What is This Device Called? (Real-time map of daylight/darkness)

Some years ago I was at a Rand McNally store and they had this gizmo that was a map of the world, with shadows moving over it in real time, indicating the parts of the world that were in light/darkness at any given moment. It updated with the rotation of the earth and with the seasons.

Does anyone know what such a thing would be called? And better yet, where I can download one as a screensaver and/or an iPhone app?

the wall-mounts are called “geochrons” named for the manufacturer


Desktop Earth is the computer equivalent. free program, generally awesome


if you’re feeling exceedingly lazy tonight :wink:

I find this one quite interesting.

I’ve tried Googling it, but with no success. How does a mechanical geochron work? Specifically, how do they get the dark and light regions to change over the year?

my guess would be there is a screen over the light source which is projected onto the map’s rear. that screen rotates (probably more accurately tilts) as the date changes. if you look at a geochron on an equinox, there won’t be a “slope” to the daylight boundary (other than a slope conforming to the spherical nature of the earth

it’s all mathematical how that slope moves so it’s easy to build a mechanism that changes the exposure of the light source per day. but i’m no astronomer.

That .gif is hypnotic. And awesome.

But how are you getting a shadow that smoothly varies between one curved shadow and two straight ones (at the equinoxes), and where the curved one has many different shapes? The S-shaped shadow boundary will have curvatures that vary.