When our planet plunges in that "gas" of particles of all sizes, from ones as small as micrometric specks of dust to ones as large as boulders, a shower occurs. It may last a few hours to full weeks, and it typically reaches a few hours-lasting maximum when the highest density of bodies enters the upper atmosphere, then fading away. Most meteoroids ignite as they reach an altitude of about 100 kilometers, but what we see from the ground is not exactly a combustion. Rather, it is the light emission resulting from the conversion of kinetic energy of the meteoroid as it progressively slows down in the thickening medium.
As Peter Jenniskens well exemplifies on page 46 of his foundational book "Meteor Showers and their Parent Comets", even if the efficiency of the energy-to-light conversion is a small 1%, a body with a tiny 0.008 grams mass moving at a speed of 71.6km/s will shine with 10000 watts of luminous power for a whole second, so at 100km distance it will be visible as a medium-brightness, mag. +3 star. Pebbles with a weight of a gram or more will produce awesome fireballs in the night sky.
While all dust particles in a cometary trail slowly move in random directions, having scattered away from their original position after one revolution around the Sun, what we observe from Earth is a coherent shower, where all meteor trails seem to originate from one single point in the sky, the so-called "radiant". Of course, in order to appreciate directly this phenomenon you would need rates of meteors large enough to enable youto observe more trails at the same time. That is very rare! It is something that only happened to me once in a lifetime of meteor observations, when I observed the peak of the Leonid showers and got several moments when three or even four meteors were simultaneously lit up.
What instead usually happens is that you will see one meteor somewhere, then another meteor in another part of the sky, and so on, each separated by a few minutes from the next. In fact, the typical rate of meteors, as seen from even a dark location under a clear night sky, very seldom exceeds 100 per hour, even during the peak of a shower. Unde such conditions one may easily overlook the "radiant" behavior of meteor streaks in the sky.
The radiant phenomenon is due to the fact that as we sit and watch, the Earth is moving through the filament of debris, intersecting dust particles moving in random directions. Wherever they enter the atmosphere, the particles will be seen to move as though originating from one point of the sky which is the intersection of the Earth speed vector with the filament.
Nowadays, in the digital imaging era, it is very common to see long-exposure pictures of the night sky where hundreds of meteor trails have been recorded, and the radiant is clearly seen as the originating point of all tracks. But when I was a kid, no such pictures existed - I only recall one very famous image of the fantastic Leonid shower of 1966 (see below), which does not show very clearly that there be a single originating point. That event was special because the Earth was crossing a particularly dense point of the dust trail of comet Tempel-Tuttle, something that happens roughly ever 33 years.
(Above: a 12' exposure of meteors from the 1966 Leonid shower)
So what did one do in my youth days to verify the originating point of a shower, and to compare the hypothesis that a particular meteor streak was coming from one shower or another (several showers are typically active during any given night)? One used to observe the sky visually, and draw on a star chart every trail. In addition, one used to record the exact time of each meteor, plus its magnitude, colour, speed (in radians per second). All those data were then sent to astronomers, as they were the best possible way to keep track of these phenomena.
Now, Enrico Stomeo is an amateur astronomer and a friend of mine, and he has been collecting data for the Italian Amateur Astronomers Union for years. In recent years he has equipped himself with all-sky cameras which can record hours of footage during clear nights. His software allows him to extract the data and automatically produce a star chart with all the observed meteors. He did so three days ago, when the peak of the 2020 Geminids was on, and he recorded thousands of meteors. The result is the astounding picture you see below.
Different colours refer to three different cameras, that each observe a 120-degree area of the sky; the small dash orthogonal to the tracks indicates the originating point of the meteor. There can be no doubt that all these meteors came from the Geminid filament!
As for this particular shower: the Geminids are a very famous shower, and one that was discovered in December 1861 by R.Philips Greg, who observed from Manchester, England. Ever since then, the shower has been observed in mid-December with a radiant point close to the position of Alpha GEM, a bright star in the Gemini constellation. The Zenith Hourly Rate of the shower -the number of meteors one would see if the radiant was at the zenith from a clear, dark sky - has been rising steadily during the past century, and it is now well into the 100 meteors per hour at peak. This is confirmed by this year's observation. According to models of dust density along the trail, however, this trend should invert this century, as the Earth will intersect regions of the filament that are progressively less dense.
I hope these few lines have convinced you that meteor showers are not only an awesome natural phenomenon, which calls for your attention and the occasional overnight watch, but also a very intriguing bit of solar system science!
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