Meteor showers happen when the Earth’s orbit carries us through zones of debris left in interplanetary space by periodic comets (and some asteroids). Over centuries, or longer, the 1–2 gram particles, typically ranging in size from sand-size to small pebbles, accumulate and spread out into an elongated cloud along the comet’s orbit. (It’s analogous to the material tossed out of a dump truck as it rattles along. The roadway gets pretty dirty if the truck drives the same route many times!) If the Earth’s orbit intersects the comet’s orbit, we experience a meteor shower that repeats annually because the Earth returns to the same location in space on the same dates every year.
As we traverse the cloud, our planet’s gravity attracts the particles, and they burn up as “shooting stars” while falling through Earth’s atmosphere. Entering at speeds ranging from 40,000 to 256,000 km/h, the kinetic energy of each particle ionizes the air molecules it encounters, leaving a long trail of glowing gas that is less than a metre wide, but many kilometres long. Most trails occur in the thermosphere, the region of the atmosphere situated between about 80 to 120 km above the ground. Slower meteors need to reach the denser atmosphere at lower altitudes before forming a trail. As viewed from the Earth’s surface, on a clear, dark night, the trails glowing briefly as they streak in front of the distant stars.
The colours that meteor watchers see come from a combination of factors. Red light is emitted by the glowing air molecules. Meteor material itself is being burned up during its high-speed passage. Oranges, yellows, blues, greens and violets are produced when minerals — such as sodium, iron and magnesium — in the particles are vaporized. Scientists can analyze the spectrum of colour in a meteor trail and determine the composition of the material. If anything survives to land on the ground, it becomes a meteorite. An easy way to remember the difference is to recall that many rock names end with “ite,” like granite.
The duration of a meteor shower depends on the width of the zone — that is, how many days or hours it takes Earth to pass through it. A meteor shower commences on the particular date when the Earth first enters the debris field. The shower then builds to a peak when Earth passes through the densest portion of the cloud, and then tapers off — ending on the date when the planet exits the zone.
The number of meteors produced during a shower depends on whether Earth passes through the densest region, or merely skirts the edges. Since the debris is also orbiting the sun, the clouds vary in density on a timescale related to the source comet’s orbital period, producing years when more, or less, material is encountered. Planetary scientists create models of the debris distribution to help predict the intensity of showers each year.
The inherent brightness of the meteors is dictated by the average size of the grains in the cloud. Some showers are known for having fewer, but brighter meteors, while others are dimmer, but much more prolific. Most showers are global events, but certain showers are better for observers in the Northern or Southern Hemisphere due to the Earth riding high or low through the cloud.
During a shower, meteors can appear anywhere in the sky, but all the members of one shower will be traveling away from a particular part of the sky, called the radiant. This point always lies within the constellation that gives the shower its name: Orion for the Orionids, Perseus for the Perseids, and so on. The radiant simply marks the direction in the sky that the Earth is heading toward while the planet is traversing the debris cloud. (This rule can be broken when two showers are underway at the same time, and also by random meteors, called sporadics, which aren’t part of the main debris field.)
The Leonid Meteor Shower or Leonids runs from November 5th to 30th, and peaks overnight on Friday, November 16/17. The radiant point is within the sickle forming the lion’s head in the constellation Leo (the Lion), which rises in the east around midnight. The meteors are thought to be leftover material from periodic Comet 55P/Tempel-Tuttle. Leonid meteors are typically fast and bright, with many having persistent trails. The expected peak rate is approximately 15 per hour. The New Moon will leave the sky nice and dark on the peak nights.
Throughout the shower period, the best time to see meteors is when the sky overhead is plowing straight into the debris field, like bugs splatting on a moving car’s windshield. That’s because, when the constellation containing the radiant is overhead, the entire sky is available for observing. When a radiant lies near the horizon, the Earth will hide up to half the meteors. A common term used to rate the activity of a meteor shower is its ZHR, or zenith hourly rate. This is the estimated number of meteors you’ll see in one hour when the radiant is overhead. It’s based upon historical reports and computer models.
To see meteors, try to find a dark viewing location that has as much open sky as possible. Light pollution washes out the night sky, dramatically reducing the number of fainter meteors you will see. So a drive to a park or rural site away from city lights is helpful. To preserve your eyes’ dark adaptation, remember to turn your phone’s screen brightness to minimum or, even better, cover it with a red film. Disabling app notifications will reduce the chances of unexpected bright light, too.
You can start watching as soon as it’s dark. While the peak number might occur before dawn, meteors will still be visible before midnight, too. Don’t worry about looking directly at the radiant. Bring a blanket and a chaise to avoid neck strain. And remember that binoculars and telescopes will not help: Their field of view is too narrow to see the long meteor trails. If you have friends or family along, don’t look at each other while chatting. Keep your eyes to the skies!
Text credit: Chris Vaughan
