The Perseids are an annual, meteor-watching phenomenon that only occurs around February 15th.
But, not everyone may know about this year’s events by now. I’ll try to provide readers and I with some information that is only relevant to the people who work in or around the North American Space Agency or NASA.
First, the basics:
The Perseid meteor shower has both a radiant and an apparition .
The Perseid radiant (the point in the heavens between Earth and the Sun that the meteors appear to come from) usually happens on the night of or shortly after the first half of a planet’s day (or month.) When the meteor showers appear between the start of the Martian (Mars) and the start of our own (Earth), it is called “mid-morning twilight.” While the Perseid showers are most frequently active on April 20 and March 21, 2015, they can often occur in the middle of the month, just after midnight. The Perseid’s closest approaches are between August 23 and November 19, 2014. Some people have speculated about the origin of the Perseid meteor shower’s name. The name Perseids was first used by Greek astronomer Ptolemy (who lived around 200 B.C.) in his “Perseidum” treatise, written around 95 B.C. It actually means “three stars” because he thought he had found three or four brilliant stars, each about 20 light-years away. But in reality there are three celestial bodies – the Sun, Venus, and Mars combined – that form the largest planet in the Solar System. His name was used by many cultures, however, including ancient Babylon and ancient China. The names “Pseid” and “Sigurr” (pronounced “sih-durrd”), were used in Greece for the three bodies. In Egypt they were called the Milky Way and The Sun, respectively.
If you look up at the constellation Cygnus, you can see Orion and three stars called the Bracket. These are the Orion nebula which forms the radiant of the Perseid meteor shower. (The Perseids are most often visible just before or after this.) Each of these bracket stars is about 5 light-years, so that, taken in an equatorial field of view, the Brackets are visible in a little under a quarter-moon’s orbit. The brightest star in each Bracket is Betelgeuse, which is about 1 light-year away. All of these are relatively small objects compared to other globular star systems. It seems surprising that no one has been able to pinpoint the radiant by any standard astronomical measurements. However, people like Peter Ross and others have been able to deduce the position of the radiant using the so-called “satellite” method, which involves counting the number of meteors a particular region will be exposed to. It is possible for the Perseid radiant to appear farther than it actually is, and this has been measured experimentally. The method is actually not very accurate because the angle of the radiant is known at the beginning of the shower, and the angle changes dramatically at different times. It would not be surprising if the radiant was at a greater distance than the Brackets: a couple hundred light-years away, but only 30 light-years away. Since there are several hundred brightest meteors per hour in the sky during the Perseids, however, it seems likely that that location is closer to the radiant than the Brackets.
And, let us go on a tangent now. On top of these measurements, there are several other factors which influence the location of the Perseid radiant: the age of the universe, the position of Earth, the shape and orbit of Venus. For those who are interested, here is what we currently know:
The radiant is the point of light at the center of the Milky Way, in the constellation Orion. If it was located in any one of the other hundreds of locations that make up the constellation Orion, we would actually get a different result. For example, our satellite planet, Mercury, has an orbital period of about 45 Earth-days, like a planet orbits the sun. A planet does not orbit the sun but it does have a period of about 30 Earth-days. During that time Mercury is slightly closer to the Sun than Earth. This explains the difference in distance between the Radiant and the Brackets. Of course, we do know the distance between the Radiant and the Brackets by observing the distance of the Radiant at different times during the night; and we also know the orientation of the sky as an object is being viewed; but we also know the position of the Radiant from its distance to the sun. If we can have a very precise location on Earth in this manner, then it is pretty easy to tell the position of the Radiant from the Brackets. We know it is there; the only question is which location.