View previous topic | View next topic

what is a year?

Page 1 of 3
Goto page 1, 2, 3  Next

gruff5
417441.  Sat Oct 04, 2008 5:38 am Reply with quote

A day seems an unambiguous thing to measure - the sun is at its highest point at noon.

But what is a year?

1) Is it defined by when the sun (from the earth) appears against the point of its cycle against the background of stars?

2) Or is it defined by the summer/winter solstices and the like?

If it's option 1), then are the seasons drifting thru the year as the precessional cycle marches on?

 
bobwilson
417447.  Sat Oct 04, 2008 5:48 am Reply with quote

there are several different years:

Sidereal
Tropical
Lunar

etc

 
RLDavies
417449.  Sat Oct 04, 2008 5:50 am Reply with quote

Depends on what you mean by "year".

The one you're talking about, the earth's rotation measured against the background of stars, is the sidereal year.

There are plenty of others: http://en.wikipedia.org/wiki/Year

 
gruff5
417519.  Sat Oct 04, 2008 8:48 am Reply with quote

RLDavies wrote:
Depends on what you mean by "year".

The one you're talking about, the earth's rotation measured against the background of stars, is the sidereal year.

There are plenty of others: http://en.wikipedia.org/wiki/Year

I was thinking of our calendar year. Which, from your link, apparently attempts to keep the vernal equinox on or close to March 21. This is not the same as the sideral year, which would have seasonal drift over the years.

I like the idea of the Persian year, which doesn't use numerical rules, but begins on the day on which the vernal equinox falls. Cool.

 
mckeonj
417931.  Sun Oct 05, 2008 10:44 am Reply with quote

The early British, or 'Celtic' calendar was nice and simple. It came from the same roots as the Persian; the year of thirteen lunar (28 days) months began at the vernal equinox, each month was named for the constellation in which the sun arose, the names of the thirteen months were also the names of the consonants of the 'beth-luis-nion' (alphabet). Vowels were treated separately; named for the five planets (wandering stars), and usually omitted in writing and inscription.
13x28=364 days; the mean solar year is just about 365 1/4 days; this left one day over at the end of each year, which made a useful festival; in a leap year you got a two day festival.
Overall, a very sensible and useful system, which got well and truly messed up by the Christians, who also supplemented and re-ordered the alphabet.
Oh yes, the early priest astronomers did know about precession and so on, and made periodic adjustments, the 'Age of Aquarius' and all that.
All this, and more, can be found in 'The White Goddess' by Robert Graves, a QI book which should be on every QI shelf.

 
Prof Wind Up Merchant
418207.  Mon Oct 06, 2008 4:11 am Reply with quote

I would say a year would be the time between subsequent winter solstices.

 
gruff5
418223.  Mon Oct 06, 2008 4:48 am Reply with quote

Thanks mckeonj, those early Celts were obviously not as dumb as we've been led to believe, as that was a very neat calendar they (we) had. Regular 28-day months would also have the effect of the month dates always being aligned with the same days of the week. So, unless you'd be a castaway, chances are you'd almost never have to ask "What's the date today"?

Yeah, Prof, it seems more 'natural' (whatever that means) to me also to start the new year on the winter solstice.

 
mckeonj
418226.  Mon Oct 06, 2008 5:06 am Reply with quote

The Analemma figure
Graphical representation
x axis - time in minutes: y axis - distance from ecliptic

Refer to the graph to see the year divided into four seasons by the x and y axes; the equinoces are on the x axis, and the solstices on the y axis.
Any calendar system could be plotted on the figure, the current one is shown in red.
Photograph of the sun's path in the sky throughout the year
The tilt is a function of latitude, it would be vertical at earth's equator.

For added interest, here is the Martian analemma; the sun images are 30 marsdays apart.

 
gruff5
418263.  Mon Oct 06, 2008 6:14 am Reply with quote

Lovely pictures of analemmas :-)

mckeonj wrote:
The tilt is a function of latitude, it would be vertical at earth's equator.

And I guess the asymmetry of the two loops of the figure-of-eight is also a function of latitude? So the "8" would be symmetrical at the equator?

 
General_Woundwort
418566.  Mon Oct 06, 2008 2:48 pm Reply with quote

gruff5 wrote:
A day seems an unambiguous thing to measure - the sun is at its highest point at noon.


But, if at either solstice the sun were to rise at 1156 hrs and set at 1204 hrs, with the the total down-time lasting eight minutes, that would be a year, right?

I assume it's measured by the period the Earth takes to orbit the Sun. Which can vary.

 
gruff5
418751.  Tue Oct 07, 2008 5:00 am Reply with quote

I'm not keeping up with you here, General.

General_Woundwort wrote:
But, if at either solstice the sun were to rise at 1156 hrs and set at 1204 hrs, with the the total down-time lasting eight minutes,

So you're suggesting a location just outside the arctic circle. Where at summer solstice you get just 8 minutes where the sun is below the horizon at midnight & at winter solistice you'd get just 8 minutes of sun above the horizon at noon. Have I understood that right?

General_Woundwort wrote:
that would be a year, right?

It's just a day - why would it be a year?

General_Woundwort wrote:

I assume it's measured by the period the Earth takes to orbit the Sun. Which can vary.

I think the orbital period of the Earth around the Sun is constant over human timescales - about 365.25 days.

 
Davini994
418972.  Tue Oct 07, 2008 11:35 am Reply with quote

How does this apply to sundials?

 
gruff5
419085.  Tue Oct 07, 2008 3:55 pm Reply with quote

Davini994 wrote:
How does this apply to sundials?

I was on holiday in Malta a few years back, was walking thru the main town, Valetta, and was intrigued to come across this on a wall in the town square.



This shows only noon time and there was a "hole device" on the opposite side of the square to throw a sun image onto the analemma

I remember Tom Hanks in the film "Cast Away" marking an analemma on his cave wall as well.

 
Davini994
419092.  Tue Oct 07, 2008 4:32 pm Reply with quote

I reckon someone could get a dissertation out of this.

 
Izzardesque
419094.  Tue Oct 07, 2008 4:34 pm Reply with quote

mckeonj wrote:

For added interest, here is the Martian analemma; the sun images are 30 marsdays apart.


I just can't stop staring at that photo. The idea that we can see the surface of another planet that clearly is just mindboggling to me.

 

Page 1 of 3
Goto page 1, 2, 3  Next

All times are GMT - 5 Hours


Display posts from previous:   

Search Search Forums

Powered by phpBB © 2001, 2002 phpBB Group