Emerald Basel

Basel is a simulation of a highly-complex mechanical analog astronomical sidereal-time watch, with 16 hands plus 12 additional indicators.

The primary hands do *not* show ordinary clock time, but instead the local apparent sidereal time; the ordinary clock time is shown a much smaller subdial at 6 o'clock.

The sidereal hour is read against the inner white dial of 24 numbers (hours of right ascension, RA), the minutes against the 60 unnumbered tick marks (minutes) on that dial.

The outer white ring is divided into the months of the year with a hairline at the beginning of each month and tick marks for every other day; the black tick marks represent even days of the month, and the spaces between represent odd days.

There are red hairline indicators over the zodiac ring (fixed at 0, 6, 12, and 18) which mark the positions of the equinoxes and solstices, and a plus sign between Aquarius and Pisces on the zodiac ring which marks the RA position of the vernal equinox in the year 2000. There are four smaller red hairline indicators over the date ring which move slowly to indicate the dates of the equinoxes and solstices. They stay close to the fixed indicators but they don't always line up exactly because as mentioned above the date ring does not match the zodiac ring except exactly at the Sun's position.

The Sun and Moon hands show their positions with respect to the the RA dial and the constellations (and each other). The red Ω-shaped hands show the positions of the lunar ascending and descending nodes (the ascending node indicator has the Ω rightside-up at 12 o'clock). When the Sun and Moon coincide and are close to one of the nodes it is possible that there may be a solar eclipse; when the Sun is close to one node and the Moon is opposite to it a lunar eclipse may occur.

The eclipse dial at the top center indicates if there is an eclipse happening at the observer's location and time.

Also, note that when the ascending-node hand coincides with the vernal equinox (at the 12 o'clock position) the Moon will reach extremes in several dimensions, including north and south declination, northern and southern azimuths of rise/set points on the horizon, and altitudes crossing the meridian.

Also shown on subdials are the apparent solar time (sundial time) and UTC (also known as GMT).

The left window (beneath the UTC subdial) shows the first two digits of the year number (centuries), the right window (beneath the Solar subdial) shows the last two digits.

Finally, just inside the RA dial is a ring which shows the sidereal times when the Sun and Moon illuminate the sky for this day. The white region is bounded by sunrise and sunset and thus shows the daytime hours (in sidereal time). The dark regions represent nighttime with gray when the Moon is up and black for full night. There are also two tiny hands that indicate the (sidereal) times of moonrise and moonset.

More information is available at the listed website.