Friday, 23 March 2012

Time To Adjust Your Sundials

A digital sundial!?
We've just had the Vernal Equinox, March 20th at 5.14 UTC (Coordinated Universal Time*).
Following closely, we are about to go into Goofy Time (aka BST) where we all have to get up an hour earlier.

That's not the reason why I'm suggesting it's time to adjust your sundials (I keep mine set to 'real' GMT all year round) but because it is good practice to adjust your sundials a couple of times a month. I'm assuming here of course that you all have a sundial (and if you don't - why not!?).

Sundials are the perfect renaissance device, an elegant combination of science and art that can be experienced many days of the year. Use your favourite search engine's image collection to see the beauty of design arising from simple to very hard celestial maths.

Here's The Science Bit
Because the Earth's orbit is not a perfect circle and the Earth's axis is tilted relative to its orbit, a solar day is only 24 hours long on 4 days of the year. The graph below shows how 'fast' or 'slow' a sundial is compared clock time (designed around an average 24hr day). The data comes from using the 'Equation of Time'.

Today (March 23) you sundial will be 6.5 minutes slow.

Interestingly, if you could plot the position of the sun in the sky at noon at different times of the year you would get the elongated figure 8 shown in the image at the top of the post, the analemma (see for further details).

The fascination of sundials or shadow clocks continues into the digital age.
A photographer/designer came up with the idea and the design for a bulbdial clock a few years ago ( A year later, Evil Mad Scientist Laboratories turned the design into a DIY kit to make for yourself (

Apart from garden sundials, I have the Pocket Sundial from on my iPhone - it always works no matter the weather!
Another great example of how location enabled devices can do all sorts of clever stuff.

I better stop now - the shadow says it's time for a lie down ...


* I know the initials should be CUT but an argument between the British and the French, who wanted TUC (temps universel coordonné), resulted in the inspired? compromise where UTC would be used (representing neither the French nor English phrase - brilliant!).

Top image from

No comments:

Post a Comment