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cristo

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The numbers changes slightly from year to year.

For the tropical year you can read this

http://en.wikipedia.org/wiki/Tropical_year

for example. It has a table towards the bottom with the actual values for several years.

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AlephZero

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One meaning is to do with calendars. That includes the Julian Year used in astronomy (exactly 365.25 days), various calendar years (Western, Islamic, etc), financial years (either exactly 52 or exactly 53 weeks long), etc. These are "defined" rather than "measured". the relevant international standard is ISO 8601.

The other meaning is to do with the earth's rotation around the sun, and as another post said that has to be measured,since it is perturbed by the gravitational attraction of the other planets etc. Because of effects like precession (the "wobble" of the earth's axis of rotation, with a period of about 26,000 years) there are several possible ways to define it which give slightly different answers.

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I may actually be able to help out some in regards to the traditional duration of a year based on the tropical year.

The orbital plane of the earth's yearly rotation around the sun stands tilted at a 23° 26' (though not a constant) degree angle towards the earth's equator. The projection of this plane on to the firmament is known as the ecliptic. The ecliptic intersects the*celestial equator* (the projection of the equatorial plane onto the firmament) in two places, the *vernal equinox* (0' Aries) and the *autumnal equinox* (0' Libra). The sun appears there in the northern hemisphere at the beginning of spring around the 21st of March (vernal equinox) and at the beginning of autumn around the 23rd of September (autumnal equinox). At both equinoxes the sun rises in the east and sets precisely in the west. So, the period of time from one sun's passage of the spring equinox till the next is called the tropical year.

But to answer your question; the exact value of duration based on the*tropical year* is 365.242 198 79 days = 31 556 925.9747 seconds.

Hope that helps!

The orbital plane of the earth's yearly rotation around the sun stands tilted at a 23° 26' (though not a constant) degree angle towards the earth's equator. The projection of this plane on to the firmament is known as the ecliptic. The ecliptic intersects the

But to answer your question; the exact value of duration based on the

Hope that helps!

Last edited:

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Thanks for the explanation. However, it seems that a year, when used by physicists, is the Julian year, or 365.25 days. One wonders why the more accurate Gregorian year of 365.2425 days was not used instead.I may actually be able to help out some on this one. You are correct in that it is actually based on the tropical year and the two events you speak of are the vernal equinox (as you mentioned) and the other is the autumnal equinox. I will try to explain this as I understand it.

The orbital plane of the earth's yearly rotation around the sun stands tilted at a 23 deg 26' 32" degree angle towards the earth's equator. The projection of this plane on to the firmament is known as the ecliptic. The ecliptic intersects thecelestial equator(the projection of the equatorial plane onto the firmament) in two places, thevernal equinox(0' Aries) and theautumnal equinox(0' Libra). The sun appears there in the northern hemisphere at the beginning of spring around the 21st of March (vernal equinox) and at the beginning of autumn around the 23rd of September (autumnal equinox). At both equinoxes the sun rises in the east and sets precisely in the west.

So, the period of time from one sun's passage of the spring equinox till the next is called the tropical year. But to answer your mathematical question; a tropical year has the duration of 365.242 198 79 days = 31 556 925.9747 seconds.

Hope that helps!

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My guess would be that it's taken on average (perhaps so as to have some form of a constant for mathematical calculations). Someone else more knowledgeable than I would likely know.

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It's astronomers, not physicists, who use the Julian year. Physicists tend to use seconds.Thanks for the explanation. However, it seems that a year, when used by physicists, is the Julian year, or 365.25 days. One wonders why the more accurate Gregorian year of 365.2425 days was not used instead.

Seconds aren't all that convenient a time scale in planetary astronomy. Years and centuries are much more convenient. Nowadays astronomers use "days" that comprise exactly 86400 seconds and Julian centuries that comprise exactly 36525 of those days. This is a fairly recent development. Prior to the 1960s or so, astronomers used Besselian years. The SOFA still provides functions to convert Besselian epoch to Julian epoch.

Astronomy has been moving away from things based on the fictitious mean sun for the last sixty years or so. For example, there is no such thing as GMT anymore, at least not officially. GMT was replaced by UTC in January 1, 1972. The tropical year is inherently based on the concept of a fictitious mean sun. It's a concept that is a bit outdated concept, at least in astronomy.

- Which tropical year? The mean time between successive summer solstices is slightly different than the mean time between successive spring equinoxes.
- In what year? The length of the mean tropical year is not constant. The Sun is slowly losing mass and slowly transferring rotational angular momentum to the Earth's orbital angular momentum. The number of seconds in a tropical year is increasing slightly.
- What unit of time? In terms of number of (solar) days per (tropical) year, the mean tropical year is getting shorter. A solar day is no longer 86,400 seconds long.

Outdated or not, it's still a concept of interest in a lay sense because the seasons follow the tropical year.

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therein, i had to do a little math, and i got the value of 365.257 Earthyear_indays. Is it

precise? I don't know. Think of it like this. The Earthmoondistance_inmiles is only precise at a specific point in time because it orbits the Earth ellipitically. So it has a minimum

and maximum distance from the Earth. When averaged out, the moon is 238,855 miles from the Earth, but then again no x number of sources can even agree on this. So maybe then this pertains to the length of the year also.

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tropical earth year = 365.24219879

sidereal year = 365.25636042

365.249279605 - 365.25636042 = 0.01416163/ 2 = 0.007080815 + 365.24219879 = 365.249279605 or 365.25

The sidereal year is the space of time between the sun's passages of a certain star and is about 20 minutes longer than the tropical year due to the retrograde motion of the point of vernal equinox.

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Since you started this, I have a different question which someone suggested as an April fool joke. But it got me thinking. Things would be much simpler if we adapt a different time keeping system. Yes, a decimal or a metric system instead of base-60 time keeping.

100 sec = 1 minute

100 minutes = 1 hour

10 hours = 1 day (12-hour)

10 months = 1 year, months will have more days.

Is this idea totally impractical?

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