Sundial

[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/6/1498...na-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