Introduction to Astronomy
Darshan Joshi
As an introduction to Astronomy it is always good to differentiate it clearly from Astrology. ‘Astronomy’ is the most ancient science which deals with planets, stars and other heavenly objects. Whereas ‘Astrology’ deals with effects of these objects (planets, stars and other heavenly objects) on human life. Whatever may be our personal views regarding Astrology, we shall not discuss it while dealing with Astronomy. Feynman says- “In nature where there is emotion there is corresponding complexity and mystery about it!” I think the converse of this statement is also equally true. The complexity of the Universe and its various mysteries must have ignited the minds of primitive man with a strong emotion- the emotion of curiosity! I think this curiosity was the key to open the entire field of Astronomy. Alright! But after all what motivated people to study this? I think the periodicity of sun, moon etc. must have urged the contemporary human mind to recon and record time. Calendars, festivals etc. were outcome of this early use of astronomy. However apart from this many other things led the wheels of astronomy roll on. Now after this brief philosophical overview let’s discuss highlights of Astronomy. Since this is just an introductory talk on astronomy, we will concentrate on basic terminologies helpful to us throughout astronomy rather than dealing with astrophysics. Now imagine our sky as a huge sphere with the earth at its centre. All stars, planets etc. appear to be projected on this huge spherical screen (although we know stars & other objects do not lie at same distance from earth!). This is called the ‘celestial sphere’. If you imagine a large circle on it concentric to our equator then you are looking at the ‘celestial equator’. With our naked eyes one can see around 6000 stars (which are quite homogenously distributed in the sky) in the sky. The entire sky is divided into 88 constellations (group of stars) by the ‘International Astronomical Union.’ Sun, moon and the planets of our solar system move on the background of these constellations. Now for time being look at sun’s motion relative to earth. We can see that sun traces a specific path (a circle centered at earth) on the celestial sphere over a period of one year (the actual picture is that earth takes about a year to complete a revolution around sun) on the background of stars. This path is called the ‘ecliptic’. Ecliptic is inclined to the celestial sphere at an angle of 23.5 degrees (since inclination of earth to orbital plane is 23.5 degrees). You all must be familiar with the 12 zodiac signs (known as ‘rashi’ according to Indian terminology). These zodiac signs are 12 constellations on the ecliptic with approximately equal sizes. Hence we can conclude that sun stays on background of one zodiac sign for almost one month.
I think the Indian Astronomers were interested in defining the ecliptic more precisely. This is evident from the fact that Indians divided the ecliptic into 27 equal constellations which are together known as the ‘Nakshatras’. Now just to mention it, one zodiac covers area equivalent to the area covered by about 2.25 Nakshtras. Now let me ask you a simple question: Which is the brightest star in the sky?.......Sirius? Are you sure?.... Here’s the catch, Sirius is the brightest star in the night sky! Sun- our nearest star- is the brightest star in the sky! O.K.! Given that astronomy is an ancient science, you will notice that most of stars and constellations are named after hero, warriors, princess etc. according to Greek or Indian or some other mythology. The early astronomers were keen enough to make a catalogue of known stars. However ‘Tyco Brahe’ was the one who brought about a revolution in astronomical observations by recording and documenting observations systematically so also by introducing sophisticated tools and instruments. It was his observations that aided ‘Kepler’ to arrive at his crucial laws.
