The Perseids

The Organizers
Catch a Star!
Historical Information
Method of Observation
Ann Astachonok
Christina Karchevskaya
Andrei Pobiyaha
Dubrouski Sergei
AstroClub "Cirrus"
About Shower
Activity: 17 July - 24 August
Maximum: August, 11-12
ZHR: 100+
Radiant: α =46°, δ=58°
Velosity: 59 km/s
r: 2.6
Perseids is the most well known and most active contemporary meteor shower. In the long history of observations this meteor shower vacillated between hardly noticeable and very high activity (the latter corresponds to the meteor shower). The increased activity of Perseids was expected in 2004.
With the aim of investigating the shower activity the Gomel group of observers organized a field trip. Different methods of observation were used. Attention was paid to photographic observation with the fixed cameras. The results were two photos with three meteor images: two Perseids and one Cassiopeid. Based on these pictures the equatorial coordinates of the starting and ending point of meteor flights were determined, and also the origin point of flight was found, its coordinate α r =38.07° δ r =57.5°. The detailed example of processing the negatives is enclosed in the work.
Since the beginning of time "shooting stars" have caught peoples' attention and this sight has amazed the majority of Earth's inhabitants. Indeed it is a very peculiar phenomenon when a fallen star suddenly shoots a bright streak across the sky. However, it is not the star that falls, but the little particles of matter called meteors. These particles are about 1 mm or more in diameter; they hit Earth's atmosphere at high speed and entirely burn there causing the flash to emerge in the night sky. Some of those bodies do not have time to burn up completely, so that the debris, called meteorites, while atmospheric entry, can reach the Earth's surface. Meteors are quite often guests in the Earth's sky. About 100 million meteors brighter than the 5th magnitude burn up in the Earth's atmosphere daily! However the vast majority of them remain undetectable by Earth observers.
Sometime Earth in it orbital movement comes across the meteoroid swarm (continuous mass of meteor substance on the orbit around the sun); in that case the meteor shower can be observed - numerous meteors that seem to be emanate from the same spot on the sky; usually, this is a time limited event lasting from several hours to several days. It means that Earth meets not a single meteor particle but a swarm or the cloud of them. Special features characterize shower's meteors. If traced backward, the meteor trajectories seem to emanate from the single spot in the sky called radiant. This illusion occurs due to the perspective effect: in reality, meteor particles enter the high layers of atmosphere by parallel trajectories.
In certain days Earth comes across the orbits of several abundant meteor showers. In this time one can observe frequent shooting stars appear in the certain region of the sky. The meteor shower is called for the name of the constellation where the radiant of the shower is located. There are numerous showers known, although only some of them are characterized by high activity. Very rarely Earth comes across with the very dense swarm of particles that can result in extremely strong stream of particles with ten or hundred meteors per minute. Usually a good regular shower gives about fifty meteors per hour.
Historical Information
Perseids is the most popular meteor shower ever. The major portion of meteors observed in summer time belongs to this shower; it has amazingly strong activity! The earliest records about this shower are found in Chinese archives. For example, one dated as 36 A.D. says: "… more than 100 meteors came from there in the morning…" plentiful data is found in Chinese, Japanese and Korean archives dated 9th, 10th and 11th centuries. In the 12th - 19th centuries, descriptions of Perseids activity were rare. Nevertheless, August has been called meteor month for its high meteor activity. In the Medieval period Perseids got the name "St Laurence tears" for the name of the holiday that is celebrated in the time of the meteor shower maximum.
Quetelet (Brussels) was the first to bring to the public notice the 1835 meteor shower observation results and to show the correlation between meteors and Perseus constellation. Eduard Heis was trying to determine the hourly rate of this shower - a number of meteors observed during one hour, thus indicating the activity level of the meteor shower. According to these observations, activity of the meteor shower was 160 meteors per hour.
Figure 1 - Radiant drift of the Perseids shower.
Heis and other observers from all over the world continued to observe Perseids almost every year; estimated activity varied between 37-88 meteors per hour. In certain times (e. g. 1835) activity increased to 78-102, and even to 215 meteors per hour in 1863. During the 19th century the meteor shower activity was evaluated as an average one.
Figure 2 - The orbit of the comet Swift-Tuttle.
In 1867 Italian astronomer Giovanni Virginio Schiaparelli (1835-1910) stated that the meteors with a radiant in the Perseus constellation had the same orbit as the Swift-Tuttle comet (discovered in 1862, with a 120 year - long period of revolution). At the same time the first trials were done to correlate the 1861-1863 increase in Perseids activity and passage through the orbital perihelion by the comet. Numerous returns of the comet spread out meteor particles along the entire orbit. Since the region next to the comet nucleus is enriched in meteoroid particles, meteor shower activity increases when the comet approaches the perihelion.
