Thursday, October 6, 2022

The Portable Greyline Map

Knowing where there is daylight and darkness over the earth at any given time is somewhat of a necessity when DXing shortwave or mediumwave. There are online maps, sure. But what if you had your own personal daylight/darkness map you could save to your computer and click on any time you wish?

Look for the download at the upper right of this page.

The Daylight/Darkness Greyline Reference Map is produced by my Radio Data MW program.

Included is a GoogleMap-based, HTML-driven map which shows the current daylight/darkness state over the earth. Being a Google-based map, it is zoomable and scrollable. You may choose map view or satellite view. If desired, a home location may set by scrolling the map and clicking "<<Set Home". This will allow you to return to the same reference point as long as the current map page remains open. Depending on your brower, latitude and longitude may be saved across map restarts.

Draw tracks, check distances and bearings to points, all displayed in the marker tooltip

You must have an internet connection to view the map.

The file download is at the upper right of this page. It's a small 19 KB. For more details on usage, be sure to read the readme.txt file contained in the download.

The Portable Greyline Map


No install necessary! The map is HTML-based, so no regular install is necessary. Simply unzip the downloaded file and click on the map file to run. The map will open up in your web browser. The map is self-contained. Again, you must have an internet connection to view the maps.


Greyline propagation, or propagation along the earth's sunrise/sunset terminator, is very noticeable on the 160, 80, and 40 meter ham bands and lower shortwave bands. Medium wave signals can and do travel that path as well. 

A night/day overlay can be drawn or removed by toggling the Greyline button at the top of the map. The overlay will show the areas of the earth in darkness, including the earth's terminator - the band of twilight surrounding the earth.

The night/day overlay shows the width of the earth's terminator and its transition from total darkness to the sunrise and sunset edge. The actual sunrise or sunset point is the edge where the overlay disappears. The lightest band of the overlay is civil twilight, where the sun is just below the horizon to a point 6 degrees below the horizon. The next darkest band is nautical twilight, where the sun is between 6 degrees and 12 degrees below the horizon. The next darkest band is astronomical twilight, where the sun is between 12 degrees and 18 degrees below the horizon. The darkest area is where total darkness exists, where the sun's position is greater than 18 degrees below the horizon. Medium wave and other signals, as described above, can propagate along the terminator at greatly reduced signal attenuation - and are often received at astonishing distances.

The night/day overlay gets UTC from a software call to Javascript. Javascript calculates UTC from your computer clock, as it knows the time difference in local hours to UTC. So you must have your computer clock set correctly and with the proper time zone. Long story short, the night/day overlay is always synced to UTC (at map page load time). The overlay is static until you refresh the page or press the "Update" button, where it will move to its new position.

For mediumwave enthusiasts: Some US stations transmit at reduced power during a period called "CRITICAL HOURS". The FCC defines Critical Hours as the first two hours of daylight after sunrise and the last two hours of daylight before sunset.

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