The Earth, along with us, takes 365 days 6 hours 9 minutes and 10 seconds to complete one trip around the Sun. Our location on Jan 1st – the New Year’s Day – is opposite the constellation of Sagittarius (the Archer) in this celestial circle at present.
But.. why “at present”?
Because the position of Jan 1st recedes slightly each year. The reason is that the present ‘Gregorian’ calendar system reckons 1 year a little shorter at 365 days 5 hours 49 minutes and 12 seconds – around 20 min less than the actual orbital time (or the ‘Sidereal Year’).
This is done not arbitrarily, but for a very good reason: a gentle sweep or precession of the Earth’s rotational axis, which causes seasons to arrive slightly faster than they otherwise should each year by around 20min. This slightly shorter year is called the ‘Tropical Year’ in contrast to the ‘Sidereal Year’. The Tropical Year is, for all practical purposes, what we mean by one year.
This ‘axial precession’ or ‘precession of the equinoxes’, which takes around 26000 years for a full cycle, was known since antiquity – ancient Sanskrit astrological texts called it the ‘ayanamsa’.
So every year, on an average, we celebrate Jan 1st around 20 min earlier than the previous year, taking around 26000 years (365x24x60/20) to come back full circle.
In addition, there is the annual difference of ~5h:49m continually corrected by the leap year cycle that adds February 29th every 4th year – except on century years other than those divisible by 400.
This curious century year exception, known as the Gregorian reform, was added by Pope Gregory XIII in 1582 to correct the Julian calendar introduced by Julius Caesar in 45BC which only had leap years, and thus an average tropical year of 365 days and 6 hours: an error of ~11 minutes, which however lead to a cumulative error of 15 days by 1582! Because of this rule, 1700, 1800 & 1900 were not leap years, but 2000 was – being divisible by 400.
To summarise: Jan 1st does a regular 4-year hop around, with an occasional interruption of the forward leap in three out of four century years, while it gently recedes along the Earth’s orbit over a 26000 year period.
Jan 1st is a moving place!
Welcome once again to Jan 1st my co-travelers in this cosmic merry-go-round, and may our next round be a jolly good one 🙂
Happy New Year!