Famous Leonid meteor shower arrives
One of the most famous and historically prolific meteors showers — the Leonids — will peak on the overnight period of Nov. 16 and 17, with the best viewing (weather permitting) during the midnight to pre-dawn hours of
Nov. 17.
The Leonids' radiant — the apparent point of origin in the night sky — lies in the constellation of Leo — the Lion, a prominent late autumn/early winter night sky constellation.
The Leonids are debris from Comet 59P/Tempel-Tuttle, a comet that takes about 33 years to orbit from deep space, around the sun, and back to deep space. The comet was named for astronomers Wilhem Tempel and Horace Tuttle, who independently discovered the comet in 1865 and 1866, respectively. The comet's last perihelion (closest approach to the sun) was in 1998, with the next scheduled for 2031.
The Leonids have been known since 902 AD when Chinese astronomers noted that the stars fell as rain, and determined that the shower's radiant lay in the Leo constellation.
The Leonids have provided some amazing meteor storms (when the number of falling meteors exceeds 1,000 per hour). In 1630, a Leonid meteor storm of more than 1,000 per hour fell just two days after the funeral of the great German astronomer Johannes Kepler (a fitting tribute), while, in 1799, the Prussian scientist Alexander von Humboldt recorded thousands of bright Leonid meteors in the space of four hours, including countless fireballs with long smoke trails, while travelling in what is now Venezuela.
In 1833, as the Leonids fell over North America, numbers ranged from 100,000 per hour on the east coast to over 240,000 per hour on the west coast. In 1866, when Comet Tempel-Tuttle was confirmed, it was the Leonid shower/ storm (as hundreds of meteors per hour fell over America, and thousands per hour fell over Europe) that ultimately led to the Leonids being associated with this comet.
Thousands of Leonids fell in just 15 minutes in 1966, and again in 2001 and 2002 (the last Leonid storm).
WHAT IS A COMET?
Think of a comet as a dirty snowball, composed of countless dust and small, rocky particles mixed with primordial ices, the remnants of when our solar system was first formed billions of years ago.
The vast majority of comets are located within the Oort Cloud, a vast ring of icy bodies located millions of kilometres from the sun, at the farthest edge of the solar system. Periodically, these comets get nudged from their domain by perturbations caused by our solar system's other planets (most notably Jupiter) or some other distant celestial body, whereupon the sun's gravity draws them into the inner solar system, and, ultimately, around itself.
As they swing in around the sun, they often get caught in a perpetual loop orbit, which causes them to periodically re-orbit the sun (in TempelTuttle's case, approximately every 33 years).
As comets circle the sun, approaching perihelio (the point they are closest to the sun), the surface ices sublimate (go directly from solid to gas), releasing the dust/rock particles. These particles are then pushed by the sun's radiation out behind the approaching comet, eventually forming a conveyor belt-like stream of material that follows the comet around the sun.
Each passage uncovers a fresh lair of debris; some passages leave large amounts of debris, while others only a relatively small debris stream. This creates a series of dense and less-dense sections in the debris stream orbiting the sun, which explains why the number of meteors seen in any given year often varies significantly. Large numbers signify interception of a dense part of the debris stream, while low numbers a less-dense section.
For example, the famous Leonid meteor storm of 1866 occurred when Earth passed through a dense debris section left by Tempel-Tuttle's 1733 passage. As the Earth intercepts the conveyor belt of comet debris moving around the sun, it slams into the debris particles. As the debris passes through the Earth's atmosphere, it heats up and disintegrates, appearing as meteors. The size of the particle passing through the atmosphere determines the brightness and longevity of the meteor.
The speed at which the Leonid debris particles hit the Earth's atmosphere is an astonishing 260,000 kilometres per hour, with the smaller particles sketching a fleeting bright streak in the night sky, and the larger particles creating brighter, longer-lasting fireballs (some measured at -1.5 magnitude), some of which leave smoke trails or trains behind as they burn up in the atmosphere.
WHAT TO EXPECT
Unfortunately, astronomers do not anticipate Earth will pass through a dense section of the Comet Tempel-Tuttle debris stream this year, and are only predicting an average zenith rate (when the radiant is at its highest point in the night sky, normally after midnight), of 15-20 bright meteors visible away from urban lights.
The waxing, crescent moon (one day after the new moon) will be absent from the night sky on the overnight period of Nov. 16 and 17. The predicted number of meteors/hour is a best guess estimate. Low numbers have, upon occasion, turned into significantly high numbers, so you could be pleasantly surprised at what this year's Leonid shower may offer.
THIS WEEK'S SKY
Mercury (magnitude -0.7), now moving closer to the sun, is a barely discernible predawn object this coming week. Rising around
5:40 a.m., it reaches 11 degrees (a little over a hand's width at arm's length) above the southeast horizon before fading from view as dawn breaks around 6:55 a.m.
Venus (magnitude -3.9) is visible in the eastern, predawn sky, rising about 4:30 a.m., and reaching 22 degrees above the southeast horizon before it, too, fades with the break of dawn.
Mars (magnitude -1.6) becomes visible in the evening sky as dusk deepens around 5 p.m., 20 degrees above the eastern horizon, reaches 48 degrees altitude above the southern horizon around 9:25 p.m., and remains observable until about 3 a.m., before sinking below eight degrees above the western horizon.
Jupiter (magnitude -2.1) appears 20 degrees above the eastern horizon about 5 p.m., then sinks towards the western horizon, setting around 8:30 p.m.
Saturn (magnitude +0.6) appears a short time after Jupiter, 21 degrees above the southern horizon by 5:20 p.m., before heading towards the horizon and setting shortly before 9 p.m.
Look for the waxing, crescent moon to the left (east) of Jupiter and Saturn on the evening of Nov.19.
Until next week, clear skies.
EVENTS
• Nov. 17 — Leonid meteor shower peak (pre-dawn)
• Nov. 22 — First quarter moon Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. His column, Atlantic Skies, appears every two weeks.