Bifilar Sundial Applet

 Enter latitude in decimal degrees (northern positive, southern negative), enter longitude in decimal degrees (eastern positive, western negative),  then press "Apply input". Key commands: you may use the keys "h", "d", "m" to increase the hour, date, or month, or Shift key and "h", "d", "m" to decrease the hour, date, or month ! Click the applet first !

 Click into the grid plane to change the gnomon position. This sundial has two non-intersecting threads parallel to the horizontal dial plane. The upper thread (distance a above the plane) is oriented North-South, the lower (distance b) is orthogonal to the first. The intersection of the two threads' shadows gives the time. The lines of hour angles are equidistant if b = a sin(latitude) and intersecting at P with PQ = a sin(latitude).

More of my sundials:

 An analemma is the figure-of-eight shape that results if the Sun's position in the sky is recorded at the same time of day throughout the year. As a consequence the gnomon draws a similar figure. Watch an animation. The red part of the analemma is used to read the time from summer solstice (June 21) to winter solstice (Dec. 22), the blue line from winter solstice to summer solstice.

 The diurnal lines are painted for (data 2002): 6/21 (summer solstice) 6/1 8/1 9/1 9/23 (vernal equinox) 10/1 11/1 12/1 and 5/1 4/1 3/20 (spring equinox) 3/1 2/1 1/1 12/22 (winter solstice)

The grid size is b=a cos(latitude).
With "Upper Thread 10 cm" and an elevation angle of 45° the shadow is at y=b.

Click into the grid to change the position of the gnomon.

Choose "Current" from the "Analemma..." menu to show the current position of the sun, the analemma for the current time and the diurnal line for the current day:

An analemma is the figure-of-eight shape that results if the Sun's position in the sky is recorded at the same time of day throughout the year. As a consequence the gnomon draws a similar figure. Watch an animation.

More of my sundial applets ...

Please visit my pages:

and animation

 Books Savoie, Denis: La Gnomonique, Les Belles Lettres, Paris 2007 Michnik, H.: Theorie einer Bifilar-Sonnenuhr. Astronomische Nachrichten, Band 217, 1922, p. 81-90 (PDF) Web Links The Analemma (Bob Urschel) Ausführliche astronomische und mathematische Darstellung Shadows (François Blateyron) A great sundial program for Windows (download, freeware) The Dialist's Companion DOS program that calculates a wide range of data elements of interest to dialists and others who are concerned with phenomena associated with solar time.

Please visit my GeoAstro Applet Collection

Last Modified: 20.10.2023

© 2010-2023 Juergen Giesen