The perseids, or tears of San Lorenzo, they are a meteor shower that every year appears in the constellation of Perseus. Those who look up between the 9th and 13th of August approximately will see a multitude of luminous lines in the night sky..
It is the best-known meteor shower, which at its peak can produce up to 80 meteors per hour or more, depending on the geographical location and the atmospheric conditions of the moment, but it is not the only shower..
Throughout the year there are showers of stars in various points of the sky, however, the Perseids, apart from having a high rate of meteors / hour, occur on pleasant summer nights in the northern hemisphere, which is why they are so popular among the observers.
The Perseids were already known to the Chinese around AD 36. At some point in the Middle Ages, Catholics baptized this annual meteor shower with the name of the tears of Saint Lawrence, deacon of the church of Rome, martyred in that city on August 10, 258, under the reign of Emperor Valerian..
Naturally there were debates about their origin and also about that of the sporadic shooting stars. The general consensus for a long time was that they were simply atmospheric phenomena, but in the early 19th century, several astronomers correctly identified them as a celestial phenomenon..
Meteor showers are named after the constellation from which they appear to come, an effect due to perspective, since the meteor trajectories being parallel, in the view of the observer on Earth, they seem to converge at a point called radiant.
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Towards the beginning of the 19th century, scientists such as Alexander von Humboldt and Adolphe Quetelet hypothesized that meteor showers were atmospheric phenomena..
The discussion about the true nature of shooting stars intensified after the Leonids, another shower that regularly appears in November, was particularly intense in 1833, in the eastern United States..
After painstaking studies, American astronomers Denison Olmsted, Edward Herrick and John Locke independently concluded that meteor showers were caused by fragments of matter that the Earth encountered while traveling its annual orbit around the Sun..
Some years later, in 1866, the Italian astronomer Giovanni Schiaparelli discovered the link between the orbits of comets and the meteor showers, proving that the orbit of Comet Tempel-Tuttle coincided with the appearance of the Leonids..
In this way, he proposed the hypothesis that the rains were nothing other than the encounter of the Earth with the remains left by comets whose orbit took them close to the Sun..
Thus, meteor showers like the Perseids have their origin in comets and also in asteroids, objects that, like planets, also belong to the Solar System. They are fragmented by the gravitational attraction that the Sun exerts and the remains are scattered in the form of dust throughout the orbit..
This powder is made up of particles of different sizes, almost all the size of a micron more or less -one thousandth of a millimeter-, although there are fragments with a much more appreciable size..
When colliding with the Earth's atmosphere at high speed, the ionization of the molecules in the atmosphere produces the trail of light commonly called Shooting Star. In the case of the Perseids, they meet the Earth at an approximate speed of 59-61 km / s. The higher the speed, the greater the luminosity of a meteor.
The comet that gave rise to the Perseids is 109P / Swift-Tuttle, discovered in 1862 and with an approximate diameter of 26 km. The time it takes for this comet to travel its elliptical orbit around the Sun - the period - is 133 years.
It was last seen in December 1992 and calculations indicate that it will pass very close to Earth around 4479, and it is already a matter of concern for some, since its diameter is more than double that of the asteroid that is believed to have caused the extinction of the dinosaurs.
The Perseids begin their activity in mid-July and end in mid-August of each year. The maximum of activity generally coincides with the feast of San Lorenzo, around August 10.
Or point of the celestial sphere from where the trajectory of the shooting star seems to originate. The radiant of the Perseids is in the boreal constellation of Perseus.
Or zenith rhythm, whose value is in meteors / hour and depends largely on the conditions of the observation. For the Perseids, it is estimated at 75-100 meteors / hour in dark and well-clear skies..
The Perseids occupy the third place among the major showers of stars, although as said, the zenith rhythm can vary to a greater or lesser extent. This is the case of the Leonids, which from time to time go from being rains to true meteor storms.
During the passage of the associated comet, the zenith rate of a shower can increase considerably. It also happened with the Perseids in 1993, after the return of the Swift-Tuttle.
With which the brightness profile of the meteor swarm is obtained. It depends on the mass and speed of the incident particles.
The population index is denoted as r. Values of r between 2.0 and 2.5 mean brighter than average swarms, and as the value of r increases, brightness decreases.
The Perseids are well known for the number of fireballs o fireballs that produce. Instead of settling to leave a trail of light in the sky and disappear, the racing cars are accompanied by great explosions of light, color and even sound.
In addition, fireballs are much brighter than an ordinary shooting star, being able to equal in brightness to Venus or Jupiter, that is, they have apparent magnitudes greater than -3.
The fireballs are due to encountering much larger than average particles. The large number of Perseid fireballs is explained by the huge nucleus of Comet Swift-Tuttle, which leaves behind fragments - known as meteoroids- of considerable size.
Although fireballs are almost never a great danger, some very massive ones that have hit the ground have caused considerable damage. The Tunguska event in Siberia in the early 19th century is believed to have been caused by a fireball impact..
More recently, the 2013 Chelyabinsk fireball in the Urals caused property damage and numerous injuries. The sound of the impact could be recorded even in Antarctica.
Fortunately, observing the Perseids does not require the use of instruments. The best observations are made with the naked eye, but the chosen place must meet certain conditions, such as being away from light pollution and from trees and buildings that obstruct the visual field..
Make sure the moon is low on the horizon, otherwise you can barely make out shooting stars. The most appropriate time is after midnight, usually two or three hours before sunrise, since at that time the Earth runs directly into the meteors..
The radiant should be high in the sky, so it is recommended to observe the rain reclining in an extendable chair or lying directly on the ground, but it is not necessary to look directly at the radiant. Meteors are coming from all directions.
You have to include everything that contributes to making observation comfortable, since it is a labor of patience, so you have to bring food, drink, lanterns with dim light, insect repellent and a smartphone with astronomical applications.
These are a great help to locate in the night sky and find the radiant, they also provide important data and some even offer advice on photographing the event for a memorable experience..
For those who want to combine their love of astronomy with photography, here are some tips to get good shots:
-Choose a dark area with little light pollution. The moon should not be high in the sky at that time.
-The radiant of the meteor shower should be above the horizon, at 40 ° or a little more, preferably.
-Use a single lens reflex camera to regulate the exposure time, or a compact camera with manual mode and good quality.
-With a wide angle you can capture more space of the sky and increase the chances of recording more shooting stars.
-Bring spare batteries, especially if the night is cold.
-The use of a tripod is mandatory to avoid vibrations.
-Have a trigger cable, to avoid touching the camera and adding unwanted vibrations. Simply program the trigger and enjoy the view of the sky without interruptions. It is recommended to set the shooting interval between 2 and 5 seconds.
-A large aperture should be used to capture as much light as possible.
-High ISO to record objects with lower brightness.
-The Earth moves, so the exposure time must be taken into account, so that the stars appear as points and not as lines.
-The hyperfocal distance It is important, it is the distance at which the largest focused area in the image is obtained, and with it a greater depth. There are applications to obtain the optimal value.
-Establish a good white balance, depending on the lighting conditions.
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