香港市區可見的明亮行星
Bright Planets Visible in the Urban Areas of Hong Kong

 

香港高度城市化,令黑夜猶如白晝。但只要天氣情況良好,我們仍能看到天空中最明亮的天體。今期通訊會向大家介紹四顆行星,它們一般都較為明亮,在市區亦可清楚看到。但由於各行星和地球之間的距離不斷改變,因此它們的位置及光度亦會不斷變化,觀測前必須掌握它們的出沒規律。

Hong Kong has a high degree of urbanization and the night sky is lit up by artificial light. However, under good weather conditions, bright celestial objects in the sky are still visible. In this issue, we are going to introduce four planets to you. They are relatively bright and can be clearly seen in the urban area. Since the distances between the planets and the Earth keep changing, the position and brightness of each planet as viewed from the Earth keep changing as well. We have to understand the movement of these planets before planning an observation.

金星 Venus       

當金星接近「大距」時,它會成為天上最明亮的星星,其光亮程度更會令人誤以為它是「不明飛行物體」!金星位於地球軌道之內,因此它只會於日落後或日出前分別出現於西方或東方近地平線的位置。在金星「大距」時,金星和太陽的夾角最高達45度至47度左右。

When Venus is near to its greatest elongation, it will become the brightest star in the sky. Some people may mistakenly think that it is a "UFO" due to its brightness! Since Venus is located within the orbit of the Earth, it will appear near the western or eastern horizon only during sunset or sunrise. During its greatest elongation, the angle between Venus and the Sun reaches a maximum of around 45 to 47 degrees.

Venus

地球Earth

金星Venus

太陽 Sun

金星東大距
Greatest Eastern Elongation

金星西大距
Greatest Western Elongation

地球軌道
Earth's orbit

金星軌道
Venus's orbit

「大距」分為兩種,當金星在太陽以東達至大距時,稱為「金星東大距」,這時金星在日落後出現在西面的半空中,並顯得非常光亮。

There are two types of greatest elongation. When Venus reaches its greatest elongation to the east of the Sun, it is called the "Greatest Eastern Elongation of Venus". At that time, Venus will appear in the western sky after sunset and become very bright.

「金星東大距」的日期
Date of Greatest Eastern Elongation of Venus

「金星西大距」的日期
Date of Greatest Western Elongation of Venus

12/1/2017

3/6/2017

18/8/2018

6/1/2019

25/3/2020

13/8/2020

在2016年11月至2017年2月期間,金星的距角幾乎達40度或以上,並且十分光亮。在日落後的西邊半空中,很容易就會看到耀眼的金星。

During the period from November 2016 to February 2017, the elongation of Venus will reach almost 40 degrees or above and Venus will look very bright. The bright Venus is visible in the western sky after sunset.


火星 Mars

火星是一顆紅色的星星,觀測它的最佳時機是每隔約26個月才發生一次的天文現象「火星衝」**之時。而由於火星軌道呈橢圓形,每隔15至17年,「火星衝」更會於火星位於近日點位置附近時發生,故又稱為「近日點衝」,那時火星會較接近地球而且顯得更光亮。下次火星的「近日點衝」將在2018年發生。

Mars is a star that appeared in red color. The best time to observe it is the time during the astronomical phenomena "Mars Opposition"**, which occurs about every 26 months. Since the orbit of Mars is elliptical, for every 15 to 17 years, an opposition will occur when Mars reaches its closest position to the perihelion, which is called the "perihelic opposition". At that time, Mars will be much closer to the Earth and it looks much brighter. The next perihelic opposition of Mars will occur in 2018.

Mars

地球 Earth

太陽 Sun

火星 Mars

 

 

在2003年至2018年的火星衝期間,地球和火星皆在不同位置,有時兩者會較為接近。

During the Oppositions of Mars from 2003 to 2018, the Earth and Mars are at different positions. Sometimes they are closer to each other.