By the beginning of the 20th century Perseids activity had steadily declined. According to the observational data of Denning the meteor number was 50 meteors in 1901-1910. Only 4 and 12 meteors per hour were observed in 1911 and 1912 respectively. Denning was very surprised by this drastic decrease, but in the following years the hourly rate returned to normal. In 1920 there was a meteor shower in which the hourly rate increased to 200 meteors per hour! It was very uncommon considering that the comet was located close to aphelion in this time! In the1920's the shower activity was slightly below average but in 1931 and 1945 there was an increase to 160 and 189 meteors per hour respectively.
In 1973 Brian Marsden predicted a return of the comet to perihelion by the 9th or 16th of September, 1981 (± one year), which stimulated a vigorous observation activity. Their great attention was rewarded: an average hourly rate as high as 65 meteors per hour in 1966-1975 suddenly increased to 90 in 1976-1983, comprising 187 meteors per hour in 1983. The Swift-Tuttle comet itself was never found and was considered to be lost.
After 1983's activity maximum, there was no increase in Perseids' average hourly rates. For instance, in 1984, with a full moon during a predicted maximum, the Dutch meteor society announced approximately 60 meteors per hour. In 1985 meteor activity declined to 40-60 meteors per hour, which remained the same in 1986.
In 1990 Marsden published the new prognosis. Since the Swift-Tuttle comet was observed in 1737, it should reach perihelion in December 1992. The comet was rediscovered at the end of 1992. Meteor observers were waiting impatiently for the 1993 Perseids; as expected, the maximum fell on Central Europe. The observers from all over the world came to Central Europe; meteor activity comprised 200-500 meteors per hour. High hourly rates were still being observed in 1994, the maximum fell on the U.S.
In 1860`s together with the determination of current shower activity observers also determined the radiant of the meteor shower by plotting meteors on the star chart. William Denning appeared to be the luckiest: he observed 2,409 Perseids in 1869-1898. He was the first to discover the daily drift of the radiant coordinates and published the precise radiant ephemeris in 1901. According to the analysis of the last photographic data, daily movement of radiant is equal RA=+1.40°, DEC=+0.25°
It is worth mentioning that together with the main radiant, the activity of secondary radiants near the η Perseus was reported earlier. Denning in 1879 states: "I discovered two other meteor showers from the χ and the γ Perseus". Activity of secondary radiants was determined well enough during the last meteor stream visually, although before it had been observed only with the telescope. The results are listed below:
  • August 12th, 1921 Ernst Öpik observed 9 meteors from oval area with size 5.7°x2.2° with center in RA=40.0°, DEC=+55.6°.
  • August 11, 1912 Adamson discovered the secondary radiant south of the main one. It was described as a prolong region with sizes 6°x1° with center in RA=46°, DEC=+57.5°.
  • August 10, 1931 C. B. Ford and B. C. Darling discovered the telescopic radiant with coordinates RA=40.9°, DEC=+54.4°.
  • August 8, 1932 Öpic discovered radiant RA=39°, DEC=+54°.
  • August 12th-13th, 1934 Ford observed telescopic meteors of radiant RA=43.1°, DEC=+55.2°
The multipart structure of Perseids complex has been observed during 3 years (1969-1971) by observers from the Crimea. In addition to the main radiant near to the η Perseus, they proved the existence of secondary radiants near to the χ and γ Perseus, and also discovered radiants near to the α and β Perseus. Meteor activity of the discovered radiant was unstable and the daily drift repeated the movement of the main radiant. The data of secondary radiant location is listed below.
  • γ Perseids. Active period August 11-16; the average coordinates of the radiant is RA=41°, DEC=+55°; the diameter of the radiant region is about 2 degrees. The increase and decrease in activity level depends on the main radiant. The data concerning the secondary radiants is listed below.
  • χ Perseus. The period of activity is August 7-16. Average coordinates of radiant RA=35°, DEC=+56°, the diameter of the radiant diameter is about 2 degrees. The maximum of the shower can be observed on August 9-11.
  • α Perseus. Activity period is August 7-24, average radiant coordinates are RA=51°, DEC=+50°. The radiant diameter is 1.5 degree. The maximum of the shower can be observed on August 12-17.
  • β Perseus. Activity period is August 12-18, average radiant coordinates are RA=47°, DEC=+40°. The radiant diameter is 1 degree. The hourly rate is unstable, which is the weakest part of the Perseids complex.
Many secondary active centers were discovered, commonly by visual observation; only a few were proved by the radio-echo research method. The α Perseids was the only one to be proved with the highest probability level. In 1951 and 1953 the diffuse radiants were discovered with the radiation region of 8 degrees and the center in RA=54°, DEC=+48°. The maximum activity of the shower fell on August 8-11, with the highest hourly rate of 37 meteors per hour. In general, the Perseids main radiant is about 50 meteors per hour.
