In the vast span of our Solar System there are swarms of sand bits and dirt clods known as meteoroids. When these "space debris" cut into the Earth's atmosphere, air friction brings them to flames, forming momentary bright streaks that blaze across the sky. These are the meteors that we see from time to time. Meteors often appear as sporadic lonely streaks that traverse somewhat random paths in the sky. This sporadic activity, which persists throughout the year, can account for about 10 visible meteors every hour on a clear night.
Photo Courtesy NASA
A "meteor shower" occurs when our home planet encounters a relatively dense region of meteoroids in space. An unusually large number of meteors can then be seen flying out from a certain point in the sky known as the "radiant". Most astronomers believe that meteoroids are of cometary origin, which are leftovers from periodic comets or are even fragmented comet nuclei. These meteoroids share more or less the same orbits with their parent comets, which remain relatively unchanged over eons. Our Earth makes periodic rendezvous with these meteoroid swarms, thereby enjoying a regular display of these celestial fireworks with predictable patterns. To name a few, we have the Lyrids in April and the Geminids in December.
It is common that meteor showers are unspectacular at all, with the number of observed meteors barely surpasses the normal hourly rate. This can be disappointing to stargazers who hold high expectations especially with occasional boasts in the media, but it is definitely an unforgettable experience for those who are lucky to witness a truly magnificent outburst.
On 13 November 1833, people in Boston of the United States were terrified to see the sky dazzling with thousands of meteors and fireballs streaming from the constellation Leo. It was estimated that over 150,000 meteors were visible per hour. Many religious devotees believed that the Judgment Day had finally arrived, some even went so far as to commit suicide.
The recent outburst came in 2001 of Leonid, which over 1,000 meteors can be seen per hour. Hong Kong was one of the best locations for the 2001 Leonid Meteor Shower. On 19 November 2001, seven to eight meteors per minute (about 500 meteors per hour) were seen in sub-urban areas. Some of the meteors were fireballs with magnitudes below 0. Many meteors even had colours and red heads and the green tails of meteors could be observed on that day.
Major Meteor Showers in 2021
|Shower|| Activity |
| Maximum |
|Zenith Hour Rate (ZHR)||Suggested Observation Period||Local Observation Condition|
|Quadrantid||28/12 - 12/1||3/1, 22:00||110||4/1, 1:00 – 6:00||Fair|
|Lyrid||14/4 - 30/4||22/4, 21:00||18||22/4, 23:00 – 23/4, 5:00||Fair|
|η-Aquariid||19/4 – 28/5||6/5, 11:00||50||6/5, 3:30 – 5:00||Bad|
|Perseid||17/7 - 24/8||13/8, 3:00||100||12/8, 23:00 – 13/8, 5:00||Excellent|
|Orionid||2/10 - 7/11||22/10||20||22/10, 22:30 – 23/10, 5:30||Fair|
|Leonid||6/11 - 30/11||17/11, 17:30||10||18/11, 00:30 – 6:00||Fair|
|Geminid||4/12 - 20/12||14/12, 15:00||150|| 13/12, 21:00 – 14/12, 6:00; |
14/12, 21:00 – 15/12, 6:00
Source of data: International Meteor Organization
* Local Observation Condition is divided into five levels, which are "Bad", "Unfavourable", "Fair", "Favourable" and "Excellent". The following factors would determine whether the observation condition is good or bad:
Observation Time: A darker sky is more favourable to meteor observation. If the meteor shower peaks in the period when the Sun has not set, such as midday, or during sunrise or sunset, it is harder to see the trails of the meteors as they will be obscured by the bright Sun. Also, due to the motions of the Earth, the meteoroids collide head-on with the atmosphere at relatively higher speeds after midnight, causing them to burn up vigorously. Thus, the meteors observed at dawn are usually brighter and easier to observe.
Moonlight: Moon is the second brightest object in the sky after the Sun. When the occurrence of a meteor peak is close to a full moon, the bright moonlight will obscure the dimmer meteors and affect the number of meteors we can see with our naked eyes.
Radiant: Radiant is a point in the sky where the meteors appear to originate. As there are many skyscrapers in Hong Kong, when the radiant lies close to the horizon, the shower will be blocked by buildings easily, plus there is more haze near the horizon. Conversely, when the radiant is close to the zenith, it favours the observation of meteors.