There float in the vast reaches of our Solar System swarms of sand bits and dust clods known as meteoroids. When these "space debris" tear into the Earth's atmosphere, air friction causes them to burn, 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 streaming from a certain point in the sky known as the "radiant". Most astronomers believe that meteoroids are of cometary origin. They are leftovers from periodic comets or are even fragmented comet nuclei. This at least accounts for the fact that a particular group of meteoroids shares with its parent comet more or less the same orbit, which can remain relatively unchanged over eons. An immediate consequence is that our Earth makes periodic rendezvous with this meteoroid swarm, thereby culminating in a regular display of these celestial fireworks. To name a few, we have the Lyrids in April and the Geminids in December.
It is not uncommon that meteor showers are not spectacular at all, with the number of observed meteors barely surpasses the normal hourly rate. Needless to say, this may disappoint stargazers who hold high expectation, but it is definitely an unforgettable experience for those who have the luck to witness a truly enormous outburst.
During the small hours of November 13, 1833, people in Boston of the United States were terrified to see the sky blazed with thousands of meteors and fireballs streaming from the constellation Leo. (It was estimated that over 150 000 meteors were visible per hour). While many religious devotees believed that the Judgment Day had finally arrived, some even went so far as to commit suicide.
Major Meteor Stream of 2018
|Shower|| Activity |
| Maximum |
|Zenith Hour Rate (ZHR)||Suggested Observation Period||Local Observation Condition|
|Quadrantids||28/12 - 12/1||4/1, 6:00||110||--||Poor|
|Lyrids||14/4 - 30/4||23/4, 2:00||18||23/4, 1:00 – 5:00||Favourable|
|η-Aquariids||19/4 – 28/5||6/5||50||--||Poor|
|Southern δ- Aquariids||12/7 - 23/8||30/7||25||--||Poor|
|Perseids||17/7 - 24/8||13/8, 4:00 – 16:00||110||12/8, 21:30 – 13/8, 5:00||Excellent|
|Orionids||2/10 - 7/11||21/10||20+||21/10, 2:00 – 5:30||Fair|
|Leonids||6/11 - 30/11||18/11, 6:30||10-20||18/11, 2:00 – 5:30||Fair|
|Geminids||4/12 - 17/12||14/12, 20:30||120||14/12, 19:30 – 15/12, 6:00||Excellent|
Source of data: International Meteor Organization
Local Observation Condition is estimated by the effect of moonlight during maximum and whether the observation period is favourable to Hong Kong. It is divided into five levels, which are "Poor", "Unfavourable", "Fair", "Favourable" and "Excellent".