Wish Upon a Shooting Star

(This is article here was published as a book-let from Inter-University Centre for astronomy and Astrophysics (IUCAA) in 1999, prior to the Leonid shower of that year)

When we see a streak of light in the sky, rather like a bright little star falling down, we call it a shooting star. Astronomers call it “meteor”. When an interplanetary particle traveling at high speed enters into the Earth’s atmosphere, it ablates due to the energy produced by its friction with the atmospheric constituents. Such a particle, called a meteoroid, can be about as large as a pea, a few millimeters in diameter. Smaller grains may produce a meteor so faint that it can be seen only through a telescope. On the other hand, a bigger meteoroid can produce a meteor as bright as Venus. Such bright phenomena are called fireballs, which can be seen in broad daylight. (Remember Venus can be seen even when the Sun is above the horizon.) Sometimes a fireball also makes a noise as it rushes through the atmosphere of the Earth. Some of these fireballs may explode in the sky – they are then called bolides.

Extraterrestrials of different kinds
Normally a meteoroid completely burns out in the atmosphere of the Earth as a meteor, a fireball or a bolide. But sometimes it may survive the plunge, and part of it may hit the surface of the Earth. It is then called a meteorite. They were sometimes called uranolith – the mineral that fell from the sky.

It is believed that meteoroids were formed at the time of the formation of the solar system and meteorites can give us clues of the condition prevailing during that period.

Meteorites are classified into three broad categories : Stones, Irons and Stony irons. Stony meteorites account for nearly 93% of all meteorites but owing to their close resemblance to the terrestrial stones they are difficult to discover. A freshly fallen stony meteorite may have a fusion crest which would be produced by its outer surface melting during the plunge.

Iron meteorites account for about 6% of meteorites. They contain about 90% iron and about 9% nickel. In 1620 an iron meteorite was picked up at Jalandhar, Punjab. It weighed about 1.6 kilograms. Emperor Jahangir asked his artisan to make weapons out of it. The artisan found that the composition of the meteorite was “rather less dense” than required to make strong enough weapon. He added one part of iron to three parts of meteorite material to make two swords, one knife and one dagger.

Stony iron meteorites consist of about equal amounts of rock and iron. They look like small pieces of stone set in iron. Some carbonaceous chondrites, which belong to a class of stony meteorites, contain amino acids, which are the organic building blocks of life-sustaining proteins. It has been suggested that they may have played a role in the appearance of simple organisms on our planet about 4 billion years ago.

How meteoroids burn
We observe meteoroids as “shooting stars” as they burn in the Earth’s atmosphere. Usually a meteoroid starts burning at the height of about 140 km above the surface of the Earth and burns out completely in the atmosphere. A fireball may travel down to an altitude of about 50 km before it disappears.

Meteoric phenomena lasts from a fraction of a second to a few seconds. A meteor may leave behind a trail of ionized gas. This trail can be seen for as much as half an hour under undisturbed atmospheric conditions. The speed of a typical meteor in the atmosphere can range from 12 km per second to 72 km per second.

On any clear dark night, one is expected to see a meteor every few minutes. It can come from any random direction in the sky. However, there are certain times of the year when meteors can be seen coming from a particular direction in the sky in large numbers.

Beginning of meteor shower astronomy
Four hours before dawn on November 13, 1833, people of eastern North America saw what no one had seen ever before. The sky was almost lit up by large number of meteors. It was a heavenly firework. Among those who witnessed the display was one Denison Olmsted. He observed that meteors were coming from a particular point in the Leo constellation. He estimated that up to 150,000 meteors were observed in an hour. He then further investigated observations of this extraordinary display. He found that similar display was seen at other places too. In Jan 1834 he presented result of his investigation. He in his report he rightly suggested that the meteors had originated from a cloud of particles in space. Olmsted, however, no explanation for the nature of the cloud.

Later, astronomers discovered that the display of November 13, 1833 repeats, if not with the same intensity, every 33 years or so. They also found that this display occurs close to the perihelion passage of comet Tempel-Tuttle. There was a connection between meteoric activity and comets. This was the birth of new astronomy – astronomy of meteor showers.

When a comet approaches the Sun, volatile material trapped inside its stony shell sublimates, breaks the shell and escapes forming a cloud of gas and dust, called coma, around the comet. Fragments of the shell gets scattered in different directions. But most of the debris go to form the tail of the comet and follow the same path as that of the comet. Over a period of time a ribbon of debris is formed along the comet’s orbit.

If the orbit of the comet and the Earth intersect or lie close to each other, then every year when the Earth passes close to or through this point it would experience a surge comet’s debris entering into its atmosphere. Of course, the probability of the Earth colliding with the comet itself may be negligible.

