My Science School

            Billions of stars which we see in the sky every night are made up of hot gases. Even the Sun is a star. However, there are stars which are million times more shining than the Sun, but they don’t appear so because of their greater distances from the earth. The stars are of varying sizes.

            We measure the distance of stars in units of light years. A light year is the distance travelled by light (its velocity being 3 x  km or 3 hundred thousand km per second) in one year. The star nearest to the earth (other than the Sun) is Proxima Centauri. Its distance from the earth is 4.28 light years. This star is visible only in the Southern Hemisphere. The nearest star visible in the Northern Hemisphere is Sirius. Its distance from the earth is 8.8 light years. In addition to this, another neighbouring star is Alpha Centauri which is 4.37 light years away from us.

            The farthest star from the earth which is visible with our naked eye is more than eight million light years away from us. If we use powerful telescopes, we can even see stars that are 1000 times more distant than this one. Some stars are so far away from the earth that light from them takes more than 1,000 million years to reach the earth.

            Scientists have developed various kinds of optical and radio telescopes to study stars. They have gained considerable knowledge relating to the heavenly bodies with the help of these telescopes. 

            In ancient times, when people looked up at the sky in the night, they observed many bright stars arranged in a certain pattern or group. Each group is called a constellation. It means a group of stars. Studies of movements of these constellations led to the discovery that they move in space in fixed directions. The number of constellations visible during the year at different times was found to be 12. The Sun and the Moon were always seen rising and setting in the same directions in which these twelve constellations were located. These 12 constellations were called the signs of the zodiac.

            The hypothetical part of the sky through which the Sun, the Moon and other planets seem to be passing is called the zodiac. Each of the 12 constellations situated at an angle of 30°, is named as a different sign of the zodiac. In this way the 12 signs of the zodiac have come into existence. Each sign is connected with one constellation and the shape of each constellation resembles certain earthly body or substance. The names of these signs and the position of the Sun are as follows:

1. Aries: The Ram - 21 March to 19 April


2. Taurus: The Bull - 20 April to 20 May


3. Gemini: The Twins - 21 May to 21 June


4. Cancer: The Crab - 22 June to 22 July


5. Leo: The Lion - 23 July to 22 August


6. Virgo: The Virgin - 23 August to 22 September


7. Libra: The Balance - 23 September to 23 October


8. Scorpio: The Scorpion - 24 October to 21 November


9. Sagittarius: The Archer - 22 November to 21 December


10. Capricorn: The Goat - 22 December to 19 January


11. Aquarius: The Water Bearer - 20 January to 18 February


12. Pisces: The Fish - 19 February to 20 March

     

Continue reading "What is the Zodiac? "

            When we stand in the sunlight, the rays of the Sun fall on us and we can see our shadows. Exactly like this, shadows of the Earth and the Moon are also formed in the space due to the Sun rays falling on them. Since the Earth and the Moon are spherical in shape, their shadows are conical in shape. These shadows are very long. The larger the distance of a body from the Sun, the longer will be its shadow. Lunar eclipse is darkness on the Moon due to the Earth’s shadow over a portion of it.

          While making their revolutions, the Sun, the Earth, and the Moon come in a straight line and the Earth sometimes comes in between the Sun and the Moon, thus the shadow of the Earth falls on the Moon. In other words, the Sunlight does not fall on the Moon when the Earth comes in between the two. The portion over which the shadow falls becomes dark. This is called lunar eclipse. Such a situation occurs only on Purnima (full Moon) days. Therefore, the lunar eclipse takes place only on that day. If the Earth’s shadow covers the entire Moon, it is total lunar eclipse. If the shadow covers only a part of the Moon, it is partial eclipse. Generally, there are three lunar eclipses in a year, out of which one is a total lunar eclipse.

             Now the question arises: when the full Moon comes every month, why does lunar eclipse not take place every month? The reason is that the plane of the Moon’s orbit makes an angle of 5° with the plane of the Earth’s orbit. Consequently, the Moon revolves either above or below the Earth’s shadow. So, it is only twice or thrice a year that all the three - the Sun, the Moon, and the Earth - come in one straight line. Hence the lunar eclipse does not take place every month. Astronomers, using mathematical calculations, easily predict the time and duration of the lunar eclipse.