usually show only the aspects of the Sun or Moon. On the Torre dell'Orologio, Brescia clock in northern Italy, the triangle, square, and star in the centre of the dial show these aspects (the third, fourth, and sixth phases) of (presumably) the moon. Dragon hand: eclipse prediction and lunar nodes The Moon's orbit is not in the same plane as the Earth's orbit around the Sun but crosses it in two places. The Moon crosses the ecliptic plane twice a month, once when it goes up above the plane, and again 15 or so days later when it goes back down below the ecliptic. These two locations are the ascending and descending lunar nodes. Solar and lunar eclipses will occur only when the Moon is positioned near one of these nodes because at other times the Moon is either too high or too low for an eclipse to be noticed from the earth. Some astronomical clocks keep track of the position of the lunar nodes with a long pointer that crosses the dial. This so-called dragon hand makes one complete rotation around the ecliptic dial every 19 years. When the dragon hand and the new moon coincide, the Moon is on the same plane as the Earth and Sun, and so there is every chance that an eclipse will be visible from somewhere on Earth. Historical examples Su Song's Cosmic Engine The Science Museum (London) has a scale model of the 'Cosmic Engine', which Su Song, a Chinese polymath, designed and constructed in China in 1092. This great astronomical hydromechanical clock tower was about ten metres high (about 30 feet) and featured a clock escapement and was indirectly powered by a rotating wheel either with falling water and liquid mercury, which freezes at a much lower temperature than water, allowing operation of the clock during colder weather. A full-sized working replica of Su Song's clock exists in the Republic of China (Taiwan)'s National Museum of Natural Science, Taichung city. This full-scale, fully functional replica, approximately in height, was constructed from Su Song's original descriptions and mechanical drawings. Astrarium of Giovanni Dondi dell'Orologio The Astrarium of Giovanni Dondi dell'Orologio was a complex astronomical clock built between 1348 and 1364 in Padova, Italy, by the doctor and clock-maker Giovanni Dondi dell'Orologio. The Astrarium had seven faces and 107 moving gears; it showed the positions of the sun, the moon and the five