They began to cross the night sky in the second half of July, but the real spectacle of the Perseids takes place in mid-August. They receive the popular name of 'tears of San Lorenzo' because it is easy to see them on the 10th of this month, the festivity of the Spanish martyr of the same name, but in reality their peak activity takes place between the 11th and 13th. Specifically, this year in Europe it will be the night of Friday 12 to Saturday 13 August, with a peak around 00:03 (Spanish peninsular time), according to the Institute of Astrophysics of the Canary Islands (CAR).
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The Perseids are a shower of meteors, commonly called "shooting stars", of cometary origin and visible from the entire northern hemisphere in midsummer.
Its activity rate can reach up to 200 per hour, although it is usual to see around a hundred, and this year, in addition, there will be much less because just on the 12th there is a full moon, which will make it difficult to observe.
"If the conditions were ideal, around a hundred shooting stars could be seen per hour, but the brightness of the Moon will be one of the factors that will cause the real number of visible Perseids to drop to about fifty," says José María Madiedo, researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC).
In any case, the exact number of observable meteors per hour is highly variable. Specific predictions about their specific number, depending on the day and time, are difficult to make and are usually affected by high uncertainty, clarifies the National Astronomical Observatory (NAO-IGN).
The full moon will make it difficult to observe the 'shooting stars' this summer, so it is advisable to look towards the darkest areas in the opposite direction when it is present
Radiant towards the constellation of Perseus
The Perseids can be seen anywhere in the sky, although they seem to come from a specific area, its radiant: the constellation Perseus (hence its name). For observers located in the Iberian Peninsula (latitude 40º north), the radiant of the Perseids is located to the northeast above the horizon throughout the night.
To enjoy these 'shooting stars' it is not necessary to use telescopes or any other type of optical instrument that limits the field of vision. It is enough to observe the sky from a place that is as dark as possible and away from the light pollution of the populations. It is preferable that there are few obstacles to the view, such as buildings, trees or mountains.
Look for dark skies
It is convenient to direct the gaze also towards the darkest areas, in the opposite direction to the position of the Moon if the observation is made when it is present. The most comfortable thing is to lie down and wait for your eyes to get used to the darkness.
"As in previous years, it is necessary to find a place far from urban centers, fix your eyes on a point in the sky and wait patiently to see some of the light traces of the Perseids," says Miquel Serra Ricart, an astronomer at the IAC, who confirms that it is not a good Perseid year: "The full moon will make it difficult to see the weakest meteors, so their frequency will be lower and we will only observe the brightest ones, which will continue to be impressive."
Serra-Ricart, in collaboration with Portuguese institutions, will rebroadcast the night of August 12 to 13 the maximum activity of this shower of 'shooting stars' from La Palma and Madeira, a joint initiative to raise awareness among the population about the problem of light pollution.
The trail of Comet 109P/Swift-Tuttle
The Perseid shower is produced by the impact of particles in our atmosphere left by comet 109P/Swift-Tuttle, which has a period of 133 years and last passed close to the Sun in 1992. This object was discovered in 1862, and, with an approximate size of 26 km in diameter, it is the largest that periodically approaches the Earth.
As it describes its orbit around the Sun, the comet leaves behind a trail of gases, dust, and debris (rocky material). Every summer the Earth runs into this comet trail and some of the rocky fragments (meteoroids) are caught in its gravitational field. During this encounter, some particles disintegrate as they enter the Earth's atmosphere at high speed, creating the colorful luminous streaks that receive the scientific name of meteors.
“Most of the meteoroids that break off from this comet are as small as a grain of sand, or even smaller. When they cross our planet, they enter its atmosphere at more than 210,000 kilometers per hour, which is equivalent to traveling across our country from north to south in less than twenty seconds”, compares José Luis Ortiz, researcher at the IAA-CSIC.
At these speeds, the collision with the Earth's atmosphere is so sudden that the temperature of these particles increases to about 5,000 ºC in a fraction of a second, for which they disintegrate, emitting a flash of light called a meteor or, more popularly, a shooting star. It is therefore not true stars, but incandescent particles.
The disintegration occurs at high altitude, usually between 100 and 80 kilometers above ground level. Larger particles (pea-sized or larger) can produce much brighter shooting stars, called bolides or fireballs.