Monthly Archives: February 2003

Why is the Sky blue


      Why is the sky blue? Today in VERITAS we attempt to answer this question which we have all asked as children.

      We will need to explain three colours: The blueness of the sky, the redness of sunset and the whiteness of the sky that is near the sun.

      THe atmosphere is made up of oxygen and nitrogen molecules. These molecules interact with the light that comes from the sun. These molecules absorb the light and then emit it. The light that comes to us from the sun is absorbed. So the absorption is direction specific( it happens to the light that comes to us from the direction of the sun). However after absorption of light the atom may emit this light in any direction. This phenomenon is called scattering of light.

      Scattering of light is a complex phenomenon and cannot be fully understood without the help of modern Quantum Mechanics. And there are several different kinds of scattering depending on the properties of the incoming light and the atoms. The most prominent scattering phenomenon that occurs in the sky due to nitrogen and oxygen molecules is called Rayleigh scattering. In this the different colors of the light from sun are scattered differently. The scattering is inversely proportional to the 4th power of the wavelength of light that strikes the atom. What this means is that a light with smaller wavelength is scattered much much more that the light of a larger wavelength.

      We know that blue has a shorter wavelength and red has the longest wavelength. So the blue light is scattered away by the molecules. Blue light is scattered about 10 times more than red light. That means that blue light is splashed all over the canvas of the sky. The red color however more or less maintains its incoming direction. So whereever you see you will see blue in the sky. The color of blue would be deeper the further you look from the sun.


      But why isnt the color of the sky violet when we know that violet has the least wavelength? Should it not get scattered everywhere and color the sky?  Yes! Violet gets scattered even more than blue. But our eyes are more sensitive to blue than violet. And sometimes if you look carefully you would see a tinge of violet in the sky.


      What happens during sunset. The sun ‘s direction is at the other end of the atmosphere. The rays of the sun have to travel a larger distance to reach us and they encounter more of the molecules on the way. So when we look at the sun we look at the light that has been least scattered. The light that has been most scattered would appear in the color of the sky. The light that has been least scattered appears in the direction of the sun. What light is this? Red of course! (max wavelength = least scattering). That is the reason why sunsets appear red in color.

      Now lets try to explain the whiteness of the sky near the sun( except at sunset). This is explained by anothet kind of scattering called Mie scattering.     Mie scattering is not wavelengh dependent. All wavelengths get scattered equally and the direction of the scattering is mostly in the forward direction( the direction of the incident light). So when we look at the sun we will see the forwaded light of the Mie scattering and it is a mix of all possible colors( white).


       A sense sublime

Of something far more deeply interfused,

Whose dwelling is the light of setting suns,

And the round ocean and the living air,

And the blue sky, and in the mind of man.


William Wordsworth.


      Does this answer all our childhood questions. Yes some of them.. but it poses many more : ” What if I go to another planet… would the sky be blue? or some other color?  What if the atmosphere was made of some other gases? …. “

      Isnt is more fun when you carry your childhood curiosity with you?



  Go, wondrous creature! mount where Science guides:                 

  Go, measure earth, weigh air, and state the tides:                 

  Instruct the planets in what orbs to run,                          

  Correct old time and regulate the Sun;                              



The path finder

This post was written on  Wednesday, February 05, 2003.


Imagine …. You are born into a very poor family. Your mother leaves you when you are three. You suffer from small pox before you are five and are nearly about to die. You suffer from serious fever attacks, painful skin eruptions and stomach disorders throughout life. You want to become an astronomer but you have a very poor eyesight and cannot see the stars. Employment is always uncertain for you. You have to earn a living by doing something you do not believe in. You are a deeply religious person but throughtout life you are persecuted for following your faith.
Your wife dies at a young age. Half of your children die before the age of 10. Your superiors treat you badly. It is a time of conflict and war. Sometimes you have to do your work with soldiers living in your house. Your mother is accused of witchcraft and you have to spend several years defending her in court. You are banished from two cities and your property is confiscated.

