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If you need an excuse to bundle up, sip hot chocolate, and stare at the sky for a few hours, the Geminid meteor shower delivers. Every December, this reliable shower turns the night sky into a flickering light show with bright, colorful streaks flying from the direction of the Gemini constellation. Unlike most major showers that make you stay up until after midnight, the Geminids start early, making it a great event for younger sky-watchers…….Continue reading….
Source: Discover Magazine
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Critics:
A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth’s atmosphere at extremely high speeds on parallel trajectories. Most meteors are smaller than a grain of sand, so almost all of them disintegrate and never hit the Earth’s surface.
Very intense or unusual meteor showers are known as meteor outbursts and meteor storms, which produce at least 1,000 meteors an hour, most notably from the Leonids. The Meteor Data Centre lists over 900 suspected meteor showers of which about 100 are well established. Several organizations point to viewing opportunities on the Internet. NASA maintains a daily map of active meteor showers.
Historically, meteor showers were regarded as an atmospheric phenomenon. In 1794, Ernst Chladni proposed that meteors originated in outer space. The Great Meteor Storm of 1833 led Denison Olmsted to show it arrived as a cloud of space dust, with the streaks forming a radiant point in the direction of the constellation of Leo. In 1866, Giovanni Schiaparelli proposed that meteors came from comets when he showed that the Leonid meteor shower shared the same orbit as the Comet Tempel.
Astronomers learned to compute the orbits of these clouds of cometary dust, including how they are perturbed by planetary gravity. Fred Whipple in 1951 proposed that comets are “dirty snowballs” that shed meteoritic debris as their volatiles are ablated by solar energy in the inner Solar System. Because meteor shower particles are all traveling in parallel paths and at the same velocity, they will appear to an observer below to radiate away from a single point in the sky.
This radiant point is caused by the effect of perspective, similar to parallel railroad tracks converging at a single vanishing point on the horizon. Meteor showers are normally named after the constellation from which the meteors appear to originate. This “fixed point” slowly moves across the sky during the night due to the Earth turning on its axis, the same reason the stars appear to slowly march across the sky. The radiant also moves slightly from night to night against the background stars (radiant drift) due to the Earth moving in its orbit around the Sun.
See IMO Meteor Shower Calendar 2017 (International Meteor Organization) for maps of drifting “fixed points”. The geocentric velocity of the meteors can vary considerably between showers. For example, the velocity is around 27 km/s for the Taurids and 71 km/s for the Leonids. (Compare to the Earth’s average orbital velocity of 29.8 km/s.) Incoming meteors produce a measureable light curve along their trajectory, which varies in brightness by the rate of ablation.
The observed heights for meteor ionization is from 70 to 110 km, where the atmosphere is sufficiently dense to heat the projectiles. A typical meteor in a shower has a diameter of 5 μm with a density of 2 g cm−3. It starts to melt at a temperature of around 1800 K. As the Earth rotates, the shower rate will be low when the radiant point is near the horizon, then it will rise to at least 50% of maximum when the radiant point reaches an altitude of 30° above the horizon.
Optimum viewing is when the radiant point is at an angle of 45°, or half way up the sky, as the meteors are still passing through a thicker column of air. The longer, more prominent trails will then be observed 30–60° away from the radiant point. Most meteor showers improve their visibility after midnight, as the observer’s position becomes more oriented toward the direction of the Earth’s orbit around the Sun.
For this reason, the best viewing time for a meteor shower is generally slightly before dawn — a compromise between the maximum number of meteors available for viewing and the brightening sky, which makes them harder to see. 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.”t
Hence, meteors radiating from near the star Delta Aquarii (declension “-i”) are called the Delta Aquariids. The International Astronomical Union’s Working Group on Meteor Shower Nomenclature and the IAU’s Meteor Data Center keep track of meteor shower nomenclature and which showers are established.The peak rate of a meteor shower is measured by the zenith hourly rate (ZHR), which is the expected number of meteors visible to the naked eye when the radiant point is at the zenith;.
That is, at the highest point in the night sky. For younger streams that are more clumpy, the rate can vary year to year with peak meteor “storms” occurring with the orbital period of the stream. Some storms have measured hundreds and even thousands of meteors per hour. The showers typically showing the highest ZHR are the Perseids (75/hr), Geminids (75/hr), and Quadrantids (60/hr).
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