The Perseids is an unusually rich meteor shower that is one of the 10 biggest meteor showers of the year. It is also a sky watcher’s favorite, not just for the abundance of its meteors but because, unlike other showers which can exceed it in maximum output (such as the Quadrantids and Geminids), the Perseids enjoy a prolonged period of enhanced activity that can be viewed over more than a week.
The Perseids also take place for up to six weeks between mid-July and August, the warmest time of the year for northern hemisphere residents, and a pleasant time for camping out and viewing this spectacular celestial event. Its radiant, or point of apparent origin, lies within the constellation of Perseus, with the name “Perseids” derived from the Greek word“ Perseides”, referring to “the sons of Perseus” in Greek mythology.
Quick facts about the Perseids
- Discovery: 36 AD (First recorded)
- Parent Body: Comet Swift–Tuttle
- Constellation: Perseus
- Right Ascension: 03h 04m
- Declination: +58°
- Date of Peak: August 12 (July 23–August 20)
- Velocity: 58 km/s
- Zenith: hourly rate 80
Origin of the Perseids
As comets approach the Sun, they sacrifice varying amounts of dust, gas, and other debris as they heat up. This process is known as “sublimation”, and the stream of dust and small-diameter debris released can persist in the orbit of the progenitor comet for thousands of years. In the case of the comet linked to the Perseids, Swift-Tuttle, the stream of debris is known as the Perseid cloud, and the shower occurs as the Earth passes through it during its orbit around the Sun.
Comet Swift-Tuttle is a relatively short-period comet with an orbital period of 133 years, which means that the debris stream that is estimated to be about 1,000 years old is replenished on a “regular” basis. However, in 1865 a filament of dust separated from the main stream and as a result the Earth now passes through two streams. In some years, this provides a second, albeit less spectacular peak about one day before the main shower.
When to observe the Perseids
Depending on where in the debris stream the Earth crosses it, the Perseids become visible from around the middle of July each year. The peak period falls anywhere between the 9th and the 14th of August when up to 60 or more (up to 80) meteors per hour can be seen. However, even though the meteors arrive from almost all points in the sky, the track of the Swift-Tuttle’s orbit means that more meteors are seen from the northern hemisphere than from south of the equator.
As is the case with many, if not most meteor showers, the best time to view is in the hours just before dawn, when the side of the Earth that is rotating into the Sun also enters the main stream of meteors. However, no meteors from the Perseids are known to have reached Earth’s surface since most burn up at heights above 50 miles (80 km).
History of the Perseid Meteor Shower
Although the first recorded account of the Perseids appears in Chinese manuscripts that date from the first century A.D., credit for the discovery of this shower is generally given to Belgian astronomer Adolphe Quetelet, who reported it in 1835.
In some Catholic traditions, the Perseid meteor shower is also known as “the tears of St. Lawrence”, most likely because the date at which the shower peaks, 12 August, very nearly coincides with the date on which the Saint achieved martyrdom on August 10th. The Perseids are also associated with the Roman god Inuo-Priapus, who was believed to have fertilized the fields by ejaculating on them once a year on the date the shower peaks.
Nonetheless, as was the case with many pagan festivals, the early Church appropriated the festival by substituting the name of St. Lorenzo for that of the goddess Larentia, who not only happened to be the female version of Priapus, but whose name was deemed to be phonetically similar to “Lorenzo”. The transition to a Christian festival was no doubt made easier, if not relevant, by the fact that the festival of Larentia was also celebrated on August 10th.