The first astrolabes are known to have existed before 150 B.C. and served mainly two purposes; to observe and to display. Observation of the sky had mainly one purpose: to determine the correct date and time. Thanks to the watch movement inside the Ulysse Nardin Astrolabium, the purpose of observation has become obsolete and the main purpose today is to display the sky and its celestial bodies above us. It can also be used to predict astronomical events, for example the next eclipse of the sun or the time of sunset at any given date. It is the first watch that sensitizes us for the things happening in the sky and makes us become a better observer of nature and the stars above: The Dragon hand circles around the Ecliptic once every 18.611 years! Seconds, Minutes, Hours and even Dates are therefore becoming almost irrelevant. What counts is the movement of Earth, Sun, Moon and the Stars. Perpetual Calendar is exactly synchronized with the movement of the Earth around the Sun of 365 days, 5 hours, 48 minutes and 46 seconds. It therefore is the only Perpetual Calendar which has never to be adjusted!

The difference between Sun Hand and Moon Hand corresponds to the time the Moon takes to circle around the Earth once: 27 days, 7 hours, 43 minutes and 11.5 seconds The Dragon Hand moves almost at the same speed as the Ecliptic, but it makes one extra turn every 18.611 years. It indicates solar or lunar eclipses, if it is aligned simultaneously with the sun and moon hands. Ecliptic represents the firmament and is divided into the 12 signs of Zodiac. It makes one turn every 23 hours, 56 minutes and 4.1 seconds (exact time that the earth needs for one rotation around its own axis) The Planisphere or Watch-Dial represents the earth in a planispheric (flat) form. The horizon line separates the visible part of the sky (silver-coloured upper part) from the invisible part of the firmament, hidden behind the earth. The Planisphere is calculated according to the latitude of the specific place. This example is 46 degrees North (Geneva and Chicago etc.) The Equator represents the calendar with monthly graduations. It makes one turn every 365 days, 5 hours, 48 minutes and 46 seconds (exact time that the earth needs for one rotation around the Sun). This takes into considerations all leap years of the Gregorian calendar and has never to be adjusted. The Bezel has a 12-hour graduation (Roman numerals) for the legal or normal time and a 24-hour graduation (Arabic numerals) for the solar or local time and all astronomical data. For better clarity, the 24-hour graduation has been omitted at the location of the Roman numerals.

Legal Time (or Normal Time) is shown by the hour and minute hands and the Roman numerals on the Bezel. On the example, it is 12:15 am or pm. Legal Time is the same for all cities within the same time zone, but might change during daylight saving times (ahead by one hour) The Month is shown by the tip of the Sun hand on the Equator. The date can be estimated. The lines between the months indicate the first day of the month.

The Astrolabium does not only show the Moon-Phases, but also whether the Sun and/or Moon are currently visible on the sky. Since the Sun-Hand turns a little bit faster (one turn in 24h) than the Moon-Hand (one turn in 24h 49’), the Moon is waning whenever the Sun-Hand is behind the Moon-Hand.

Summer Solstice (June 21) is the longest day in the Northern Hemisphere: 12 Noon at Summer Solstice: Sun Hand and Ecliptic intersect with Tropic of Cancer. The Sun reaches its highest position (almost at Zenith, 90° above Horizon). 6pm at Summer Solstice: The intersection of Sun Hand and Ecliptic is still in the light grey zone. It will take several hours before the Sun sets. Winter Solstice (December 21) is the shortest day in the Northern Hemisphere: 12 Noon at Winter Solstice: Sun Hand and Ecliptic intersect with Tropic of Capricorn. The Sun is at its lowest position (less than 30° above Horizon). 6pm at Winter Solstice: The intersection of Sun Hand and Ecliptic is just passing the twilight zone. The Sun has set a few hours ago and Night has fallen.

Temporal (or Apparent) Hours were used in the Medieval Age when clocks and watches were not widely available and when, for practical reasoons, the Natural Night and the Natural Day were equally divided into 12 equal parts. Since a night in winter lasts much longer than a night in summer, the night hour in winter lasted longer than 60 minutes and shorter than 60 minutes in summer. Only during Equinox (in March and October), when day and night are equally long, the Temporal Hours last exactly 60 minutes. Temporal Hours were only abandoned after accurate mechanical clocks became commonly available. The Ulysse Nardin Astrolabium shows Temporal Hours only for the night, because the dial would otherwise be too cluttered.