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Here Comes the Sun

"GeoChron, the Gift of Presidents"

When I was a kid, I would pass by a cool glowing map hanging in the hallway outside of my dad’s office.  It was a "GeoChron" and it showed the areas of day and night over the earth.  If you watched long enough you could see the shadow creep westward.

Many/most of our clients have interests worldwide.  Emergency preparedness and response is an important issue to the enterprise and believe it or not, it is important for coordinators to have an idea of the day/night context of the coworkers with whom they are communicating.  In the event of an emergency, nighttime is a pretty important factor.  Gerald understood this; look how happy he is with his.

We built a MOSS 2007 portal for a client who was interested in seeing their risk data mapped and needed the context of day and night.  Additionally, the map incorporated a series of world clocks, like you would see at an airport or a broadcast station.  The beauty of the interactive map is that there are so many interaction options for this type of feature.


The Seasonal Shadow-Blob Shift

The earth’s axis rocks back and forth over the year: 23.5 degrees one way — to straight up and down — 23.5 degrees the other way, then back.  Every year (but it wasn’t always 23.5 degrees).  So a visual representation of daylight and nighttime on a map will look quite different depending on the time of year.  It is not just a column of light moving west once a day.

So what’s the deal, why is the day/night shadow such a weird shape?  Well, when you unwrap the round earth into the Mercator projection, that’s what you get.

The illustration on the left shows the angle of the earth in June, when the Northern Hemisphere gets more light.  The result on a Mercator map is a shadowy blob that covers the Southern Hemisphere in more hours of darkness.

Here it is in September, the same time of day.  This time of year the direct rays of the sun are around the equator; North and South get equal amounts of daylight, so the day/night shadow is more vertical (the map is actually showing the earth a little before the equinox so the north is still getting a little more light than the south).

In December, the shadowy blob is favoring the Northern Hemisphere, while the Southern Hemisphere enjoys more hours of daylight.  Good for Australia, bad for me.  So it goes.

How It Looks

I live in Lansing, MI, in the Eastern time zone and I just took the screen capture on the right (it’s about 11:30 am).  I am in that highlighted zone and I can see that sunrise passed me by several hours ago and is currently moving westward across southern Alaska.  It is still dark in Hawaii but not for long.  The sun is setting in Afghanistan and has already set over Madagascar.

You can tell it is summer here because the wider part of the daytime band is in the Northern Hemisphere.

The already great context of maps is enhanced by the visualization of day and night as it shifts through time and seasons, particularly to those with global interests -not just Presidents.


John Nelson / IDV Solutions / john.nelson@idvsolutions.com



One response

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