Year

Year

New postby Peter Blunsdon » Sat Jan 24, 2009 1:05 pm

A little understood quantity of fundamental importance to horologists and astronomers. Without further qualification the term year is meaningless.
Two definitions are in common use:

A solar year takes its basis from the time taken for the earth to travel round the sun.
A sidereal year which is the time taken to travel round a star. In this case a fictitious star called The First Point of Aries.

A solar day is the time taken between the sun crossing the earth's meridian twice. It varies in length according to the time of the year when it is measured. This is because the earth's path is not a circle but an ellipse and also the earth is inclined to the sun. It varies about 30 minutes, being faster in November and slower in February. A sun dial shows this variation in time whereas a clock does not. (You can see this on some sundials when the Equation of Time is shown).

To overcome the problem of varying solar days an artificial point in space was defined, The First Point of Aries, which gives us the property of a year having constant sidereal days.

A sidereal day is shorter than a mean solar day by nearly four minutes.
There are 365&1/4 solar days to 366&1/4 sidereal days in a year.

We love to use solar days in our daily lives because of the influence of the sun. Astronomers love sidereal days because their measurements relate to the relative movement of heavenly bodies.

It is a strange thing that very few clocks show days, ie are '24' hour clocks given its fundamentality to our life.

Peter
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Re: Year

New postby Charlie Sides » Mon Jan 26, 2009 2:33 am

Interesting information there Peter. thanks for posting.
Probably have to read it a time or two until it sinks in.

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Re: Year

New postby Fortunat Mueller-Maerki » Mon Jan 26, 2009 11:49 am

There are many historically significant clocks out there that show sideral hours rather than solar hours.

------------------------------------------

Of course most clocks dont show real solar time either, but 'mean solar time'.

The length of solar hours varies throughout the year (because of the equation of time) .


There are many historic clocks that show real solar time in addition to mean solar time with a seperate minute hand (the two are close enough that you never need a sperate hour hand).

Also found frequently is a display on a clock that just shows the difference between the two.

One of my favorites is a clock at the Geymueller Schloessel in Vienna (Sobeck collection) thatstrikes mean solar time on bells and real solar time on gongs.


Another horological particualrity are clocks whose pendulum beats in real solar seconds rather than mean time, in other words the pedulum length gets varied constantly over the course of a year.



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Re: Year

New postby Peter Blunsdon » Mon Jan 26, 2009 5:08 pm

Thanks Charles,
Thanks to Fortunat.

I have never seen the clocks Fortunat describes, although I know of them.

Here's a sun dial on a cathedral in France that I have seen where the mean and real solar times are shown albeit very roughly.

More clocks Fortunat?

Peter
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Re: Year

New postby Fortunat Mueller-Maerki » Mon Jan 26, 2009 7:38 pm

Peters last picture is of a miniature meridian line, and yes it is in a way a sundial, but it does NOT show time, it shows the DATE and it shows when it is noon


Meridian lines are quite common in European cathedrals, on a one week trip through Italy last year I saw about a dozen of them.

They work like this: Somewhere high up on the roof or in a wall there is a small hole. Exactly south of that hole on the oposing wall (or on the floor) is the figure shown on the picture (called an enemela). When the sunspot crosses the center line it is noon. but h0w far up the line the sunspot crosses indicates the date. Depending on the geometry of the building some meridian lines are accurate to within a day Some noon lines run for hundreds of feet all the way across a church floor.. The illustrated instrument just has a small gnomen with a hole very close to the ( very short) noon line.

The best english language book on meridian lines is:

# Title: Sun in the Church
SubTitle: Cathedrals as solar observatories
# Author: J.L. Heilbronn
# Publisher: Harvard University Press
Other Keywords: meridian meridiane easter enemela suncalendar
ISBN: 0 674 85433 0 -- Library of Congress: QB29.H33 1999 99-23123 -- Dewey: 520'.94-dc21
Language: ENG
Notes: Probably the most comprehensive english language book on the subject of meridian lines and solar calendars, countless illutrations, index, bibliography, scholarly text but quite accessible.
Edition: 1999, 1st edition -- Copyright: 1999
Kind: Book
Type: Sundials
Geographic area: Italy
Topic: Astronomy
Organization: NA/other
Pages: 366 -- Height in cm: 25
Print Status: 1 (1 means in print - 2 means out of print)
BHM ID: 443



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Re: Year

New postby Peter Blunsdon » Tue Jan 27, 2009 11:06 am

Fortunate, Thanks for the detailed explanation, I had not meant to imply that you could tell time, loose language, it of course shows the equation of time over the year.

Interesting side line to our principal hobby, but I expect some folks study nothing else.

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