The point in the sky from where the meteor shower appears to come from is called the radiant of the shower and the shower is named after of the constellation in which its radiant lies. The meteor shower of November due to the Earth’s passage across the orbit of the comet Tempel-Tuttle is known as Leonid because the radiant lies in the Leo constellation.

Observations of meteor showers are ideally suited for those who are unable to invest inexpensive observing apparatus. The study of meteor showers can give us a lot of information about the parent comet and its constituents. All you need is a good eye. There are about a dozen meteor showers which we know are good to observe so you can make your plans well in advance. The observations can be carried out in a very comfortable position. In fact you simply lie down and look up. Such a project can be undertaken by individuals or by a team of observers. However, if you do not choose the proper time to observe your first shower, you are likely to find this game very boring: so one must make a good start. Once you get used to it, it is fun.

Observing Meteor Shower
The most common method of observing a meteor shower is counting the meteors visually without using any optical aid. But astronomers have been using different observing tools such as photographic, radio and video recording of the meteors. Here we will discuss the visual observing techniques in detail.

Preparing for the Observations

1. Site selection – It is a common experience that one sees fainter stars from the countryside than from light-polluted cities and towns. It is, therefore, essential that one goes to a dark place, free from lights of streets and buildings. A place about 40 to 50 km from a town, preferably away from a major highway, will be good enough, but one should check it out beforehand.

You should also be cautious about selecting a site for observation. The astronomical consideration is that the highway or a road with heavy traffic should not be in the direction of your watch – East and Northeast in the case of Leonids. It will be very annoying to get one’s eyes blinded every now and then by headlights of approaching vehicles. Personal safety is equally important – a place which looked good during the day to you may also have looked good to some not-so-good people. It is our experience that such people can certainly disturb your observing schedule in many unforeseen ways.

The terrace of a school or dharamshala offers a safe site but by and large such places are not too far from busy roads. Places of worship may also be explored. You should check (with the authorities and with the local chowkidar) whether you will be allowed to switch off the light in the open premises while your observations are in progress.

Most often people are very cooperative if you explain the purpose. A farmhouse of a friend (or friend’s friend) could also be check out.

If you are a group of a young girls and boys not accompanied by an older person then please carry some means of identification – just in case. You should also inform your guardians exactly where you are going and what your schedule will be. Even if you are grown-ups it is better to inform the local police or people in charge of security in the nearest village of your purpose. We know of a case in which motorcycles of a group of amateurs were taken into custody by local villagers who had thought that they (amateurs) were a group of terrorists.

2. Clothes -You should be adequately dressed for the occasion. We know that the nights are always colder than the day but in open and away nights are colder still. You should have plenty of warm clothes. Instead of one thick piece of clothing many layers of thin cotton ones work better, for the air trapped between two garments acts as a good insulator.

Wear warm socks and shoes. Chappals and sandals are out. Have a pair of hand gloves, two pairs if required and practice writing with the gloves on during the day. Also carry a good cap and a muffler or scarf to cover your head, neck and nose. Try to avoid loose clothing which might keep falling off your body.

Carry a thick bed sheet, a pillow and blanket or a sleeping bag. The golden rule is to be “be comfortable while observing”. Carrying a folding easy chair to the observing site is not a bad idea.

3. Food and Drinks – Food should be nutritious and not heavy on your digestive system. Do not carry messy food such as dal or sambar. Instead take stuffed parathas, sandwiches or idlies (avoid dosas – they are oily). Avoid rice – it makes you feel sleepy. Carry enough of drinking water and do not depend on on-the-spot supply. Carry approximately half a liter of milk per person, sweetened if you like and/or with drinking chocolate added to it. Take your milk just warm enough, not piping hot. Avoid drinking tea or coffee and smoking. These stimulants might reduce your ability to see fainter meteors. And a BIG NO to alcoholic drinks please. Thanks you!

Have your first bite at your regular eating time. Eat just a little less than a full stomach – wash your food down with warm milk. Try nibbling on chocolate bars in between. These give lot of energy.

4. Insect repellent– Carry a good insect repellent of the body lotion type. Burning coils won’t help in the open. Even though the mosquitoes will keep you awake it is certainly not a pleasant experience.

5. Light – Though you should be avoiding sources of extraneous light most of the time, you would require one small torch which which has its bulb covered with red gelatin paper for reading and writing. You should also have a slightly brighter one for moving around, e.g. while attending nature’s calls.

6. Medicines – Carry a first aid kit and your regular medicines. In the army they say don’t kill your enemy – injure him badly so that the enemy manpower will decrease by at least two if not more. If you fall sick not only your observations will get affected, but someone else will also have to miss them, having to look after you. So take care of yourself.

7. Paper and pencils – To note down the observations you must take a pad of blank sheets and pens or pencils to write with. Make sure that your writing material is in order and will work in the night. Bad paper can absorb moisture on a damp night and refuse to be written on. Ink pens can cause smudges. Some ball point pens may not work and you might just forget to take your pencil sharpener (or extra lead for clutch pencils).

