That the earth’s axis is tilted around the plane of its orbit was known even to ancient civilizations who had no understanding that the earth moves around the sun. After all, the noontime sun moves polewards in summer and in the opposite direction in winter. As a result, days are longer in summer, and nights longer in winter. Even a layman could see that. It was also fairly easy in the temperate zones of the earth to connect these motions to four distinct seasons. This was the beginning of ancient astronomy, and its off-shoot, which is the modern calendar.
Doubt thou the stars are fire,
Doubt that the sun doth move,
Doubt truth to be a liar,
But never doubt I love.
–Hamlet (Act 2, Sc 2) William Shakespeare
But today, as the world begins another mad celebration of the arbitrariness of this calendar, and the decimal system of writing numbers (the end of the year, and, mistakenly, the end of the decade), I was moved to ask whether any special meaning could be given to a calendar. In the Elizabethan era, even as Shakespeare was writing about the fixed nature of the earth as an eternal truth, the earth was displaced from the center of the cosmos. It was realized that the earth orbits the sun, and that the sun was but one out of many stars. It was realized that the earth moves in an elliptical orbit around the sun. The difference between the long radius and the short is just about 1 percent of the radius, so this difference is not easily observed.
It remains that from the same principles we demonstrate the form of the system of the world.
— Principia Mathematica (Book 3) Isaac Newton
Small it may be, but the ellipticity is there, to mark special points on the orbit: special dates of the year. There is a date when the earth is furthest from the sun, and one when it is the closest. Between January 3 and 4 the earth comes closest to the sun. I will mark the beginning of a new orbit around the sun, a new year, on the coming Friday. How great a coincidence it is that this will come 12 days after the birthday of Isaac Newton, the man who understood that the reason for a ripe apple falling from a tree is the same as that which forces the orbit of the earth to be an ellipse. And what is special about the coming year is that it ends the 4th century after Johannes Kepler’s discovery (by 1619 CE) of the laws of planetary motion; that’s not the end of a decade, it is the end of the century. 2020 CE may as well be called 401 Keplerian Era.