DaylightMap shows the pattern of night and day on a Google map, for any area of the Earth, for any date and time. It also allows you to select up to ten locations on the map; in addition to seeing at a glance whether it's daytime there, you can show their local time, sunrise and and sunset times, and length of day. These locations can be remembered for future visits, saved as a bookmark, or sent to other people as a web link. This site can thus function as a graphical world clock, and can be used to show relative times of any future or past event.
It can even show just the night-time city lights, correlated to the base Google satellite view (on better browsers). Which has nothing to do with daylight per se, but is pretty cool to look at nonetheless.
There's also a near-real-time view of global cloud cover available, updated every hour from weather satellite imagery. It's interesting both for looking at the big-picture patterns, as well as a more localized picture of what the weather is doing in your part of the world. A specialized cloud-cover view is available as well, my Hurricane Watcher.
More recently, I've packaged the DaylightMap + Clouds functionality as an iGoogle Theme, meaning you can now wrap your whole Google home page in a DaylightMap.
Something interesting that I found while developing DaylightMap is that there are many different definitions of sunrise and sunset, mostly to do with just what part of the sun's disc is (or is not) visible above the horizon. So if the apparent sunrise/set shown here doesn't precisely match the times you find at another source, that's probably why. Also, I'm currently not accounting for the varying elevation of the Earth's surface, which affect local sunrise and sunset times. Please note as well that DaylightMap gets the current time from my server, and while this is pretty accurate, it's not an ultra-precise atomic clock.
This all means that the sunrise and sunset you see here shouldn't be relied upon if you need a high degree of accuracy. This website is for fun, and is more about showing the shape of the Earth's shadow on its own surface than for highly-accurate times of specific sunrises and sunsets.
Finally, this entire site, including all its content and software (but excluding the Google base maps and API code), are © 2006 and beyond by Udell Enterprises, Inc., and may not be reproduced without express permission. The daylight plot may be used on other Google maps, however; see the Get Your Own page for details.
By expanding the Options bar, you can select whether you want to show daylight for the current date and time, or for another date and time that you specify. If you do set another date/time, be sure to click Apply afterwards to update the map.
This is also where you can add map markers, by any of the methods shown in the Add dropdown. Once you have added a marker, click on it to see its full details.
If you check the Save in Browser box, any markers you add to the map should still be here the next time you visit this site. In order for this feature to work, you'll need to have cookies enabled in your browser (check your browser's Help system for more details).
You can link to any DaylightMap by using the Link To This Map tool under the page's title. Here's how:
- Use the map controls to get the display you want. Remember, if you want a specific daylight line, select Other Date/Time (rather than Current) under Options.
- Right-click on Link To This Page
- To keep the map for yourself, select Add To Favorites / Bookmark This Link
- Or, if you select Copy Shortcut / Copy Link Location, you can then paste the map's web address anywhere you want, such as into an e-mail.
The Link To feature also allows you to craft a specific link to daylight maps of interest by hand if you wish. Here are a few examples:
See the Technical Notes page for full details.
My best reference in developing the sunrise, sunset, and Earth-shadow aspects of this site was the list of formulae at Paul Schlyter's pages. I found the Basics of Positional Astronomy helpful as well.
I also owe an algorithmic debt of gratitude to the open source at EarthView, which generates some similar daylight-shadow views to this one. You'll notice their curves are a different shape than mine — that's because they use a different map projection than Google's Mercator. This holds true for many of the other terminator plots you'll find floating around the infosphere as well.
Oh, and that reminds me, MathWorld has a very informative page on the Mercator projection and its associated equations.
The city light imagery shown on the night side of the Satellite and Hybrid views is from NASA's Visible Earth catalog. Although I'm using the highest resolution they offer, it's still only acceptable through about the first half of the Google Map's zoom range, so beyond that I suppress the city lights.
Which brings up another point: Although this site shows a real-time day/night terminator, the satellite imagery underlying it is not real-time. The city light photos date from 23 October 2000, and Google Maps' satellite photos are of varying ages (for more information, see Google Maps Help on the topic).