8. A good watch – Your observations are useful only when one knows the time of the observations. Have at least one good watch which has been set with either GMT from the BBC or IST from the AIR. Just before the news on both the stations you can hear time beeps. Other time services (such as TV) may not be reliable.

9. Sleep – Before you start take a good two-hour nap soon after your meal so that you will not feel sleepy in the night. We also advise that all other preparations be done before you fall sleep, since waking times are notoriously unpredictable.

Filling in the observing log
Do not depend on your memory and commit every possible detail in connection with observations on the paper. Before you actually start observing, there are a few things you must note down. Write your name and the name of your organizations if you belong to one. Latitude and longitude of the place you are observing from. You can get this information from Survey of India maps. If you do not get this information immediately then at least record full details of the place so that you can reach it again.

You must record the date on which the sun set and the following date. For example, if you went for observing on the November 17th and returned on the 18th then you will write 17/18 Nov. 1998. Since the date changes at midnight, it can be confusing if both dates are not recorded.

Write down the weather conditions and record the presence of clouds including how heavy they are, wind, mist or fog etc.

If you have not set your watch then write down how fast or slow your watch is as compared to that of somebody else who might have reliably set his/her watch.

Carefully choose your observing direction. It should not be covered by trees etc. or illuminated by lights. Don’t look directly at the radiant. An angular distance of 20 to 40 degrees from the radiant is good enough. Record the area of the sky you will be looking at. Write the name of one or two stars close to the centre of this area. It is better to keep watching the same field so that you get familiar with it as the time passes.

{Click for charts for selecting the field centre charts GEM }

One would like to know the faintest object that your eyes can detect – this is “lm”, the limiting magnitude of your eyes. You can estimate your 1m by finding out the faintest star that you can see using a star map. Another method is to count the number of stars in one of the areas shown on the maps including the corner stars. Then using the table find the limiting magnitude.

The International Meteor Organization has identified 30 such fields. First allow your eyes to get adapted to the darkness, it takes about 15 – 20 minutes after you’ve looked at a bright source of light. Then before you start observing record the limiting magnitude ‘lm’.  Limiting magnitude tables are available at http://www.imo.net/visual/major/observation/lm

Remember no two observers are likely to report the same 1m. Moreover you should not make efforts to see the faintest star to stretch you 1m as this cannot be repeated for the fainter meteors.

Normally the shower observations are to be carried when the radiant is more than 50 degrees above the horizon.

Taking the observations

Our task is to observe the number and brightness of meteors seen during a certain time interval. Before you start observing record the start time. Every time you see a meteor trace its path backwards. If it reaches the radiant then it is a shower meteor else it is a sporadic one. You may write ‘S’ for shower and ‘N’ for sporadic or non-shower. Or make a tick mark for shower and cross for a sporadic one. Estimate and write down the magnitude of every shower meteor observed. To get the estimate of the magnitude of a meteor see the table below. Accuracy within one magnitude is good enough. But when the activity increases you may not get time to write down the magnitudes.

Table to estimate the magnitudes of meteors

Mag.

Object

Mag.

Object
-13 Full Moon 1.6 Castor
-10 Quarter Moon 1.9 Alhena
-4.5 Venus (average) 2.2 Beta Leo
-2.5 Jupiter 2.4 Beta UMa
-1.6 Sirius 2.6 Gamma Leo
0.5 Procyon 3.1 Beta CMi
1.2 Pollux 3.2 Epsilon Gem
1.3 Regulus 4.6 Theta UMa

Note the colour of the meteor if you notice any. But remember colours are not perceived for meteors fainter than +2 magnitude. This information is of least value of all the data and can be omitted during your first few observations. Finally, write down the duration of the trail if the meteor leaves one.

When you are about to get tired or would like to relax – stop. Record the time when you stopped observing. In any case take a few minutes’ break every 30 minutes or so. You may also write any remarks on the brightness and trail that might have been left behind. After the observations add up all the sporadic and shower meteors for every observing interval.

You can tape all this information on a tape recorder and play it back the next day and fill in the report form. Be sure to carry enough of tapes and batteries for the tape recorder. Make sure you record the starting time every time you restart the tape.

You may also work as a part of a team. One of you may observe and the other may take notes and record the time. From time to time these roles can be exchanged.

You must remember not to influence each other by announcing ‘Ah here I see brilliant one’ or ‘wow! look at that beauty’ or similar things that only non-serious amateurs would say. The persons most likely to get distracted is the timekeeper.

How to differentiate between shower meteors and sporadic meteors

1) Shower meteors appear faster the larger their distance is from the radiant and the higher their elevation is above the horizon. Near the radiant or near the horizon shower meteors generally seem slower.

