With this program, a rete can be rotated over the plates of the traditional and equinoctial astrolabes, and a spherical form of the rete can be rotated around the body of the spherical astrolabe. After the rete has been rotated into position, the plate or body allows you to find the positions in the sky of those celestial bodies shown on the rete which are above the horizon (and much more). With the universal astrolabe, the stars and ecliptic circle are drawn on the plate, and their positions in the sky can be found on the rete. These four types of rete are described on this page: Rete of the Traditional Astrolabe Rete of the Equinoctial Astrolabe Rete of the Universal Astrolabe Rete of the Spherical Astrolabe   Rete of the Traditional Astrolabe The procedure for setting a traditional astrolabe was to find the position of the Sun along the ecliptic circle using the back of the astrolabe and then to rotate the rete to align this point on the ecliptic circle of the rete with the current time on the rim. To avoid this procedure, the method used with a modern planisphere has been adopted with this program. Around the outer edge of the rete displayed here is a calendar scale. The rete is rotated into position by aligning the current date on this scale against the current time shown on the rim around the plate. The outside edge of the central section of the rete is used to represent the celestial Tropic of Capricorn, so all the significant stars to the north of this tropic can be shown on this astrolabe, as well as the ecliptic circle. The ecliptic circle is normally marked with a Zodiac scale but you can choose to display a calendar scale instead which, without using the scales on the back of the astrolabe, will allow you to find the position of the Sun around the ecliptic circle on each day of the year. You can choose which of these alternatives you would like by using the '4' key on the keyboard, or you can use the menu: Menu: Astrolabe /Rete - ecliptic circle /Zodiac scale Menu: Astrolabe /Rete - ecliptic circle /Calendar scale You will note that the calendar scale around the outside of the rete doesn't quite align with the calendar scale within the ecliptic circle. The outer scale sets the rete correctly for the stars. The calendar scale within the ecliptic circle is used to find the position of the Sun, and the difference between the two is known as the Equation of Time (EoT). In theory, one could mark the 'figure of eight' diagram for the EoT on the pointer, between the tropics, but the distortion caused by the projection made it appear unconvincing. A button marked 'star names +/-' is provided on the button panel which allows the star names to be displayed and removed. The '5' key on the keyboard also adds and removes the star names, or you can use the menu: Menu: Astrolabe /Rete - Star names/Show star names Menu: Astrolabe /Rete - Star names/Omit star names   top of page Rete of the Equinoctial Astrolabe On an equinoctial astrolabe, the rete shows the stars and the ecliptic circle. It differs from the rete of a traditional astrolabe in that the outside edge of the central section of the rete shows the equator, not a tropic. Stars to the south of the equator are shown mirrored inwards over the stars which are to the north of it. Thus, the north and south celestial poles are superimposed at the centre of the astrolabe. This allows a selection of stars of the complete celestial sphere to be shown, and the ecliptic circle appears as two symetrical arcs. To differenciate between the normal and mirrored stars, those to the North of the celestial pole are shown as small filled circles and those to the South as small open circles. The ecliptic circle is marked with the signs of the Zodiac, and each sign is graduated. After using the back of the astrolabe to find the equivalent Zodiac positon for any date, this position on the Zodiac scale of the rete shows the position of the Sun. (You can't select a calendar scale as an alternative for the Zodiac scale.) Around the outer edge of the rete is a calendar scale. The rete is rotated into position by aligning the current date against the current time found on the 24-hour clock dial drawn around the outer edge of the plate. As with the conventional astrolabe, a button is provided on the button panel which allows the star names to be displayed and removed. The '5' key on the keyboard also adds and removes the star names, or you can use the menu: Menu: Astrolabe /Rete - Star names/Show star names Menu: Astrolabe /Rete - Star names/Omit star names   top of page Rete of the Universal Astrolabe On the universal astrolabe, the rete doesn't show the stars and ecliptic circle. Instead, it has a grid similar to that of the plate of the universal astrolabe. The universal rete has a variety of uses. The most common is to show the horizon and the Zenith together with the Azimuths and elevations (almucantars), the plate under it showing the stars and ecliptic circle. To set your latitude, the outside edge of the rete has a pointer which can be set against any angle of the latitude scale on the outer edge of the plate. This latitude scale is shown in the four quadrants. Southern latitudes are shown on the left and Northern ones on the right. Similarly, rotate the rete so that the pointer points to the latitude shown in the top half if you want the midnight position of the stars to be located on the rete in their Summer Solstice positions, and in the lower half for their Winter Solstice positions. If you are interested in the location of the Sun, you can use either the Summer or Winter Solstice positions, following the track of the Sun along the parallel of the plate and using the hour angle scale on the lower half of the plate appropriately. With any of these settings, the pole of the rete represents the Zenith and the straight line across the diameter of the rete is the horizon. The centre of the horizon line (the centre of the astrolabe) represents the east and west point on the horizon, and the north point is denoted by that end of the horizon line which is closest to the top of the astrolabe, the lower end being the south point. When the rete is set for the Winter Solstice, stars shown on the plate as filled circles are to the west, and those shown as open circles are to the east. and conversely for the Summer Solstice. When used for this purpose, coordinates beneath the horizon line are of little interest apart from the -18 degree twilight arc. When the Universal Astrolabe is used to convert between the equatorial coordinates (RA and declination) on the plate and the ecliptic coordinates (longitude and latitude) on the rete, the rete must show the coordinates of the sky both to the north and to the south of the central equator line, so a full grid is necessary. For this purpose, the pointer on the rete to be found against the 0 degree parallel should be set against a mark against the 23.4 degree position which is indicated on the scale of the upper right quadrant of the plate. However, you can display the plate over which is the rete set permanently to the 23.4 degree position. This is most easily set by using the menu: Menu: Univ.Astr /Rete at 23.4 If the universal astrolabe is used to perform spherical trigonometry, a full set of coordinates is sometimes needed on the rete, although quite often only a half-grid is required.   top of page Rete of the Spherical Astrolabe On a spherical astrolabe, the rete is the transparent sphere which rotates over the body sphere, and shows the positions of the stars and the ecliptic circle. It also shows the celestial equator, and the two celestial tropical circles. Around the centre of the rete is a calendar scale, and you will find a 24-hour clock scale around the body sphere. To set the rete, rotate it around the body until the current date is shown against the current time. The body then shows the Azimuth and elevation of any star on the rete or any point on the ecliptic circle. The ecliptic circle is marked with the signs of the Zodiac, and each sign is graduated. This allows the position of the Sun to be located according to its position in the Zodiac, which is most conveniently found against the date scale on the back of a (subsidiary) traditional astrolabe. However, the point on the ecliptic circle which is closest to the date shows the position of the Sun with only a small (and usually quite acceptable) error.

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