Kepler’s laws and Venus

Before we proceed to discuss the remaining parts of planet Venus, I have a promise to fulfil. Modern science owes much to Johannes Kepler (1571-1630). He was born in a region that is nowadays included in southern Germany, to very poor parents. Unfortunately, he had an unhappy childhood in which he remained distraught. He had a long drawn dispute with inquisition (in this case it was a German religious court set up by church to punish people for heresy) defending his mother charged for witchcraft for three years. To top it all, he lost a young son when the child was only eight. Although Kepler in turn owes much to the methodical but whimsical Tycho Brahe’s observation of planets, he devised his famous laws of planetary motions which have served as the bulwark of astronomy ever since. By the way, Tycho (he is usually remembered by his first name rather than his surname), the astronomer from Haven, Denmark, was the last of the great astronomers of the pre telescope era. In short, Kepler’s laws of planetary motions are: 1.    The orbits of the planets are not circular but ellipses with the Sun not at the centre but away, at one focus or place. 2.    A line from the planets to the Sun sweeps over equal areas in equal period of time. 3.    A planet’s orbital period in years squared is equal to its average distance from the Sun in astronomical units cubed. Kepler’s First Law states that the orbits of the planets are ellipses (elongated circles) with the Sun lying at one focus (place). Though Kepler was able to determine the elliptical shape of the planetary orbits, the orbits in their case are nearly circular, yet they conform to the first law just the same. Of the planets known during Kepler’s time Mercury has the most elliptical orbit, but only slightly. According to Kepler’s Second Law, a line from the planet all the way to the Sun sweeps over equal areas in equal intervals of time. That means that when a planet is closer to the sun and the line connecting it to the Sun is shorter, the planet must move rapidly if the line is to sweep over the same area. But when the planet is farther from the Sun the planet’s motion would carry it differently, rather slowly, but the area would remain the same. Kepler’s Third Law states that a planet’s orbital period squared are proportional to its average distance from the sun cubed. It relates the orbital period to orbital size. Remember that the stars in a galaxy or the binary stars system also follow Kepler’s Laws. Kepler’s skin (life) was saved despite his scientific aberrations and beliefs which were considered wayward by the church, because Northern Europe where he carried out his work was beyond the powers of the church. Simply speaking, new laws would henceforth govern the mechanics of the Solar System. One, that planets do not orbit in a circle but in an ellipse (a slightly squashed circle) and that the sun is not at the centre, but of the centre, a little to one side — how so ever slightly. Secondly, it implies that during any one period (six months or six days), a planet covers the same distance in the sky not more not less. Thirdly, the relation between a planet’s orbital period to its average distance from the sun; that of 2:3. I do appreciate that to many of my young friends, Kepler’s laws might appear to be larger than life and rather difficult to follow. But then so are Newton’s laws (which will come later on) and those of Einstein as well as the atomic nature of matter and also the stars which are essential for the understanding of quantum physics. Some of you might pursue this as a field of study in future as did Professor Salam and Einstein. What you may do is understand Kepler’s laws of planetary motion with the help of the diagram and leave the rest for the morrow when you grow up a little and reread it here or elsewhere. In any case, I am always available to help you out as I do to those asking for it frequently. As far as planet Venus is concerned, we should leave it for the future and take it up in the next issue. Suffice it to say that many secrets remain to be revealed about this, the second planet out from the sun. Adieu till then!

The writer is a professional astronomer and a former head of PIA Planetaria. He can be reached at astronomerpreone@hotmail.com