Home Search GeoAstro Applets Astronomy Chaos Game Java Miscel- laneous Physics Quiz Who is Who ?

Sidereal Sundial Applet
This horizontal sundial is showing the local sidereal time

 Check your time zone offset and the local time. Click the applet first! Enter latitude in decimal degrees, northern positive, southern negative, enter longitude in decimal degrees, eastern positive, western negative, then press "Apply input. You may use the keys "m", "d", "h", "n" to increase the month, date, hour, minute, or Shift key and "m", "d", "h", "n" to decrease month, date, hour, minute ! Press shift key and click into the sundial area to change the gnomon position.

More of my sundials:

Analemma Sundial

Sidereal time is defined as the hour angle of the vernal equinox.

(Local) Sidereal Time = (Local) Hour Angle + Right Ascension

θ = τ + α.

This type of sundial has a vertical rod (gnomon, height H) casting the shadow on a horizontal plane. The shadow of the gnomon's top (point P) is used to read the local siderel time.

Any point of the horizontal plane is crossed by two sidereal hour lines, one for the first half of the year, and one for the second:

Berlin: Feb 24 at 10:51 CET: Local Sidereal time is 21:00

Berlin: Oct 17 at 11:21 CEST: Local Sidereal time is 12:00

On the day of the vernal equinox, at local noon, the local sidereal time is 00:00.

On the day of the summer solstice, at local noon, the local sidereal time is 06:00.

On the day of the autumnal equinox, at local noon, the local sidereal time is 12:00.
On the day of the winter solstice, at local noon, the local sidereal time is 18:00.

The grid size is 2.5 m (independent of the gnomon height).
With "Gnomon Height 10 m" and an elevation angle of 45° the length of the shadow is 10 m:

Click into the grid (shift key held down) to change the position of the gnomon.

Choose "Write Data" from the "Details" menu:

Choose "Write Sidereal Time Month" or "Write Sidereal Time Day" from the "Details" menu:

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.

 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. 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.

and animation

 Web Lin Compute Local Apparent Sidereal Time (USNO) The Analemma (Bob Urschel) Ausführliche astronomische und mathematische Darstellung Shadows (François Blateyron) A great sundial program for Windows (download, freeware)

Please visit my GeoAstro Applet Collection