the history, science and mathematics of indian (हिंदू hindu) and other calendars (english edition)  the book is about the evolution and history of different calendar systems from sumerian to gregorian calendars. the logic, the history, science and mathematics, and why and how they have changed and have been adopted differently across the world. the solar, seasonal, sidereal, lunar and lunisolar calendars and their basic difference have been described vis-a-vis the motion of earth and moon, many allied topics such an eclipse are explained in a simple way such that anyone can tell the approximate date of the next eclipse. easy, real life examples are given to describe motions of heavenly bodies. some very interesting facts of the earth and universe are covered, which you always wanted to know, but never got all the facts at one place. <br> <br>though the book has been written mainly to cover indian and hindu calendars, more than half the text is about other calendars of the world. very interesting facts like, why february has 28 or 29 days and why a month has 30 or 31 days are explained with reasons and chronology. <br> <br>the book is a must for all indians, especially hindus who will find answers to almost all queries regarding hindu calendar dates and why they are so different. hardly any book is available which has covered both facts and faith about hindu calendar systems, and how inspite of different calendars, the festivals are celebrated on the same day. <br> <br>taking about indian calendars, systems followed by different religions, hindus, muslims, sikhs, christians, buddhist, persians, bahai etc. are covered as well as that of neighbouring countries, who are also having similar calendar systems. <br> <br>a few extracts from the book are given below: <br> <br>"a festival falling on different dates in different years is a common feature of all religions. for hindus, the hōlī festival may occur on any day from march 1 to march 29; for muslims the īd may happen on any day of the year; whereas for christians, the easter sunday may occur from march 22 to april 25 or as celebrated by some orthodox churches from april 3 to may 10. this does not happen arbitrarily, but there is a history, science and mathematics behind this." <br> <br>" ii. to actually see the new moon (i.e. the crescent moon, visible normally on the 2nd day after new moon) and adopt the same evening or the next morning as the first day of the month. <br> <br>the latter system had a problem, as it depended upon weather and visibility by a person. so, it was decided that the sighting will be acceptable only when announced or confirmed by some designated people (e.g. present day hilal committee in islam, “hilal” meaning crescent in arabic). some countries have now resorted to calculations also. " <br> <br>" this causes the time difference between solar and sidereal year, such that a full cycle will be completed in 25771.5 years. around 2100 ce the north pole will point towards polaris and in 14000 ce towards another star vega, replacing polaris as the north star. again around 27,800 ce, the polaris will become the north star." <br> <br>"....the earth is closest (e.g. 147 million km, 2nd january 2021) to the sun at the perihelion (peri = surround, close, helios = sun) about 2 weeks after the december solstice; and farthest (e.g. 152 million km, 6th july 2021) at the aphelion (apo = apart, far) point, 2 weeks after the june solstice. <br> <br>the dates when the earth reaches the extreme points are not fixed and have drifted by a day every 58 years. in the short-term, the dates can vary up to 2 days from one year to another. the speed of the earth is fastest (30.3 km/sec) when it is closest to the sun, and slowest (29.3 km/sec) when farthest. <br> <br>if the earth is closest to the sun in january, why there is winter in the northern hemisphere.