The Perseids They are a celestial spectacle that takes place every year on the night of Saint Lawrence (August 10) due to a rain of shooting starsright? No, wrong. These are not stars but a meteor shower composed of many small meteors, that is, rocky debris. The Earth passes through this shower between the end of July and the end of August, with a peak that falls not on August 10 but on August 12th.
What are the Perseids, the “shooting stars” par excellence?
First of all, it is always worth remembering that although it is often referred to as “shooting stars”the phenomenon of the Perseids has nothing to do with the stars which shine peacefully in the night sky: this is in fact an effect due to the passage through the Earth’s atmosphere of tiny fragments Of rock And powders of spatial origin which, in their movement around the Sun, they cross theEarth’s orbit.
These fragments, called meteoroids, when they pass through our planet’s atmosphere, due to their speed they burn in a spectacular manner, leaving behind a luminous trail, called meteor. In some cases, if the fragments are large enough not to be completely destroyed by the high temperatures developed, they manage to survive when passing through the atmosphere, and then being found on Earth as space rocks, called meteorites. It is therefore a “local” phenomenon, which occurs in the atmosphere of our planet, and has nothing to do with the stars real and proper.
Someone meteors can be observed almost every night, if we move away from artificial lights and let our eyes get used to the darkness. But during some periods, which we find every year on the same dates, the number of meteors that can be observed increase particularly, up to one hundred meteors every hour. These are the “meteor showers”: this happens when the Earth, in its motion of revolution around the Sun, it passes through a region of space where fragments of dust lost by a smaller celestial body, such as a comet or from a asteroid.
For an effect of prospect, meteors that are part of the same swarm all seem to come from a specific area of the sky, called radiant. Each meteor shower is then named after the constellation where its radiant is found: the Lions from the constellation of Lion, the Geminids from that of the Twins, the Lyrids from the constellation of the Lira, and so on. In the case of the meteor shower of mid-Augustthese are called Perseids, from the constellation of Perseus.
When do the Perseids occur?
There are several traditions linked to this meteor shower. In Italy they are called as we said “tears of San Lorenzo” and are traditionally linked to the figure of the early Christian saint Lorenzo from Osca (Huesca), in Spain. According to Catholic hagiography, Lorenzo was one of the deacons of Romekilled in 257 during a persecution ordered by the emperor Valerianand is remembered in the liturgical calendar on the date of August 10th. This association, although famous, is however rather recent, and dates back to the nineteenth century.
In fact, in conditions of optimal visibility, the meteor shower of Perseids it is visible from July 17th to August 24thwith the maximum corresponding to the August 12thso 2 days after the traditional “night of San Lorenzo”. However, due to the difference between the length of the year relative to the position of the Sun (tropical yearon which the civil calendar year is also based) and the length of the year in relation to distant stars (sidereal year), the event date moves forward by 1 day every 72 years: the maximum of the Perseids therefore corresponded to the “night of San Lorenzo” between 10 and 11 August about 200 years ago, in accordance with the first scientific description of the phenomenon made by the Belgian astronomer Adolphe Quetelet in 1836.
Where do they come from?
The association between the meteor showers and the fragments resulting from a minor celestial body in orbit around the Sun is instead due to the Italian astronomer John Virginio Schiaparelli. In 1866, following the passage of the Comet Swift-Tuttle at the closest point to the Sun in its orbit a few years earlier in 1862, Schiaparelli noticed a increase of the number of meteors in the Perseid shower: he attributed this fact to the greater loss of material from the comet due to the close pass to the Sun, and therefore hypothesized in a correspondence with his colleague Angel Secchi the connection between recurring meteor showers and the orbits of some comets.
Based on this intuition, it was possible to find associations between other meteor showers and smaller objects of the Solar system, like the Leonids and the comet Tempel-Tuttle, the Lyrids and the comet thatcher, the Geminids andasteroid Phaeton, and the Eta Aquarids and the Halley’s Comet.
The comet Swift Tuttle (official designation 109P/Swift-Tuttle) is a periodic comet in the Solar System, belonging to the same family as Halley’s Comet. It has a diameter of 26 kilometers and a period of 133.28 years, and was first discovered in 1862 by Lewis Swift and from Horace Parnell TuttleIts orbit is very elongated, and the door from one distance from the sun similar to that of the Earth (1 astronomical unit), up to a distance of about 50 times greater (51 astronomical units), greater than Pluto’s maximum distance from the Sun.
