analemma and equation of time: when first understood

The analemma, photographed by Dennis DeCicco and now appearing in
schoolyards and driveways all over the world, and whose image on globes
of the earth mystified generations of school teachers, and the equation
of time that says that the sun can be ahead or behind the mean sun by upto
15mins, depend on the orientation of the earth's axis with respect to the
plane of the ecliptic and on the ellipticity of the earth's orbit.

I haven't been able to find any historical information as to when people
first knew about this. Presumably once Kepler understood that the planets
moved in ellipses (1609), it was possible to realise that the sun would
be slow or fast at different times of the year. Once clocks could measure
time to within 15mins over a period of a year (late 1700s with Harrison's
chronometers), then you could observe that the sun was fast or slow.

Possible starting points might be:

When did corrections for the equation of time first appear on sundials?

When did the analemma first appear on globes of the earth?

I remember many years ago, seeing an article in S&T about the
shape of analemmas on other planets, but this was before I subscribed
to S&T. The S&T on-line index only goes back to 1996 and doesn't show this
article. Does anyone know when it was published?

Thanks Joe

--
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!

On 2004-08-29, mack wrote:

When did corrections for the equation of time first appear on sundials?

You wouldn't have an equation of time before the development of accurate
clocks. That the movement of the sun varied with the seasons was known
in ancient times because the numbers of days in each season are not the
same, although the lengths of each season in degrees are. Originally, a
hour was the twelfth part of the day or night and therefore was flexible.
Water clocks were adjustable so the length of the hour could be changed
with the seasonal changes in the lengths of daylight and darkness.
Once clocks with reasonably accurate rates were developed the length
of the hour became a fixed quantity and you could observe that the sun
didn't cross the meridian at noon clock time.

When did the analemma first appear on globes of the earth?

I have no idea when they first appeared on globes but I'm looking at one I used
in grade school in the mid forties and it has the analemma. It was manufactured
by Replogle Globes but I can't find a date.

Regards,
Jerry

Mack requested:

I haven't been able to find any historical information as to when people
first knew about this. Presumably once Kepler understood that the planets
moved in ellipses (1609), it was possible to realise that the sun would
be slow or fast at different times of the year. Once clocks could measure
time to within 15mins over a period of a year (late 1700s with Harrison's
chronometers), then you could observe that the sun was fast or slow.

The notion of the equation of time had been kicked around for a while.
John Flamsteed eventually crystalized the concept when he computed and
published a table of values; he termed it "the equation of natural days."
I believe this happened around the year 1670.


When did corrections for the equation of time first appear on sundials?

Gnomonicists also are quite keen to learn when this first occurred. And
I'm not sure if anyone knows exactly. The earliest dials with EoT shown
thereon were made in the 1700s.


I remember many years ago, seeing an article in S&T about the
shape of analemmas on other planets, but this was before I subscribed
to S&T. The S&T on-line index only goes back to 1996 and doesn't show this
article. Does anyone know when it was published?

Harvey, David A. "The Analemmas of the Planets." _Sky and Telescope_,
March 1982, pp. 237-9.

--
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Mark Gingrich San Leandro, California

Mark Gingrich wrote in message ...

A good general reference site with animations can be found at:

http://www.analemma.com/Pages/framesPage.html

Mack requested:
I remember many years ago, seeing an article in S&T about the
shape of analemmas on other planets, but this was before I subscribed
to S&T. The S&T on-line index only goes back to 1996 and doesn't show this
article. Does anyone know when it was published?

Harvey, David A. "The Analemmas of the Planets." _Sky and Telescope_,
March 1982, pp. 237-9.

The above site also has illustrations of the analemmas on other planets.

Enjoy - Canopus

Canopus wrote:

Mark Gingrich wrote in message ...

A good general reference site with animations can be found at:

http://www.analemma.com/Pages/framesPage.html

thanks. Unfortunately the animations require proprietary codecs.
I can't imagine why animated gifs wouldn't have done the job.

The above site also has illustrations of the analemmas on other planets.

Thanks, I get the idea of the shapes at least

Joe

--
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!