Some projects are just plain fun. You get an idea, you get inspiration on how it should look, you research the maths that goes behind it and crack on until it is done. The Annual Clock is one such project for me. I actually finished it as a Flash application almost a year ago, but only just got round to converting it into a screensaver.
The Idea
Visually it was inspired by astronomical clocks of yester-year (such as the one below).
My aim however was not to portray the movement of heavenly bodies as much as it was to portray the passage of time in a more unique way than the standard hour, minute and second hands. Something about a clock that showed all layers of 'time' passing at once seemed an exciting and relatively unique prospect. My approach took a series of discs, each portraying a different element of time measurement, radiating out from the centre, with each disc moving progressively more slowly working from the instant, through seconds, minutes, hours, daylight, days, moon phases, months and seasons. I stopped there since the next levels (e.g. years, centuries, milleniums etc) would give the clock a lifespan, something I wanted to avoid. Rather than using hands I had the discs rotate instead, with the centre point representing the person - the instance of time in which we exist now.
But enough of concept.
The Maths
The code for the passage of time is pretty straightforward. You take the current computer system time and date, and use it to rotate the discs as a proportion of 360 degrees. Not too difficult. The tricky part was dealing with items that don't stick rigidly to our calendar such as Easter, the moon phases, and the solstices and equinoxes.
For the Easter calculation I did an ActionScript implementation of a formula on the BBC H2G2 website.
For the solstices and equinoxes I did an ActionScript implementation based on the astronomical algorithms of Jean Meeus (Astronomical Algorithms, 1st Ed., 1991) which someone had posted online somewhere (can't remember where now). Meeus's formulas calculate the date of the current year's solstices and equinoxes, but gives the result as a date using the Julian calendar. (In fact Meeus's formulas calculate the time of the events to within 4 minutes of actual with any error down to orbital perturbation - how cool is that?)
However a Julian date was not suitable for my needs, and meant some highly complex conversion from the Julian to the Gregorian calendar based on a formula published online by Utrecht University.
I confess I didn't understand most of the formulas (but then I am not an astrophysicist or mathematician - and I gave all sources their dues in my ActionScript comments). But that's the great thing about ActionScript's mathematical functions - if someone has written it out, it is fairly logical how you need to set up the same thing in ActionScript.
The moon phases are based on the mean moon phase duration as reported by NASA from the year 2000 (so it won't be perfect forever, but pretty good for this purpose).
So, my particular thanks go to Meeus, Utrecht University and NASA for helping me with the bit I could never work out on my own without some serious long term astronomical observations of my own. The practical upshot is a Flash based clock application that not only shows the passage of time from seconds to seasons, but also one that can predict Easter, the equinoxes and solstices, and understands that moon phases are not precisely 28 days long. Thanks folks.
Free Download
Feel free to visit the application page and download the v0.9 release as a screensaver.
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