際標凖光亮100至1,000 倍,即使於較為徧遠的環境(如濕地公園),仍比標凖光亮100倍以上。香港成為全球光污染最嚴重的地方之一。
HKU Department of Physics has been conducting light pollution research since 2003. The research team built the “Hong Kong Night Sky Brightness Monitoring Network” in May 2010. Since then, the network has monitored the light pollution conditions in different parts of Hong Kong and collected more than twenty million night sky brightness data. By analysing this world’s largest dataset, researchers found that the Hong Kong urban night sky to be 100 to 1,000 times brighter than the international dark sky standard between 8:30pm and 11:00pm. Even in remote locations such as the Wetland Park, the night sky there was above 100 times brighter than the standard. Hong Kong becomes one of the most light-polluted cities in the world.

研究團隊還發現,當公共設施和商業照明陸續地關上時,夜空光度亦逐步遞減。位於市區的監測站,在每天晚上11時、凌晨0時和凌晨1時,都錄得大幅度的夜空光度下跌,與眾多人為燈光的使用模式符合,證明人為室外燈光是光污染的元凶。
The research team also found that the night sky dims progressively when public and commercial lightings are being turned off at night. For example, the urban sky showed sudden dimming near 11pm, 12am, and 1am every night, likely caused by the timing patterns of the nearby outdoor lighting. This no doubt proves that human lighting is the source of light pollution.

全球光污染監測 Global light pollution monitoring
近年,港大的光污染研究團隊將夜空光度監測技術推廣至外地,建立了「Globe at Night – Sky Brightness Monitoring Network」。這個全球性網絡目前有23個監測站,分布在亞洲、歐洲和非洲的9個國家或地區。研究項目的網站提供了監測站的資料和實時數據。
The HKU light pollution research team recently introduced our monitoring techniques internationally by establishing the “Globe at Night – Sky Brightness Monitoring Network project”. This global network currently has 23 stations operating in 9 countries/regions in Asia, Europe, and Africa. Details of the survey and the real-time data from around the world can be found in the project website.

經初步分析,數據顯示世界各地的夜空光度差別顯著,最光的地區位處市區,最黑暗的地區位處原始國家公園。各地的市區夜空亦普遍因每晚燈光使用量的減少而逐步轉暗,情況與香港相約。數據亦顯示郊區監測站所錄得的夜空光度,在整晚大致保持平穩,反映出人為燈光使用的影響較低。
Preliminary analysis of the data shows that there are large variations in night sky brightness worldwide: urban night skies are significantly brighter than night skies in pristine national parks. In general, in urban areas the night skies get progressively darker with time as the light usages by humans decrease over the night. The situation is similar to that observed in Hong Kong. On the other hand, rural stations show steady sky brightness throughout the night, reflecting low effect of human light usages.

如何減少光污染?智慧地使用照明 How to reduce light pollution? Use lighting wisely
減少使用不必要的室外照明 Reducing abuse of outdoor lightings
原則是合理地運用照明:不要加設非必要的照明,和關掉非使用中的照明。離開房間時順手把燈關掉,也可以簡單地達到目標。從城市的角度來看,在深夜時關掉或調暗街上的霓虹廣告招牌、球場的泛光燈、郊野公園範圍內的街燈、大橋裝飾的燈泡等,都是可以減少光污染的方法。
The rule of thumb is using lighting reasonably: do not set up unnecessary lighting and turn off lights which are not in use. Turning off the light when you leave a room can also help. From the perspective of the entire metropolis, turning off and dimming the neon signboards on the streets, floodlight on sport grounds, road lights in country parks and decorative lights on bridges are all possible ways to alleviate light pollution.

改裝現有的室外照明 Modifying existing lightings
大部分室外照明的設計並不有利於減少光污染。燈罩設計不良,令部分光線向上照耀。這些光線沒有實質作用,既浪費能源,也增加光污染。優良的「截光型燈具」設計,將光線聚焦並向下照射,盡量消除向天照耀的光線,減少光污染。
A majority of the outdoor lighting currently installed are not dark-sky friendly, in which a significant portion of the emitted light is spread over a wide area and not concentrated in the targeted area. Upward-directed light is particularly useless and wasteful, and is a major culprit of light pollution. The improved “Full CutOff ” design limits the spreading of light and focuses the light downward, minimising light pollution.

選用有利天文觀測的光源 Choosing astronomical-friendly lighting sources
高壓鈉燈和光管是最影響天文觀測的照明。而低壓鈉燈則是對天文觀測造成較少影響的光源,它所發射的光線只有小部分在可見光譜中,天文學家只需用濾鏡便能輕易地將此除去。
Unlike the high pressure sodium lamps and fluorescent lamps, low pressure sodium lamps are least affecting with regard to astronomical observations. It is because the emission spectrum of low pressure sodium lamps occupies a relatively small portion of the entire visible spectrum, and astronomers can easily filter out its contributions by applying filters.

參考資料 Reference
香港夜空光度監測網絡 Hong Kong Night Sky Brightness Monitoring Network (NSN)
http://nightsky.physics.hku.hk/

都是愛迪生惹的禍:光害
作者: 林憲德、趙又嬋
出版社:新自然主義

Globe at Night – Sky Brightness Monitoring Network (GaN-MN)
http://globeatnight-network.org/

Globe at Night
http://www.globeatnight.org/

Light Pollution Simulator
http://www.need-less.org.uk/