Tuesday, October 30, 2012

Hermle Tellurium II
Astronomical Clocks and Devices

Expressions of Time offers a selection of astronomical clocks.  Our selection of  Hermle Tellurium and Astrolabium clocks reproduce the annual orbit of the earth around the sun, with the earth rotating around its own axis once in twenty four hours.  The moon, in turn, revolves around the earth in 29.5 days completing a full rotation around its axis.  This allows the reproduction of the different moon phases as they are visible from earth.  In the course of one year, the earth passes all twelve zodiac signs, months and individual days which are indicated on the flat disc.  Each clock is made in Germany.

Our most recent addition is the Ferguson’s Orrery Astronomical Device.  This astronomical device was commissioned by Sternreiter and made entirely in the U.S.A. by American craftsmen.

Fergusons Orrery Astronomical Device

This orrery is unusual because it illustrates the movement of the nodes and apogee of the Moon’s orbit, instead of the Moon’s position. The Moon’s orbit is elliptical, with an eccentricity of about 5.49%. The difference in the perceived size of the Moon between the perigee (the closest approach to the Earth) and apogee (the farthest point from the Earth) is about 12%. People noticed the difference over the years because the Moon is about 30% brighter when closer to the Earth. The precession of the apogee around the Earth is just less than nine years. The pointer fixed to the large bottom plate represents the position of the apogee of the Moon’s orbit.

The Moon’s orbit is inclined 5.14° from the plane of the ecliptic (the plane of the Earth’s orbit around the Sun). The points where the Moon’s orbit intersects the ecliptic are called the nodes. The conditions for an eclipse require that a node intersect the line of light from the Sun to the Earth and that the Moon be present at the node at the same time. These nodes recess around the Earth with a period of about 18-1/2 years. The sloped ring on the orrery represents the Moon’s orbit.

The Earth’s axis is inclined 23.5° with respect to the ecliptic and remains fixed in that orientation with respect to the zodiac. The orrery illustrates this with a zodiac inscribed on the ring representing the plane of the ecliptic. The day/night indicator mounted to the frame facilitates conversations about the different lengths of days and nights throughout the year. The calendar ring on the base illustrates both the conventional 365-day calendar and the western zodiac with 360 divisions. Ferguson’s 1764 engraving shows the effect of Britain’s adoption of the Gregorian calendar in 1752.

Check out all the Tellurium and Astronomical clocks and devices offered by Expressions of Time at http://www.clockshoppes.com/tellurium-and-astrolabium.aspx.


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