Based on a 40- yearlong Perseids' observation the following additional features were discovered. One of the most interesting statistical parameters of Perseids is the correlation between the average magnitude and the date of appearance. During certain time periods the magnitude of meteors before the predicted maximum comprise only half the total magnitude of the meteors after the maximum. For example, on August 8-13, 1953, Czechoslovakian A. Hraska established the Perseids' magnitude as high as 2.5 m ;on August 12/13 it had dropped to 2.8 m and by August 14/15 - 3.4 m. In other years the drop in meteors' brightness was less drastic. According to Zdenec Cephecha's observations of Perseids in 1956, the decrease in meteor brightness was much lower; e.g. from August 4-10 till August 10-15 the star magnitude of the meteors changed from 2.68 to 2.94 respectively.
The instability of an average stellar magnitude during the meteor shower indicates uneven distribution of particles with different masses in a meteoroid stream. The best explanation for the meteoroid stream is a filamentous structure: meteoroids of certain masses are concentrated in different filaments. Therefore, a gradual decrease in meteor brightness is observed while Earth comes across the array of filaments and there is no decrease observed when Earth moves along the filament.
Another feature of Perseids, which differentiates it from other summer showers, is the large amount of meteors with trains. According to Skalnate Pleso Observatory data, Miroslav Plavec made an attempt to investigate the phenomenon of trains' origin. He studied 8,028 meteors observed between 1933-1947 and obtained the following percentage ratio: 45% of meteors showed trains in 1933, 60% in 1936, 35% in1945 and 53.5% in 1947. His attempt to link the meteor trains' origin with sun activity was a failure. No direct correlation was found. This demonstrates that a meteor trains` origin depends on meteoroid composition and the fact that meteors are distributed unevenly thorough the orbit.
Method of Observation
In 2004 the predicted maximum of Perseids fell on August 12, 11 UT (λ = 140.01°). Esko Litinen analyzed the meteor stream dynamics and found out that Earth will come -0.0012 AU (approximately 180000 km) in proximity to the meteor particles on August 11, 2004 at 20h. 54min. This meteor stream was created by the passage of the Swift-Tuttle comet through the perihelion at 1862. In this instance one should wait for the brief maximum with hourly rate greater than 100 meteors. The most favorable conditions for the meteor shower maximum observation were in Eastern Europe, the European part of Russia, Western Siberia, North Eastern Africa, India and Eastern China. Based on a prediction of maximum increase, the Gomel group of meteor observers organized a scientific field trip outside of Gomel city in order to avoid city glow interference with the observation. Three methods were used: simple counting (to determine statistical parameters of the meteor shower), plotting the meteor streaks onto star chart (to determine the radiant of the shower), and time-lapse photography.
Photographic observations allow significant increase in sharpness of observation even when using fixed cameras; the latter case does not require paralactic mount and clock mechanism, which makes it applicable in the field. This method of observation requires lightproof wide-angled lenses and also highly light sensitive films. However, while following all these rules, still, one can perceive only the brightest meteors. When using fixed photography one can observe the images of stars as a number of curves, where each curve represents a trajectory of a daily moving star, whose image also moves across the photo emulsion, thus creating a curved line on the photograph. The meteor image appears as a linear track, called a streak, which crosses images of daily moving stars' trajectories.
To be able to process the photograph, it is necessary to mark the beginning To and the end of an exposure Te using accurate clocks. Also, while making the photograph it is necessary to close the camera lens with an opaque screen regularly for a certain amount of time (for example, every 5 minutes), and noting the opening and closing times. These breaks in exposure lead to the appearance of the white holes on the daily parallels on the stars, the so called "time marks". Time marks help to determine right ascension of meteor trajectory points. An image of each meteor appears to cross several of the star daily parallels; the following note can be used to determine interception point coordinates.
Figure 3 - Interception of the meteor pathway
image with the star daily parallel image.
The declination of the interception point M is equal to the declination of the star whose daily parallel image the meteor crosses. To determine the right ascension, one should find the time marks of T 2 ' and T 3 closest to the point M . Measuring with the microscope, one can find the values T 2 'M=a and MT 3 =b . The hourly rate of point M , (as t M ) can be measured by the daily parallel curve HM. Since moments T 2 ' and T 3 are known, one can calculate the sidereal time s 2 ' and s 3 , and the hour angle of time mark t 2 ' and t 3 using the formula t=s-α , when considering right ascension of star ( α ). In this case, the hour angle of point M can be determined using the formula (1) [7].