2) The apparent path length of a meteor shows a similar behavior in relation to angular velocity. Near the radiant or the horizon meteor paths are generally short, while they are longer when they are far from the radiant or near zenith.

This means that even if the trail came from the radiant, a meteor in the vicinity of the shower radiant does not belong to the shower if it was very fast and/or very long. .

3) If the shower becomes storm. If and when the rate of meteor reaches some thing like one per second it will be difficult to write the numbers down. You may then shift to 10 meteor counting. Make one tick mark for every 10 meteors seen.

4) If you are using a tape recorder try to “speak in” meteor with its magnitude etc. but when the rate starts increasing just speak-in beep or a short note for every meteor seen. If the activity increases even further then a beep for every 10- meteors observed. Make sure to speak in the change in beep indication.

General Remarks about observing
Meteor observing requires great concentration and quick reflexes. Your attention should entirely be devoted to the observations. Avoid talking or listening to music. As soon as you get tired take a break to walk around or take some food. In most cases this will fend off tiredness, and you will be able to continue the watch. If you do become too tired, you should stop observing and retire to bed, since the accuracy of your data will reduce as fatigue takes hold, meaning that you will miss many meteors. Thorough preparation before observing includes ensuring that you are sufficiently rested, as this is the one factor in your control that may stop you watching unlike, for instance, the weather.

Table of Meteor Showers
What follows is a brief description of those showers which have reputation of performing well – that is a meteor every 4 to 5 minutes belonging to the shower.

Shower Activity Period Max.     Radiant Vel.
km/s
ZHR
R.A. Dec.

Quadrantids (QUA)

Jan 01-Jan 05

Jan 03

15:20

+49°

41

120

Lyrids (LYR)

Apr 16 – Apr 25

Apr 22

18:04

+34°

49

18

Eta Aquarids (ETA)

Apr 19 – May 28

May 06

22:32

-01°

66

60

 

South Delta Aquarids (SDA)

Jul 12-Aug 19

Jul 28

22:36

-30°

41

20

 

Perseids (PER)

Jul 17-Aug 24

Aug 12

03:04

+58°

59

100

Draconids (DRA)

Oct 06-Oct 10

Oct 08

17:28

+54°

20

var

Orionids (ORI)

Oct 02-Nov 07

Oct 21

06:20

+16°

66

23

Leonids (LEO)

Nov 14-Nov 21

Nov 19

10:12

+22°

71

var

Geminids (GEM)

Dec 07-Dec 17

Dec 14

07:28

+33°

35

120

 

Notes on individual showers

Quadrantids – This shower can produce as many as 50 meteors per hour. The maximum activity lasts for two to three hours.

Lyrids – This shower has a variable strength of 15 to 20 meteors per hour. The maximum activity goes on for up to 8 hours. In 1982 one observer recorded a rate of 80 meteors per hour.

Eta Aquarids – This shower is due to comet Halley. Meteors are fast streaks and some leave trains (or trail).

South Delta Aquarids – This is an interesting shower to observe as one is likely to see early Perseids as well (see below) which peak on Aug 11. These meteors are rather faint.

Perseids – This is a good and very active shower but for us in India it occurs in the midst of monsoon. Perseids do come in rapid succession followed by the less active Perseids. There are other showers active during this period but the meteor rate is low. Southern Delta Aquarids and Perseids occur about two weeks apart so if one is likely to be washed out by the moonlight then you have the other.

Draconids – This shower is due to comet Giacobini-Zinner. It can give an impressive display and can result in “storms”.

Orionids – This is the second shower associated with comet Halley. Like Eta Aquarids these are fast moving meteors. One can observe concentration of fireballs about three days after the maxima.

Leonids – This shower is due to  comet Tempel-Tuttle. It’s out brust in 1833 initiated the understanding of meteor showers.  (See above)

Geminids – This is one of the finest showers, boasting up to 75 meteors per hour. This shower shows a clumping effect: the sky is silent for 4 to 5 minutes and then there is a sudden burst of 4 or 5 meteors within a minute. You can see beautiful fireballs from this shower.

Addresses
You can send your data directly to Rainer Alrt, Director of Visual Commission of the International Meteor Organization, by mail at Friedenstr. 5, 0-14109 Berlin,Germany

The following sites will have the latest information about the  showers.

Check it out.

http://www.imo.net

http://www.skypub.com

http://www.drdale.com

http://medicine.wustl.edu/-kronkg

We end this on a note that .. 

 …….when you trace back the path of a meteor to a radiant, think what you just saw. What you saw was a brilliant death of an interstellar particle which had formed almost the time that the solar system had formed. For hundreds and thousands of years it orbited the sun and now……………… !

One Response to Wish Upon a Shooting Star

  1. Makarand says:

    I understand that there are many man made objects circling the Earth (e.g.used parts of rockets). Do they enter into the Earth’s atmosphere? Do they look like shooting stars?

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s