What Is A Meteor Shower? Why Do Meteor Showers Occur? — Causes, Mechanisms, Naming, & History

January 14, 2015 in Space

Facebook Twitter Plusone Reddit Linkedin Stumbleupon Email Tumblr

What is a meteor shower exactly? Why do meteor showers occur? What are the best dates, times, and places to watch them? How are they named?

All good questions — and ones that this article will address. With an overview to be given to the modern history of the science that’s behind our current understanding of the phenomena, the mechanisms and causes behind the appearance of meteor showers in the skies, and the system of naming that the modern names are determined by.

As well as, perhaps most importantly, an overview of the (generally) best times, dates, and places to observe them at/from. Enjoy.

Perseid meteor shower

History of Modern Scientific Explanations of the Subject

While meteor showers have no doubt been observed by humans for as long as humans have been around (at least 200,000+ years), the modern/scientific views on: what they are and how to predict them; and the names that we use today; originated not all that long ago, in the early 1800s.

To be exact, to the date of one of the largest meteor storms (huge meteor showers) of recent times — the Great Leonids Meteor Storm of November 1833. By some estimates, that meteor storm saw the observation of more than 100,000 meteors an hour — other estimates are even higher, with the possibility of more than 200,000 meteors an hour being seen from the Eastern board of North America. An absolutely enormous storm, much larger than any we’ve seen in recent times.

Needless to say, this event sparked great curiosity in many people — and, in particular, in an American by the name of Denison Olmsted (1791−1859), whose explanation of the phenomena sparked further inquiries.

Leonids meteor storm shower

His explanation being based on the observations that: the (1) shower wasn’t observed from Europe, (2) that the meteors appeared to radiate from the constellation of Leo, and (3) that it was of limited duration; which led him to speculate that they were the result of a “cloud of particles” in space.

While his idea attracted support, the science of the phenomena didn’t develop much more than that until the latter parts of that century — when an Irishman by the name of George Johnstone Stoney (along with a Brit by the name of Arthur Matthew Weld Downing) attempted to calculate to an “exact” degree the position of the “dust clouds” in the Earth’s orbit around the Sun.

They did this via the study of the “dust ejected in 1866 by comet 55P/Tempel-Tuttle in advance of the anticipated Leonid shower return of 1898 and 1899. Meteor storms were anticipated, but the final calculations showed that most of the dust would be far inside of Earth’s orbit. The same results were independently arrived at by Adolf Berberich of the Königliches Astronomisches Rechen Institut (Royal Astronomical Computation Institute) in Berlin, Germany.”

And sure enough, no meteor storms were seen that season — giving some credence to the calculations used. Move ahead now to the 1980s, when an ED Kondrat’eva and an EA Reznikov of Kazan State University were the first to correctly identify the years when dust was released which was then the cause of previous Leonid meteor storms. After that, 1995 saw Peter Jenniskens accurately predict the 1995 Alpha Monocerotids outburst. Followed by Jenniskens, in 2006, publishing predictions for “future dust trail encounters covering the next 50 years” (you can search through those predictions here).

Mechanisms & Causes of Meteor Showers

Meteor showers are caused by the interlinked interactions of planets (the Earth, for instance) and streams of debris left behind my passing comets. These comets, as they make their way through the inner solar system, leave behind solid chunks, grains, dust-particles, etc, of rock that’s slowly released as the ice (frozen water, ammonia, methane, etc) is sublimated/vaporized by the radiation coming from the Sun.

These now independent meteoroids are spread out across the whole of the comet’s orbit-path — forming what’s referred to commonly as a meteoroid stream or a dust trail.

It’s recently (relatively) been noted that some of the stronger meteor showers that we experience may not be caused by the debris generated by this steady sort of release, but rather by more substantial “disintegrations” — as examples, it’s now though that the Quadrantids originate from an asteroid that broke up ~500 years ago, and that the Geminids originate from a asteroid that broke up ~1000 years ago.

Constellations big dipper

Naming Conventions & the Radiant Point

As per current naming conventions, meteor showers receive their names from the constellations that their meteors seem to originate — or radiate, from. This point of apparent radiation is known as the radiant.

This apparent origination from a particular portion of the sky is down to the fact that all of the particles are travelling in a near-parallel path (as related to the Earth’s path), and at a similar velocity. The image of the parallel lines and edges of a road all appearing to converge in the distance is down to the same (or a very similar) effect.

So, when a meteor shower’s meteor appear to originate from the constellation of Gemini, they’re known as the Geminids; when from the constellation of Draco, the Draconids; when from the constellation of Perseus — the Perseids; etc.

The exact radiant point of course drifts some over the passage of time — whether in a single night of watching, or over years, etc.

“Naming Meteor showers are named after the nearest constellation or bright star with a Greek or Roman letter assigned that is close to the radiant position at the peak of the shower, whereby the grammatical declension of the Latin possessive form is replaced by “id” or “ids”. Hence, meteors radiating from near the star delta Aquarii (declension “-i”) are called delta Aquariids.”

Meteor shower 2015 perseid

Best Times, Dates, & Places To Watch A Meteor Shower

There are good meteor showers to be seen throughout much of the year, but the best are entered around the summer and the fall/winter currently. In particular, the Perseids and Geminids offer excellent shows most years — depending on your cold tolerance you may prefer one over the other regardless of the number of meteors seen per hour. (You can find a full list of the year’s major meteor showers here: Meteor Showers 2015: Perseids, Lyrids, Geminids, Leonids, Draconids, Orionids, Etc — Dates & Times)

While the specific dates of the year to watch are rather variable, the times of day/night that are best to watch aren’t (particularly anyways) — it’s usually best to watch for meteors in the early-morning hours, a few hours before sunrise (not withstanding a bright Moon). This is down to the fact that, by that point, the radiant point of most meteor showers is approaching its highest point in the sky (this is usually the case, but not always, and varies somewhat based on latitude).

While that’s generally a good rule-of-thumb, it’s certainly no absolute — the main thing is that you want the sky as dark as possible. Which means taking into account the Moon (it’s rising/setting times, as well as its phase), and the sunset/sunrise times — as well as the facts of the meteor shower itself.

And, as always, the darker and more rural the location that you’re watching from the better — that alone, goes a very long ways. Other factors that play a part are: the humidity of the air (the drier the better), the temperature (colder tends to be better), and altitude.

References: IMCCE; NASA; Wikipedia; Memory

Image Credit: Public Domain; Perseid via Flickr CC; Constellations via Flickr CC; Perseids + Milky via Flickr CC

Facebook Twitter Plusone Reddit Linkedin Stumbleupon Email Tumblr