In order to determine right ascension using the flight moment of meteor t M , one can find sidereal time s M and use the formula (2) [7].
By this means we obtained both coordinates (α and δ) of point M . This point is situated on the curve of the big circle along which the meteor was moving. For a determination of the trajectory pole of the arc it is necessary to find also the coordinates of any other point of interception of the meteor trail and stellar daily parallels.

During the four-night-long observation session, we could made two photos with meteor images (Figure 3, 4)
Figure 3 - Constellations of Cygnus, Lacerta and Pegasus. The total exposition time is fifteen minutes, using Konica "Centuria" 1600-S film with a f=35mm, f/2.0 lens. ( Large )
Figure 4 - Constellations of Triangulum, Andromeda, and Pegasus. Total time of exposure is nine minutes, using Konica "Centuria" 1600-S film with a f=35mm, f/2.0 lens. ( Large )
Flights of two meteors (assigned as meteor 1 on the figure 3 and meteor 3 on the figure 4) were cited at 19h43m00s UT and 22h34m20s UT correspondently. The data pertaining to interruptions in exposure is listed below (table 1):
Table 1 - Photographic intervals.
of intervals
Date (UT) Beginning
of exposure
of exposure
Photo 1 (Fig. 3) 1 11.08.2004 19h26m 19h31m
  2   19h33m 19h38m
  3   19h40m 19h45m
Photo 2 (Fig. 4) 4 11.08.2004 22h25m 22h30m
  5   22h32m 22h36m
For the determination of the interception of the meteor trail and stellar daily parallel, according to the pictures, the following stars were selected:
Table 2 - Data about interception points of meteor train and stellar daily parallels.
Photo Name
of the star
m vis α 2000 ,° δ 2000 ,° Point a b
1 BSC 8421 6.14 331.3250 46.7500 A 19 21
  σ Cyg 4.23 319.3500 39.4000 B 20 10
2 56 And 5.67 29.0500 37.2500 C 25 9
  α Tri 3.41 28.2750 29.5833 D 5 27
We will demonstrate an example of determination of coordinates of points A and B on the first photo. The meteor crossed the daily parallel of BSC 8421 star at the second interval with the ratio of 19/21 and the daily parallel of the σ Cygnus star at the first interval star with the ratio of 20/10. Since the chosen stars belong to the epoch 2000.0, we need to adjust them to the moment of observation, that is, August 11, 2004 using the formulas [2]:
, where α o and δ o are equatorial coordinates of the object in the equinox J2000.0, α and δ are equatorial coordinates of the object in the moment of observation; the remaining coefficients can be determined as follows:
, where t is the number of years that passed from the equinox J2000.0, divided by 36525. 00h00m UT of August 11 comprises 2453228.5 Julian days. Then and m=0.0591, n=0.0257.
For BSC 8421 star:

