Happy New Tropical/ Anomalistic/ Sidereal Earth Orbital Time Period!

Congratulations! You have survived the Earth taking another trip around the sun in what is called a sidereal year. On average that journey takes us 31,558,149 seconds (approximately pi x 10 million = 31,415,926 seconds). Note that I measured that in seconds, because the metric system defines a second as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.” There is no single definition of a day. A sidereal day using the stars as a reference point is defined as “the time it takes for a star to go from directly overhead to directly overhead again: 23 hours 56 minutes 4 seconds = 86,164 seconds.” Each day varies from the next in length by 3 minutes, 56 seconds. The day as defined by rotation of an observer on earth with respect to the sun is 86,000 seconds, plus or minus a leap second. What about those other 164 seconds per day? Using the sidereal year we have 31,558,149 seconds. If we divide the sidereal or stellar year by that by 86,164 seconds/ day we get 366.256 days per year. In solar days, you divide the seconds in a year by 86,400 to get 365.256 days. Just as the solar day varies by about 4 minutes due to anomalies in the rotation of the Earth with respect to the sun, the stellar year varies due to anomalies in the revolution of the planet around the sun. There is additional error in the measurement, because the Earth and sun are both large, three dimensional shapes, not simple point sources. This means both the 365.256 day and 366.256 day estimates on the length of a year have error bars on them.  To make matters worse, the Earth’s orbit around the sun precesses, or slightly changes, as it wobbles, thus changing when the sun crosses the equator at vernal equinox. The tropical year is 31,556,941 seconds long, which can vary from the sidereal/ stellar year and solar year by 21 minutes per year! Over time that would move the seasons. Even if you are a diehard fan of the year you were taught in grade school, that number varies due to fluctuations in perihelion (Earth closest approach to the sun, 148 million kilometers) and aphelion (Earth furthest approach to the sun, 152 million kilometers) in its elliptical orbit. The average length of the anomalistic year that I just described is 31,558,432 seconds, or 365.26 days. 

All this just to show that when you say “happy new year”, that the precise moment of the new year can vary depending on whether you mean Gregorian, tropical, anomalistic, sidereal, solar or lunar new year.

That being said, happy new year, however you define it.

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