**While the maadha sadhurtthi falls on 2.3.2017, maadha sashti occurs the next day (3.3.2017) – Why?**

A *thiti *is a lunar day which is approximately 1/30^{th} of the time it takes to orbit the earth. It is the time it takes for the longitudinal angle between the moon and the sun to increase by 12 degrees. *Thithis *begin at various times of a day and vary in duration from approximately 19 to 26 hour. As a ‘thithi’ can be shorter or longer than a day, a ‘thithi’ can spread for two days or two ‘thithis can occur in a day. On Thursday 2.3.2017, two ‘thithis’ namely, *sadhurtthi *and *panchami*, are occurring; therefore, the ‘thithi’ which occurs on Friday (the next day), 3.3.2017, is *sashti*.

The following citations from the respective websites, the scientific phenomenon for the change in the duration of a ‘thithi’:

Jagdhish C. Maheshri writes thus: “The angular velocity of moon in its elliptical orbit around the earth continuously varies as it is affected (according to Kepler’s rule) by the relative distance between the earth and the moon, and also by the earth’s relative distance from the sun. As a result, the daily angular speed (the speed of the angle between the moon and the sun as seen from the earth) varies somewhere between 10 to 14 degrees. Since the length of a *tithi *corresponds to 12 such degrees, the length of a *tithi* also varies accordingly. Therefore, a *tithi* can extend over one day (24 hour period) or it can get skipped if two *tithis *occur in one day.” http://www.boloji.com/index.cfm?md=Content&sd=Articles&ArticleID=1089

There are three laws in the “Kepler’s laws of Planetary Motion”. According to the second law, “a line joining a planet and its star, sweeps out equal areas during equal intervals of time”. This is also known as the ‘law of equal areas’. Suppose a planet takes 1 day to travel from point ‘A’ to point ‘B’. During this time, an imaginary line, from the Sun to the planet, will sweep out a, roughly, triangular area. This same amount of area will be swept every day. As a planet travels in its elliptical orbit, its distance from the Sun will vary. As an equal area is swept, during any period of time, and since the distance from a planet to its orbiting star varies, one can conclude that in order for the area being swept to remain constant, that, a planet must vary in velocity. Planets move most rapidly when at perihelion and more slowly when at aphelion.” http://encyclopedia.kids.net.au/page/la/Laws_of_Kepler

According to dictionary, ‘perihelion’ is ‘the point in its orbit when a planet or comet is nearest the sun”, and ‘aphelion’ is ‘the point in the orbit of a planet or a comet at which it is farthest from the sun’.

Prepared by Dr.K.Thilagawathi

* Image sourced from minimela.com