What is the exact length of one year?

Answer 1

It depends on exactly what you mean by "year" and it also varies over time, so there is no single exact answer.

The short answer is either approximately 365.24218967days long (365 days, 5 hours, 48 minutes and 45.1875 seconds) on average, or approximately365.256363051 days (365 days, 6 hours, 9 minutes and 9.7676 seconds) in the year 2000, but changing slightly every year.

All scientific definitions of a "year" for our earth are within about 25 minutes of each other, but vary based on what you measure against. Everything in the universe is constantly moving relative to everything else, so there is no absolutely fixed frame of reference to measure anything against. All measurements are relative to something you choose to measure against and are only valid for the period in time in which you took the measurement. Measurements are also only accurate to the precision you can attain with the measuring equipment you use. It is impossible to measure anything with infinite precision. Every measurement is an approximation with some limited precision over a period of time where what you are measuring is constantly changing, however infinitesimally. At best you can only measure an approximate range between some limits. On top of that a "year" can be measured many different ways.

A "Tropical Year" uses the earth's tilted axis and equatorial plane to gauge the time it takes for the earth to complete one kind of cycle around the sun we call a "year". Over the course of a year as the earth orbits the sun, the earth's axis and equator change their orientation relative to the sun, cycling back roughly to the starting point once every "Tropical Year". Over these repeating yearly cycles, days and nights get longer and shorter in the northern and southern hemispheres for half of a year and then reverse back the other direction for the other half of a year. This is one primary way measure a year.

A "Tropical Year" in recent times is approximately 365.24218967 days long (365 days, 5 hours, 48 minutes and 45.1875 seconds) on average over all equatorial points. This time changes slightly from one year to the next, depends on exactly what equatorial point you are measuring the cycle of and is influenced slightly by so many factors that constantly change that we will probably never be able to predict precisely how long a "Tropical Year" thousands of years in the future will be. That far out, things could change by many seconds compared to what we do our best to estimate or predict due to tiny effects of interactions with meteors and asteroids and other things that we can not possibly predict and account for.

A "Calendar Year" is exactly 365.2425 days long (365 days, 5 hours, 49 minutes and 12 seconds) on average over any 400 year period. A year is exactly 365 days on non-leap years and 366 days long on leap years and we declare 97 of every 400 years to be a leap year. That's one in four minus one in 100 plus one in every 400 years. This is the only exact length you can cite because we define the calendar year precisely this way as opposed to measuring what actually physically happens. All other measured years can only be approximated and will always be at least slightly different from one year to the next. Even if two measured years were exactly the same, we'd never know it for sure. We'd know they were at least very, very close, but we could not know that they were the same or exactly how close they were if they were ever so slightly different by less than our ability to measure.

A "Sidereal Year" is the time it takes for the earth to orbit the sun relative to the distant stars which are not stationary, but the closest thing to stationary we can measure against so far, due to their great distance and relatively slow movement in space. The "Sidereal Year" in the year 2000 was approximately 365.256363051 days (365 days, 6 hours, 9 minutes and 9.7676 seconds).

The seasonal changes we experience each year as the earth orbits the sun are due largely to the earth's axis tilting the northern and southern hemispheres toward and away from the sun over the course of a "Tropical Year". This "Tropical Year" is what our calendar attempts to approximate by interjecting an extra day on leap years so that winter and summer occur at roughly the same time of year, year after year. If our calendar approximated the "Sidereal Year" instead of the "Tropical Year", winter and summer would slowly shift earlier and earlier in the year over time.

The severity of the earth's summers and winters also fluctuate over time partly due to the earth's orbit being elliptical. Our elliptical orbit brings us closest to the sun at one point during an "Anomalistic Year" and farthest away from the sun at another point in our orbit a half of an "Anomalistic Year" later.

An "Anomalistic Year" is approximately 365.259635864 days long (365 days, 6 hours, 13 minutes and 52.5386 seconds), slightly longer than the "Tropical Year". Because these things are not in sync with each other the distance at any given time of the "Tropical Year", such as winter or summer in the northern or Southern Hemisphere, from the earth to the sun becomes closer or farther from one year to the next and then eventually cycle back to roughly the same distance about every 21,000 years give or take a lot since all of these things are constantly changing a little.
Except in leap years 352 years but in leap years 353