Sundial

[Mike Kelly]

I have yet to carve this out, but as soon as I can get a 2' piece of corian its done.

Took 2.5 hours to calculate the lines out for my location an lay it out in Partworks.

[Brady Watson]

Cool...thanks for posting. Can you point me towards some online resources to do the calculations myself and make my own ?

Thanks!
-B

[zeykr]

Take a look at http://www.shadowspro.com

A sundial has been on my todo list for some time!

[Mike Kelly]

Brady

I used the calculator at http://www.anycalculator.com/horizontalsundial.htmhttp://

There is also some info at http://www.mysundial.ca/

Also the site/program that Ken refers to is good free program, but if you want to export to dxf it requires a $60 upgrade


Mike Kelly



Cool...thanks for posting. Can you point me towards some online resources to do the calculations myself and make my own ?

Thanks!
-B

[dana_swift]

Interesting project, I have also considered building a sundial. It will give you an appreciation of the motion of the earth which is not nearly as simple as it appears.

The problem is the analemma, which is a fancy word saying the sun moves around in the sky at the same time of day depending on the day of the year. It makes a non-adjusted sundial less accurate than you might think.

The sun moves north and south 46.9 degrees over a year, and east and west approximately 10 degrees. The east west motion results from the earths orbit around the sun not being a circle. That motion is independent of your position on the earth. It makes summer days last longer and longer as you move further from the equator, not setting at all for parts of the year above the arctic circle.

If the sundial is mounted on an Az-El mount so it can compensate for the annual analemma, the sundial can be accurate within 1-2 seconds if it is large enough to see that level of motion. Without it the error peaks at about 20 minutes.

Its no big error and shouldn't slow anybody down from putting up a sundial and having fun with it. But don't set your clocks by its indicated time, your cell phone time should be within a second.

A great conversation starter for sure..

Cool project!

D

[curtiss]

If you have your Aztec calenday next to your sundial you can correct for the analemma I think...

The little arrow thing is supposed to point at the north star which you can do at night.... or set it for solar noon in your area.

One would need two sundials, cut an hour apart, which are changed out with daylight savings time.

[knight_toolworks]

I have made a few of these. a engineer makes them and he does all the real work. I have to edit the lines to make them work and I carve them on these 24" aluminum discs that he machines flat. They show how non flat my machine is.

[khalid]

What a great topic.. I am struggling making one sundial for couple of days..using Shadowspro expert printed out one on A4 size..Now want to get the Meridian to get the direction of Pole star,,,... I will use few circle and get the shadow of the sun ...

Equation of time should be on the side of sundial to get the accurate time of the day...

[Brady Watson]

Ken, Mike & Dana - Thanks!

-B

[ironsides]

I have a Sun Dial purchased from garden shop.

It is extremely accurate ;) once a month, right after I calibrate it to match my Atomic Watch and Daylight Savings Time:)

George

[ironsides]

By the way, most of the sundials I have seen, use the Roman Numeral "IIII" rather than "IV" for the number 4.

If you Google this, you will find out that some Roman Pharaoh didn't like "IV" on the dial because it was a symbol for something he didn't like.

George

[khalid]

By the way, most of the sundials I have seen, use the Roman Numeral "IIII" rather than "IV" for the number 4.

If you Google this, you will find out that some Roman Pharaoh didn't like "IV" on the dial because it was a symbol for something he didn't like.

George

Hi George,
In Astronomy I,II,III,IV is termed as Soustractive system of Roman Numerals, whereas I,II,III,IIII...VIIII is termed as additive system..

[dana_swift]

An interesting link:

http://precisionsundials.com/renaissance.htm

This instrument is not very large and achieves 5 minutes of precision. A larger instrument using the same idea could get well under a minute.

The same site has another interesting design, with a clever solution to the analemma problem and also achieves 5 minutes of precision with a smaller instrument:

http://precisionsundials.com/schmoyer.htm

When I see clever ideas like that.. I think.. whoever designed these needs a shopbot.. then think of all the cool contributions they would make to the forum!

The more obscure- (most folks will want to stop reading here)

With optics combined with a sundial, roughly one second accuracy is possible. That is basically how we know when the earth has slowed down enough to require another leap second. I don't know what the current reference is for the actual rotation of the earth, but for a very long period of time it was (or is?) the "transit telescope" (timing star crossings) at the observatory at Greenwich England. The term "Greenwich Mean Time (GMT)" was measured by this instrument (and others) and is the original definition of coordinated civil time today across the world.

For much more than anybody really cares to know, go to the international earth reference system site in Paris France. They keep track of such details as how much the sloshing of the earths liquid core has monkeyed with rotation of the earth on any given day. They publish monthly tables indicating on each day how much the north pole has wobbled, etc.

http://www.iers.org/

Preliminary predicted information about today and the next year:

http://data.iers.org/products/6/14988/orig/bulletina-xxv-015.txt

Note its only preliminary, as all the measurements as to what is happening will not be in for several days, so we are never sure exactly which way the earth is pointing at any given moment! wierd eh? Also note the UT1-UTC which is the difference between atomic time and measured earth rotation time, which jumps from -0.57528 to 0.42465 on July 1 of this year because the earth will have accumulated down another second of error since the last correction. And notice the exact rotation time is kept to an accuracy of 5 digits! Thats 10 microseconds! Pretty good for basically a fancy sundial.

And in case you are curious why I would know or care about any of this, it used to be part of my job.

Mechanical measurements of the sky still define our measurement of time.

Change topics-

"Roman" numbers we use to day are only generally related to the ones used in ancient rome. A roman from that period would probably not be able to read most of the values they would see in modern "roman numerals".

I set out several years ago to write a program that would read roman numeral strings and convert it its integer value, with the intention of making it a homework assignment for my students. But it seems there are quite a few different ways of encoding a "roman value", and like the IIII issue, over the years there have been quite a few variations. For more info look it up on wikipedia. Not a very exciting read, but informative. It made it clear that it would only confuse the students, so I dropped the idea.

D