Leonid Meteor Display
The Leonids are a meteor shower that originates in the constellation of Leo the Lion, occurring in mid-November. At this time, planet Earth glides through a minefield of cosmic dust clouds from the tail of Comet Tempel-Tuttle. When Earth travels through the cloud we experience a Leonid meteor shower. The Leonids will peak on November 17 and November 18 in 2016, just after the Full Moon on the 16th. The waning gibbous moon means that the sky may not be dark enough to easily view the shower, but if you can get out of town, far enough away from the bright lights, you should be able to view at least some of the cosmic display.
Aside from their occasional spectacular displays, the Leonids are important for understanding the origin of meteors and their relationship to comets. They play an important role in trying to predict meteor showers themselves. This year, a moderate display is predicted.
Meteors are the light trails we see when small dust grains or rocks enter the Earth's atmosphere. Most of the meteors are caused by dust particles no bigger than a grain of sand. Meteor showers occur when a lot of dust and rock enter our atmosphere close together. These associations of dust and rock are thought to be debris ejected from comets (more rarely from asteroids) and the Leonids are associated with comet 55P/Temple Tuttle.
Dr Bill Cooke of NASA's Space Environments Group is an expert on the phenomenon. He tells us that our planet glides through the debris zone every year. It's like a minefield. Sometimes we hit a dust trail, sometimes we don't. Direct hits can spark a meteor storm, which is defined as more than 1000 shooting stars per hour. "That's what happened in, for example, 1966 and 2001," he says. "Those were great years for Leonids."
The meteors appear to radiate out from a single spot in Leo (the radiant). However, the meteors in fact are moving along parallel paths. They appear to radiate from this spot in Leo for the same reason that parallel railway lines, or any other parallel lines on the ground, appear to originate from a point on the horizon.
Comet 55P/Temple Tuttle orbits the sun with an orbital period of 33.2 years. In line with the comet's return, every 33 years or so the Leonids give an outstanding display. The last big Leonid shower was in 1966, where the number of meteors ranged from 40 per hour to an estimated peak of 200 per second!
Accounts of the Leonid storms date back to 585 AD. Each one is different, because at the return of the comet every 33 years or so, the encounter with Earth's orbit varies due to the gravitational effects of planets on the comet's orbit. This magnificent comet passed close by the Sun again on February 28, 1998.
During a storm, Earth crosses the outer regimes of the dust trail of comet Tempel-Tuttle. When the meteoric matter hits the Earth's atmosphere, new molecules and solid particles are formed that are of interest to the origin of life, the interaction of Earth and Space, and the issue of ozone depletion.
History of the Leonid Storm in Modern Times
Just before dawn on the night of November 12-13, 1833, the skies over eastern North America were lit up by thousands of meteors appearing from the region of the constellation Leo. Many thousands of people saw the event. A scientist named Denison Olmsted gathered many accounts
of the meteor storm, and, together with accounts of of an abnormal display of meteors seen in Europe and the Middle East the prior year, determined that the storm was caused by a discrete cloud of small particles orbiting the sun. That night marked the beginning of modern meteor astronomy.
A repeat of the Leonid storm ocurred on November 13-14, 1866. Shortly before this, a scientist named Hubert Newton identified Leonid showers as early as 585 AD, and he specified a period of about 33 years. In 1865 Comet Temple-Tuttle was discovered, and after its orbit had been calculated, and that of the Leonid stream in 1867, astronomers made the connection between the two.
The Leonids failed to produce a storm in 1899. It was discovered that the stream had been perturbed in intervening years by encounters with Jupiter and Saturn. A respectable display did occur in 1901, visible from the central and western United States.
A relatively weak display occurred in 1932, but enhanced activity was noted through 1939. The big event in 1966 was, however, on the way.
In 1965, the Leonids produced rates as high as 120 per hour. On the night of November 17th, 1966, observers all over the world settled in to watch. Observers across the United States, particularly in the west, were rewarded by a true meteor storm. Observers in Arizona reported rates
as high as 2400 per hour. In Hawaii, observer Mike Morrow saw rates as high as 83 per minute, or about 5000 per hour!
The Leonids approach the Earth nearly head on, and so appear very swift, often bright, with many leaving bright trails, known as trains, behind them. The visible path of the meteor will generally be long near midnight and become a bit shorter as the radiant rises higher in the sky.
Here endeth the Lesson...