For σ Cyg star:

We determined declination of point A δ=46,772° and point B δ=39,42°.
To determine right ascension we should find the sidereal time for 00h00min, August 11, 2004 in the astronomical calendar; that will be equal to s o =21h23m08s. Then we determined sideral time and hour angle of each moment in time. Calculations are listed below (Table 3):
Table 3 - Hour angle calculation of time intervals when determining coordinates of point A.
 Starting point T' k T k+1 T A
 Time, UT 19h33m00.0s 19h38m00.0s 19h43m00.0s
 Correction Δm 00h03m12.7s 00h03m13.5s 00h03m14.4s
 τ s 19h36m12.7s 19h41m13.5s 19h46m14.4s
 s o 21h23m08.0s 21h23m08.0s 21h23m08.0s
 s 40h59m20.7s 41h04m21.5s 41h09m22.4s
 RA BSC 8421 22h05m29.0s 22h05m29.0s  
 Hour angle, t 18h53m51.7s 18h58m52.5s  
Determination of the hour angle and right ascension of point A using formulas (1) and (2):

In the same way we can determine point B coordinates (table 4):
Table 4 - hour angle calculation of time intervals when determination coordinates of point B.
 Starting point T' k T k+1 T B
 Time, UT 19h26m00.0s 19h31m00.0s 19h43m00.0s
 Correction Δm 00h03m11.6s 00h03m12.4s 00h03m14.4s
 τ s 19h29m11.6s 19h34m12.4s 19h46m14.4s
 s o 21h23m08.0s 21h23m08.0s 21h23m08.0s
 s 40h52m19.6s 40h57m20.4s 41h09m22.4s
 RA σ Cyg 21h17m34.8s 21h17m34.8s  
 Hour angle, t 19h39m45.6s 19h39m45.6s  
Determination of the hour angle and right ascension of point B:

We determined coordinates of the beginning and ending points of the meteor 1 α À =22h13m07.8s δ A =46.772° and α B =21h31m17.1s δ B =39.420°
Processing of the second photo was done using the same algorithm: coordinates of interception points of the meteor 3 and stars 56 And and α Tri: α C =02h02m46.8s δ C =37.272° and α D =01h55m50.0s δ D =29.606° correspondently.
The trajectory of each meteor can be determined by the "big circle" equation and is determined by the pole of the big circle. Coordinates of the meteor trajectory pole can be determined from the system of equations (3) [7], where poles A and B are the points on the meteor trajectory:
The solution of this system has the following view:

then coordinates of the meteor trajectory pole can be determined using the formulas (4) and (5) [7]:
In the same way, we can find coordinates of the meteor 3 pole: α CD =-66.137°, δ CD =8.8829° (X CD =2.5885, Y CD =-5.8514).
To locate the radiant, one should find the interception point of two meteor trajectories; the "big circle" equation (3) can be used in the following way:
, where     and  

We can find coordinates of the radiant using the formulas (4) and (5):

Calculated coordinates of the radiant of interest are the same as that of the secondary radiant near to the χ Perseus. However, it is less likely for both meteors to belong to the same shower. Perhaps the meteors that were photographed belonged to more than one radiant, but the point of projection of their trajectories is identical to the radiant of χ Perseus. For such an abundant meteor shower as Perseids are, especially in the day of maximum, it is hardly possible to determine the radiant coordinates exactly enough while using the pictures with only two meteor images. To investigate a radiant structure one should use many photos. Unfortunately, making pictures of meteors is a very difficult task. Results demonstrated an example of determination of a meteor shower radiant from single-sided observations by means of processing of night sky pictures, obtained by fixed photography. To make such a photo is a very common task; this simple way of observations is also very efficient in obtaining data that are necessary for modern science.
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