Now at this point I would like you all to appreciate the positioning of planets in our solar system. Amazingly our planets’ positions follow a pattern which is historically known as ‘Titus-Bode Law’. If you convert distance between sun and the planet in astronomical unit (average distance between earth and sun is termed as 1 Astronomical Unit or 1 A.U.), multiply it by 10 and arrange them in order from closest to farthest you get following sequence:
4, 4+3, 4+6, 4+12, ? , 4+48, 4+96, 4+192, 4+384
The missing term (4+24) represents the asteroid belt between mars and Jupiter. In fact when this series was discovered, planets only till Saturn were known. So the series was till 4+96 along with the missing term. Astronomers wondered that there must be some object at the distance corresponding to missing term. And eventually when first asteroid ‘Cerus’ was discovered, the above law got some more importance. Then astronomers suspected of planet beyond Saturn and indeed they discovered Uranus at a distance which satisfied the next term in the sequence. Later on when perturbations in Uranus’ orbit were discovered, it was attributed to some unknown planet. This time without hesitation astronomers predicted its position according to Titus-bode law and found Neptune obeying the law happily! Let us now discuss something about the heart of astronomy- Telescopes. At the commencement of this topic let me clear a small but important misconception about the invention of telescope. Most of you might attribute invention of telescope to Galileo. However, telescope was accidentally invented by ‘Hans Lepershey’- an optician from Holland. But greatness of Galileo lies in the fact that he was the one who first used telescope for astronomical observations. With his primitive refractive telescope he discovered moons of Jupiter, sunspots, phases of Venus and recorded many other vital observations. But as you all might be familiar, refractive telescope has some major drawbacks like chromatic aberration, cost of manufacture etc. Hence the genius invention of reflective telescope by Isaac Newton became more popular among the astronomers. A small diagram below explains you the relevant features: Now days we have some other versions of reflective telescopes apart from the ‘Newtonian telescope’. We shall surely deal with them in some detail in later talks but for
time being you can understand their configuration easily by referring to following pictures: Many a times light from distant star clusters is obscured due to interstellar dust. Hence optical observations can not be useful. In such cases observation in other bands of electromagnetic waves is very helpful. For such observations we use telescope at different wavelengths. For instance to see in radio region we have radio telescope and so on. You might be aware of one such radio telescope very near to us. That’s right! It’s GMRT (Giant meter wave radio telescope) near Narayangaon.
Alright! Let us suppose that your keen amateur observations resulted into discovery of new star. Hence you want your friend- who has no knowledge of known constellationsto observe it. How will you convey the position of that star to your friend? As some of you rightly suggested, we need a coordinate system! We shall discuss here two important coordinate systems: Alt-azimuth: As the name suggests it comprises of altitude and azimuth. Horizontal angular rotation gives you azimuth and angular rotation perpendicular to horizon gives you the altitude. The point on the horizon directly below the pole star is the origin of this system i.e. north most point on the horizon is the origin. Angle measured above the horizon and clockwise w.r.t. origin is positive by convention. So if I ask you what are coordinates of pole star from pune then the answer will be: Azimuth= zero; Altitude= latitude of pune You must have surely guessed what the draw back is in this system. Yes! It is local and instantaneous. More over coordinates of a star in this system will change constantly as the earth rotates. So if you see ‘Rigel’ today at 8pm and note its altitude and azimuth, and use them to locate rigel once again at 8pm on some other day you will end up locating some other star. This happens due to revolution of earth. Try to think over this fact yourself! I am sure you will get to its understanding after a brief thought.
R.A.-Dec: To understand this system I want you all to recollect the terms ‘celestial sphere and celestial equator’. The origin point of this system is the ‘vernal equinox’ (ecliptic & celestial equator meet at two points vernal and autumnal equinox). Consider a star on celestial sphere. Angular distance between star and celestial equator is called ‘Declination’ and it is usually measured in degrees. Now imagine semicircle from celestial north pole to south pole passing through the star. This curve will intersect the celestial equator at some point. The angular separation of this point from vernal equinox gives us the ‘Right Ascension i.e. R.A.’ which is usually measured in hours. R.A. is measured towards west of vernal equinox. If you feel above explanation is too complicated then listen to this simpler version: Imagine grids of latitudes and longitudes on celestial sphere and you are at the centre. Then the latitude of a star is what is called as declination and longitude is R.A. with longitude passing through vernal equinox as the prime meridian! This system is global and changes very minutely over centuries.
So far we have sketched a very crude picture of astronomy in our minds which will be vital to help discussion on astronomy further. Let us try to imprint it clearly on our minds till the next astronomical talk. Till then keep gazing at the sky!
References:
Lectures and discussions held at Khagol Mandal, Badlapur.