       A tragic life. A life which would be a good excuse for anyone to complain and feel miserable about himself. Anyone but not Johannes Kepler! Kepler went through all this with his love for his work and he changed the way we understand the heavens. His story is the story of great determination and love for Science. He did not let the troubles of life affect his Science

       Einstein once said:” A scientist must be unencumbered by cares, must avoid all disturbing conflicts of life, must bear with all humiliations from his opponents in order to safeguard the precious something that he bears in his soul.”  Kepler did exactly this.

       Kepler was born on Dec 27, 1571 in Weil( southwest Germany). At the age of three he contracted smallpox and nearly died. His father was a soldier and his mother left with him for a long period of time leaving Johannes with his grandfather. Johannes joined the local school and later the University of Tubingen. He studied theology, mathematics, greek and latin. Kepler was on his way to become a priest. All he wanted to do was to serve God. He was deeply religious.

       In 1594 Kepler was asked to go to a Lutherian high school in Graz, Austria to replace a mathematics teacher. Kepler felt led by God to teach these students and he left the university.

       The mathematics teacher was also supposed to work as a district mathematician( create calendars, survey fields, predict weather, make astrological charts). Kepler did not believe in astrology but all his life he would be asked to make these charts. People in those times thought that study of stars except for astrological purposes is useless.


       Kepler was a deeply religious man and believed that the universe was created in a planned and orderly fashion by God. And that man could find this order. Kepler studied Euclid’s works on geometry and found that there are six regular solids:
sphere, cube, tetrahedron, dodecahedron , icosahedron and the octahedron. Also there were 6 known planets at that time. Why only 6 planets? Why only 6 regular solids? For the religious mind of Kepler there seemed relationship between the orbits of these planets and the sizes of these
perfect spheres. He wanted to find the relationship. He believed that this relationship would be a study of how God made the universe perfect!

       In 1596 he came up with a pretty wierd theory in which he explained the orbits of the planets in relationship to the sizes of these solids if put inside each other. Nobody beileved this theory. Kepler wanted more data to prove his theory. His eyesight was too weak and he could not observe the stars. He knew that the great astronomer Tycho Brahe had been observing the planets for several years and had made detailed calculations about their positions. Kepler needed Tycho’s data. But Tycho would not just hand it over to him.
Kepler took up a job under Tycho Brahe. Now Tycho was a pretty arrogant  and cranky person and used Kepler for all sorts of petty jobs as well( see the VERITAS on Tycho Brahe March 27 2000).


6 solids in a planet:


       In 1601 Tycho died. There was dispute among Tycho’s family on who would own his instruments and his observatory. In the middle of this Kepler used Tycho’s observations to sketch the orbit of Mars. He took him about a 1000 pages of calculations to come up with the final answer: the orbit is a ellipse with the sun at one of the focii. This was different from the general belief that it would be circular. Even Kepler would have liked a more “perfect” orbit. He now wanted to test the elliptical orbit theory with the rest of the planets. He also found that a planets sweep equal areas in equal times along its orbit. He published these two laws in 1609.

Tycho Brahe


       In 1611 the king of Prague died. His brother succeeded him. The king was a Catholic and Kepler was a protestant. Kepler was asked to leave Prague. His wife and son died. Kepler went to Linz and became the Imperial astrologer. His job was to predict the king and the kingdom’s future. Ten years later he came up with his third law of planetary motion. All this time he battled with his health and health/death of his children.

       Kepler decided to write a book on his three laws called: “On the Harmony of The World”. While he was working on his book his mother was charged with witchcraft. If the charge was proven she could be burnt alive. She was imprisoned and tortured and asked to confess. Kepler fought a long legal battle to save his mother.

       Kepler made several other contributions to Science: He proved why logarithms should work(Napier had proposed the idea of logrithms but he had not given any mathamatical reason). He published the Rudholphine Tables which contained Tycho’s observations and Kepler’s logarithmic calculations. He also proposed a new construction of the astronomical telescope.

       Kepler died in Regensburg in 1630. He was buried in a church. His tomb was destroyed in the thirty years war.
Kepler’s scheme to plot the earth’s orbit




 Go, wondrous creature! mount where Science guides:
 Go, measure earth, weigh air, and state the tides:
 Instruct the planets in what orbs to run,
 Correct old time and regulate the Sun;


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