Astronomical Clock

Animated Astronomical Clock

Set out below is a 24 hour clock showing the angle of the sun, moon and the planets visible to the naked eye. The moon is drawn showing the current phase. The view of the earth from either North or South pole is shown at the centre with the extent of the dark side shadow for the time of year shown. By default the positions represent the current date and time but these may be changed using the knob in the control panel. Various display modes can be set by the other controls. The whole clock movement may be speeded up to see planetary motions over the months and years by using the "motor" controls.

Guide to use of the Astronomical clock

The clock can display the position of the visible planets and the moon phase for any day from 1000BC the 3000AD. The dates are those used at Greenwich England. The Gregorian calendar was introduced in England 1752, when twelve days were dropped, so that 2 September was followed by 14 September. So the Gregorian calendar is used from 14 Sep 1752 to the present and the Julian calendar is used for date prior to 2 Sep 1752. There was no year 0 in the Julian calendar so 31 December 1BC is followed by 1 January 1AD.

Time of day

The red hour hand displays the current time on the 24 hour dial updated every 10 seconds if the "Current time" is switched "ON". The time is referenced to the host computers internal clock. The time zone is initially determined from the computer's clock but may be changed by the user.

The time or date may be varied by turning the knob. First, turn off the "Current time" function or the display will jump back to the current time every 10 sec. Select the date or time by clicking the arrow button beside the "Time" or "Date" readout. The click on the knob indent and drag to rotate. Similarly the year may be changed by clicking the up or down arrow on the "Year" spinner.

Moon phase

The moon image shows the phase or age of the moon for the date shown in the control panel. The hour number on the dial adjacent to the moon image indicates the time at which the moon will be overhead at your longitude when you are looking South (if you are in the Northern hemisphere) or looking North if you are in the Southern hemisphere.

Which planets are visible

The clock only shows the planets visible to the naked eye. They will only be visible in the night sky which is indicated by the dark grey sector of the clock face, the daytime sky is shown light blue. The hour number on the dial under each planet indicates the time at which it will be overhead at your longitude.

To accelerate the planet motion to observe relative angular velocity and retrograde motion of the inner planets a "Fast forward" function is provided. The associated readout shows the speed in days per second.

Data set used in the clock engine

The planetary positions have been calculated using the formulae and best-fit coefficients published by E. M. Standish JPL/Caltech (https://ssd.jpl.nasa.gov/txt/aprx_pos_planets.pdf). The algorithm is valid for years 3000 BC to 3000 AD. These algorithms were converted to JavaScript code by Daniel Brooke Peig (www.danbp.org).