Above is an analemmatic sundial. The idea is to orient the sundial facing south, and then place a vertical pointer on the central figure-8 track, in a position corresponding to the date. The sundial above shows a simulated shadow for 2:15 PM yesterday. It can be seen that the sundial tells the time reasonably well, thanks to the inbuilt adjustment for variation in solar position.

For large-scale analemmatic sundials, like the one below, people can stand on the central figure-8 track and act as a human pointer. A sundial like this is fun to have in the garden.

Here are blank sundials for some Southern Hemisphere cities:

The ShadowsPro software will also generate sundials like these, if anyone is particularly enthusiastic.


Sea levels in the Pacific

I recently visited Port Vila, capital of the Pacific island nation of Vanuatu (the photo above is from the Port Vila waterfront). Port Vila is the site of a sea-level measuring station. It is interesting that, although local newspapers are deeply concerned about sea level rise, the average sea level rise between 1993 and 2017 at Port Vila was essentially zero (see chart below, which uses LOESS smoothing of monthly measurements).

How can this be? Aren’t global sea levels rising at 2–3 mm per year? Well, “global sea level” is a rather theoretical concept. Ocean temperatures are not uniform. Some islands are rising out of the ocean. Others are sinking. Air pressure, and the El Niño Southern Oscillation cycle, have a huge effect on sea levels too. As they say, it’s complicated.

The NASA map below shows that some areas of the Pacific have actually seen a long-term reduction in sea level (independent of any upward or downward movement of land). Other areas, of course, have seen quite rapid increases (the increases and decreases average out to a rise of about 3 mm per year). The map covers data only up to 2008, however. Since 2008 was roughly the peak for the Port Vila data, it doesn’t quite explain the last decade of the graph above. If I had to guess, I’d assume that some of those sea-level-decrease areas on the map had shifted a bit.

Pencil charts for visualising colours

As a result of a discussion with a photographer friend of mine, I’ve been thinking (not for the first time) about visualising the colour palette of images. Consider this sunset, for example (a picture I took in Adelaide 8 years ago):

The photograph is rich in yellow and orange. However, the apparent blue in the sky is actually grey, and the apparent grey of the sea is actually brown. If we postulate a standard set of 35 plausible pencil colours, and map each pixel to the closest-matching pencil colour, we get this (I have done the comparison in RGB space):

Then we can visualise the colour palette of the image by showing the wear on the virtual pencils, if each virtual pencil has been used to colour the corresponding pixels. It can be seen that a lot of orange, brown, and grey was used (click to zoom):

Conversely, this beach scene (photographed in Vanuatu in 2016) is rich in blues:

The warm light greys of the beach don’t quite find an exact match among the pencils, but the other colours match fairly well:

And here is the pencil visualisation (click to zoom):

If, rather than using a standard set of colours, we extract the pencil colours from the image itself (image quantisation), fewer pencils will, of course, be required:

The fit to the original image will be much closer as well:

So this is a trick to remember for another day – pencil visualisations!

Molecules: a book review

Molecules by Theodore Gray

I recently purchased Molecules: The Elements and the Architecture of Everything by Theodore Gray of (this is the sequel to his superb The Elements, which I have previously reviewed). The book is packed with interesting facts about chemistry as it relates to daily life, and the photographs are absolutely beautiful, as this two-page spread shows:

The structure of the book is necessarily a little ad-hoc, lacking the obvious pattern of The Elements. However, it is still well-organised, informative, and compelling. Everyone interested in science should probably have this one on the coffee table too.

I would give this book five stars, except that nothing could be quite as good as The Elements. I should also note that Theodore Gray’s Reactions is coming out soon. I expect that to be worthwhile as well.

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Molecules by Theodore Gray: 4 stars