# What is This Device Called? (Real-time map of daylight/darkness)
**URL:** https://boards.straightdope.com/t/what-is-this-device-called-real-time-map-of-daylight-darkness/518221
**Category:** Factual Questions
**Created:** [November 19, 2009, 11:48pm UTC](https://boards.straightdope.com/t/what-is-this-device-called-real-time-map-of-daylight-darkness/518221 "2009-11-19T23:48:19Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@HeyHomie](https://boards.straightdope.com/u/HeyHomie)
#### Post date: [November 19, 2009, 11:48pm UTC](https://boards.straightdope.com/t/what-is-this-device-called-real-time-map-of-daylight-darkness/518221/1 "2009-11-19T23:48:19Z")
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?
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### Author:  [@Rumor\_Watkins](https://boards.straightdope.com/u/Rumor_Watkins)
#### Post date: [November 20, 2009, 12:00am UTC](https://boards.straightdope.com/t/what-is-this-device-called-real-time-map-of-daylight-darkness/518221/2 "2009-11-20T00:00:45Z")
> [@HeyHomie](#):
>
> 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
[http://www.geochronusa.com/new\_geo/](http://www.geochronusa.com/new_geo/)
Desktop Earth is the computer equivalent. free program, generally awesome
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### Author:  [@Rumor\_Watkins](https://boards.straightdope.com/u/Rumor_Watkins)
#### Post date: [November 20, 2009, 12:08am UTC](https://boards.straightdope.com/t/what-is-this-device-called-real-time-map-of-daylight-darkness/518221/3 "2009-11-20T00:08:05Z")
[http://codefromthe70s.org/desktopearth.aspx](http://codefromthe70s.org/desktopearth.aspx)
if you’re feeling exceedingly lazy tonight 😉
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### Author:  [@Cunctator](https://boards.straightdope.com/u/Cunctator)
#### Post date: [November 20, 2009, 12:30am UTC](https://boards.straightdope.com/t/what-is-this-device-called-real-time-map-of-daylight-darkness/518221/4 "2009-11-20T00:30:49Z")
I find [this one](http://www.worldtime.com/cgi-bin/wt.cgi?cnt=29515&ID=9a145278f6&land=17) quite interesting.
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### Author:  [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)
#### Post date: [November 20, 2009, 12:46am UTC](https://boards.straightdope.com/t/what-is-this-device-called-real-time-map-of-daylight-darkness/518221/5 "2009-11-20T00:46:28Z")
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?
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### Author:  [@Rumor\_Watkins](https://boards.straightdope.com/u/Rumor_Watkins)
#### Post date: [November 20, 2009, 12:53am UTC](https://boards.straightdope.com/t/what-is-this-device-called-real-time-map-of-daylight-darkness/518221/6 "2009-11-20T00:53:23Z")
> [@ZenBeam](#):
>
> 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.
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### Author:  [@Kobal2](https://boards.straightdope.com/u/Kobal2)
#### Post date: [November 20, 2009, 2:01am UTC](https://boards.straightdope.com/t/what-is-this-device-called-real-time-map-of-daylight-darkness/518221/7 "2009-11-20T02:01:43Z")
> [@Rumor\_Watkins](#):
>
> [http://codefromthe70s.org/desktopearth.aspx](http://codefromthe70s.org/desktopearth.aspx)
>
> if you’re feeling exceedingly lazy tonight 😉
That .gif is hypnotic. And awesome.
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### Author:  [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)
#### Post date: [November 20, 2009, 2:44am UTC](https://boards.straightdope.com/t/what-is-this-device-called-real-time-map-of-daylight-darkness/518221/8 "2009-11-20T02:44:01Z")
> [@Rumor\_Watkins](#):
>
> 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.
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.