Java applet page:
The traditional astrolabe displayed by this program
shows an almucantar arc at 18 degrees beneath the horizon.
This arc indicates the time of astronomical twilight.
When the Sun is more than 18 degrees below the horizon,
its light is usually insufficient
to affect astronomical observations.
To find the times of twilight for any date, first set the date using the buttons in the time/date panel. Then adjust the time setting until the position of the Sun on the rete is over the part of the twilight line which is beneath the east side on the horizon. The time you have set indicates the time of morning twilight, or daybreak. If you adjust the time setting until the Sun is over the twilight arc beneath the west side of the horizon, the time setting will indicate the time of evening twilight, or nightfall.
Additionally, some ancient astrolabes showed arcs which were at 6 and 12 degrees below the horizon. These were the 'civil' and 'nautical' twilight arcs, the former indicating the beginning and end of the day in some parts of the world, and the latter indicating the time when the position of the horizon between the sea and sky couldn't be reliably identified.
When these arcs were not drawn on an astrolabe, the twilight times were found using the 6, 12 and 18 degree altitude arcs (almucantars). For this, the point on the ecliptic circle was first found which was diametrically opposite the Sun. The time when this point crossed the 6, 12 or 18 degree altitude arc drawn above the horizon indicated the appropriate twilight time.
On the universal astrolabe displayed by this program, the horizon rete also has an 18 degree twilight arc. After the rete has been rotated into either of the two positions (it makes no difference whether you choose the ws or ss setting) to suit your latitude, find the position of the Sun on the ecliptic line drawn on the plate. Visually, follow the parallel arc from this point to the twilight arc, and from here read off the time from the polar arcs, using the scale towards the bottom of the plate.
The plate of the spherical astrolabe could also have a twilight arc but this hasn't been included on the two cutout diagrams.
The equinoctial astrolabe shown here uses a table of horizons and, because it doesn't have almucantars, it doesn't show twilight arcs. If an option had been provided to show almucantars to suit specific latitudes instead of a table of horizons, a suitable twilight arc could have been found among the almucantars drawn beneath the horizon.