Fireworks in the sky

Fireworks in the sky

Last year, just after the festival of lights, the skies opened up with fireworks of a different sort, this time for a select few. Meteors showered to the amusement of amateur astronomers. Earlier, in 1998, Kargil soldiers were taken by surprise by beautiful showers.

Generally, any dawn will show about 5 -10 meteors per hour. These are sporadic meteors and a good chance of spotting them is on a moonless, clear sky. However on some specified days of the year, the number increases to almost 300 per hour. This is called a shower. It builds up gradually for about a week and then the decrease also is gradual.

It is a matter of great interest to know why the meteor showers observe the solar calendar. They occur on specified days of the year. The puzzle was solved about 110 years ago, after pooling data of showers and some “missing” comets. The natives of South America were the first to identify the frequency and inform two botanists, who in turn communicated the excitement to the world via The Gentlemen’s Magazine in 1799. The showers recur every year on November 17 with a grand show once every 33 years. This is associated with the comet Tempel - Tuttle which visited us in 1992. Simple calculations show that the escape velocity on a comet is indeed a very small number, owing to its small mass.

You can just walk off to space, if you happen to land on one of them. Most of the material evaporated from the comet will be lost this way and they are left behind all along the orbit.
 
When the earth and debris meet
The orbits of comets are inclined to that of the earth providing only one or two chances for the earth to near this debris of cometary material. The small dust particles fall on the earth with very high velocities. As they enter the atmosphere, they get oxygen and burn up in the sky. It is this action of burning that produces the light streak.
This also explains why the showers occur on specific days. Two showers occur, one in April and another in October, when the earth encounters the debris of comet Halley. The dust of comet Tempel - Tuttle shower on the earth every year during November. Each visit of the comet (once in 33 years) replenishes the dust.

The showers are assigned names of constellations like Aquarids, Perseids and Leonids. This refers to the specific direction in the sky from where they appear to come. A cross check with the names and associated dates will show that the specific constellations rise only at dawn. Successive visits of comets over several centuries have created blobs of dust material which are slowly drifting. By systematic observations of the showers, it has been possible to identify these blobs of dust - their location along the orbit.
Each is assigned the number of the year of apparition. The dust accumulated in 1886, 1833 and 1899 are clustered and the earth passed through this group in 1999. Clustering due to 1767, 1733 and 1799 was also close by and created a second peak. Last year, the earth passed through the edge of the debris of 1466 and created a sharp increase in the number - about 100 per hour.  This year, it will pass through the centre of this dust blob and hence there is an increased activity. Meteors can cause a hiss on the radios too; this is also a method used for counting the number of meteors. The time of the peak activity is favourable for Asia. A second peak is also predicted.

For a glimpse of meteor showers...
Leo rises by 2:00 am; however you will see some meteor streaks even before that. Go to the terrace and lie down flat, this ensures that you get a good view of the entire sky. It is difficult to capture the streaks on camera; however you can expose it for a long time. The stars leave trails and the meteors will be recorded as streaks criss-crossing the star trails. You will get to see the two planets Mars and Saturn on either side of Leo. If you have a pair of binoculars,  you can search around Mars for this cluster.
Low level activity begins by November 14 and reaches its peak in the early hours of November 18. Then it fades again and ends by November 21.
(The author is with the Jawaharlal Nehru Planetarium)

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