naspis

Simulator of the astronomical dial of the clock

There are many reasons for the creation of models and simulators of individual functions of the Prague Astronomical Clock. It may be an effort to simulate natural processes by means of modern technology and compare this model with the work of old masters. Or, it may be an endeavour to manage to realize by one’s own wits the same thing that people have already admired for six hundred years, and to do so at rest with a computer. We are encouraged mainly by the effort to show the principles of individual time indicators as clearly as possible. All model creators probably have one thing in common – admiration and reverence for the ancient works and a desire to make a contribution to it, though remotely.

On this page, we present a text simulator of the astronomical clock, which shows what the Prague Astronomical Clock should indicate at this moment. At the same time, we will again try to show how to read these data on the clock. The simulator does not show the position of the Sun and Moon towards the ecliptic nor the moon phases. You can try reading of time on the graphic simulator by Mr. Sandler, which is constructed much better. Our calculations on this page are simplified. For better clearness, seconds are indicated here although the astronomical clock does not indicate them.


What should the astronomical clock indicate right now:

CET SEČ The time is indicated by the golden hand attached to the Sun arm on the dial written on the astrolabe in Roman numerals. Note that DST is not set on the astronomical dial.
Sunrise Východ It is only displayed in the moment when the symbol of the Sun passes over the horizon line. Here it is calculated based on the mean solar time.
Sunset Západ It is displayed in the moment when the symbol of Sun passes over the horizon line. It can also be read at the position of the 24th hour on the 24?hour dial.
Old Czech Time Staročeský čas The day has 24 hours counted from the sunset. Simply speaking, it is just a time shift. On the clock, it is indicated by the golden hand on the largest rotated dial – 24?hour dial.
The calculation is simple: CET – time of sunset
Planetary hours Planetní čas The day is divided into 12 hours with changing duration of the hour in the course of the year. The light day commences at the moment of sunrise and ends by the sunset. We convert it to CET by means of the ratio of the duration of light day and the period of 12 hours. Night planetary hours are not displayed on the clock.
Sideral time The only time on the clock that is not related to the Sun. The day has 24 equal sideral hours. The sideral day is approx. 4 minutes shorter and is counted from the culmination of the vernal point. Therefore it should be displayed on a 24?hour dial rotated by 12 hours compared to the dial of a common clock. On the astronomical clock, it is read on a dial of 12+12 hours, which is a little confusing. The sideral time is counted from the upper culmination of the vernal point, i. e. from the moment when the arm with the star points upwards. Let us add that it is the other way round than with the Sun arm. When the Sun arm points upwards, it is noon; the sideral “noon” is at the bottom.We used a simplified calculation from CET, date and geographical position. The sideral time is always a local time.


The best way to verify the values is to look directly at the astronomical clock by means of a web camera (located in the ( Grant Hotel Praga ). if the illumination and quality of the display allow this.

Graphic simulator of the clock

We are glad to introduce the program made by Mr. Karel Sandler, which provides a graphic representation of the model of the astrolabe. The picture is updated every 5 minutes and thus shows the actual situation on the clock a short while ago. The date and time for which the picture has been generated is in the lower left corner. The program is based on slightly different calculation methods; see the detailed description in Czech and English by clicking the corresponding flag.



Text, Java script and PHP support: Petr Král
Programming of graphic animation and explanatory text: Karel Sandler

e mail