tag:blogger.com,1999:blog-4844458687369955274.post4791717353629238517..comments2018-07-15T00:14:54.349-07:00Comments on mapHead: The International Date Linenatcasehttp://www.blogger.com/profile/18058664776852941599noreply@blogger.comBlogger1125tag:blogger.com,1999:blog-4844458687369955274.post-25076251705158386292010-12-13T06:24:31.484-08:002010-12-13T06:24:31.484-08:00Craig has pointed out to me:
"You wrote: [Th...Craig has pointed out to me:<br /><br />"You wrote: [The Sun] is always due east or west at 6 o'clock (either one) local time.<br /><br />I do not believe that's true year round. Twice a year (at the equinox), the sun rises/sets due east at all latitudes. Sailors who want to "swing" their compass (adjust deviation error) use Amplitude tables to calculate WHEN the Sun is due east or west, and a simple SunStick to sail directly east/west. After allowing for local magnetic variation, they can then adjust compensator magnetics to remove any ship-induced deviation."<br /><br />Earlier, Criag had pointed to the interesting (and ancient) device of the <a href="http://en.wikipedia.org/wiki/Analemma" rel="nofollow">analemma</a>. This also means that "local time" using a 24-hour clock varies over the course of the year. Apparently this is due to the elliptical nature of Earth's orbit, which leads me to wonder how the analemma effect was explained in a pre-Copernican system. If it was explained at all. Was the analemma observed before modern timekeeping? How?natcasehttps://www.blogger.com/profile/18058664776852941599noreply@blogger.com