日期
Date

火星和地球的距離 (百萬公里)
Distance between Mars and Earth (million kilometre)

火星視大小 (角秒)
Apparent size of Mars (arc second)

視星等
Apparent magnitude

9/4/2014

92.9

15.1

-1.5

22/5/2016

76.1

18.4

-2.1

27/7/2018

57.7

24.3

-2.8

14/10/2020

62.7

22.4

-2.6

8/12/2022

82.3

17.0

-1.9

 

1角秒為1/3600度,因此即使在近日點衝時,火星的視大小也不到0.01度,肉眼是不可能看到火星的圓盤的。其實,並非一定要等到「衝」的時候才能見到火星,只是其餘時間這顆紅色的星並非十分明亮而已。

1 arc second is equal to 1/3600 degree. So even during the perihelic opposition, the apparent size of Mars is less than 0.01 degree. It is impossible to see the disk of Mars with the naked eye. In fact, it is not necessary to wait for the opposition to occur to observe Mars. At other times, Mars is still visible though it is not very bright.

 

木星和土星 Jupiter and Saturn

和火星一樣,木星和土星皆位於地球軌道以外,因此當「木星衝」或「土星衝」發生時,就是觀測它們的最佳時機。但它們和地球皆十分遙遠,因此每次「衝」的距離並無明顯不同。木星和土星的公轉周期較長,因此「木星衝」和「土星衝」分別每隔約400天和378天就會發生,幾乎是每年皆有的天文現象。

Same as Mars, Jupiter and Saturn are located beyond the orbit of the Earth. Therefore, the best time for observing Jupiter or Saturn is when the opposition of Jupiter or the opposition of Saturn occurs. However, as both planets are far away from the Earth, the differences between their oppositions are not significant. Also, since the orbital periods of Jupiter and Saturn are long, an opposition of Jupiter and an opposition of Saturn occur about every 400 days and 378 days respectively, which can be counted as two annual astronomical phenomena.

木星衝
Opposition of Jupiter

土星衝
Opposition of Saturn

日期
Date

視星等
Apparent magnitude

日期
Date

視星等
Apparent magnitude

8/3/2016

-2.5

3/6/2016

0.0

8/4/2017

-2.5

15/6/2017

0.0

9/5/2018

-2.5

27/6/2018

0.0

10/6/2019

-2.6

9/7/2019

0.1

 

由於太陽系各個行星幾乎位於同一平面上,因此從地球觀察,會發現各行星總是在黃道(又即是地球的公轉平面)附近。

Since the Sun and the planets almost lie on the same plane, the planets are always near the ecliptic (equivalent to the plane of Earth's orbit) when viewed from the Earth.

夜空中,行星和其他一般的星星有一項不同之處,就是它們並不會像星星般閃爍不停。由於行星和地球距離比較近,行星的光其實是一條足以讓人測量其面積的細微光柱,一般的大氣擾動不足以令該光柱出現明顯變化,因此行星的星光能保持穩定。如果今晚天氣良好的話,你會否試試出外看看行星?

In the night sky, there is a difference between the planets and other stars. The planets do not twinkle as other stars do. It is because the Earth and the planets are much closer. The light from a planet is actually a tiny beam with a measureable area. Normal atmospheric turbulence will not cause changes in that beam, and therefore the light of a planet is steady. If tonight's weather is fine, would you like to go outdoors and take a look at the planets?

Star, Planet, Earth’s atmosphere

恆星 Star

行星 Planet

地球大氣層 Earth’s atmosphere

地球大氣層對行星及一般星星的光線影響有不同的後果

The effect of Earth’s atmosphere has different consequences on the lights of planets and stars.

 

* 視星等是描述星星亮度的數值,數值愈低代表星星愈光。在夜空中較明亮的恆星大多介乎0等至1等。天狼星是夜空中最明亮的恆星,視星等為-1.4。滿月的視星等為-13而太陽的視星等則為-27。
Apparent magnitude is a value to describe the brightness of a star. A lower value represents a brighter star. Most of the bright stars in the night sky are between magnitude 0 to 1. Sirius, the brightest star in the night sky, has a magnitude of -1.4. The magnitude of a full moon is -13, and that of the sun is -27.

** 當外行星和太陽、地球接近連成一線時,外行星和太陽位於地球的相反方向,稱為「衝」。
When the Sun, the Earth and an outer planet are nearly in a straight line, and the Sun and the outer planet appear on the opposite sides of the Earth, it is called “Opposition”.

 

數據來源:美國海軍天文台
Source of data: The United States Naval Observatory (USNO)

本地觀星活動
Local stargazing activities