Marble is an open source digital globe application that is capable of displaying a number of local and Internet data sources. Intended for educational applications, Marble could be likened to an "open source Google Earth".

Marble is cross-platform and has versions for the KDE environment and Qt.

The KDE Education Project seeks to produce a range of free educational software using KDE, focusing on schoolchildren of age 3 to 18. KDE Education already includes a range of education programs covering languages, mathematics, geography, and science. These include the KStars desktop planetarium, and Marble. Marble is a desktop globe that can use local map data or internet map data.

Marble has two application versions: Marble KDE and Marble Qt. KDE recommend the native Marble KDE version, which requires KDE version 4. Users of earlier versions of KDE, should use Marble Qt. Marble Qt requires teh Qt toolkit, but can also run on MS Windows and Mac OSX systems. A Live CD ISO version, called "Marble in a Box", includes a usable set of map data and is intended for systems without internet connections.

A Marble Widget is also available for developers to incorporate into their own applications. This widget is available as both a QWidget and a KDE 4 KPart. Both the application and the widget are distributed under the Lesser GPL license.

The latest version (0.9) was released on February 9th 2010, and is available for download. v0.9 added support for plug-ins (eg. for weather overlays), custom tile URL schemes, and multi-touch support. After using Microsoft Surface's multi-touch support with Bing Maps, I expect this will be a popular feature for touch pad systems such as the iPad.

Marble is generally easy to navigate. The globe is easy to drag and zooms with the scroll wheel (something Google Map refuses to do). Although it might possibly be annoying in the polar regions, Marble's globe remains oriented. This contrasts with Google Earth which has a tendency to turn its globe as you pan. This is simply an artifact of the Earth being spherical. Google's approach is technically more correct but it means you have to keep rotating your globe back to "north is top".

Marble does not take advantage of any possible hardware acceleration. This does not generally affect performance, although zoom updates are not as smooth as Google Earth, Google Maps, or Bing Maps. Data updates are fairly good - faster than most homebrew MapServer applications, and faster than OpenStreetMap's own tile servers.

As an open source project, Marble does not have the financial and infrastructure support of Google or even NASA. Hence the resolution tends to be less. KDE defend this by pointing out that it is sufficient for the intended educational mission.

As an example of the resolution limits, here is the satellite image layer of Big Island, Hawai'i, fully zoomed in:

Marble comes with a number of 'themes' (ie. tile layers). These include the satellite view (see above), OpenStreetMap, atlas (with 'Times Atlas' style elevation coloring), Earth at night, Historical 1689 map, and climate data (precipitation and temperature for June and December). Some of these are a little unusual in a basic map application, but are obvious interest in an educational application.

Here is the Earth at night layer at maximum zoom (note that I have turned all the  captions and grids have been turned off):

Further historical maps are planned. So far, only G. de L'Isle's map of 1720 is available. I am glad to see Waldseemuller's map of 1507 on the 'to do' list. The imagery is available from the US Library of Congress (who own the only surviving copy) but Waldseemuller's projection will complicate the transformation. It would also be great if an early globe was added to the list of early maps - perhaps one of Mercator's.

As well as a legend and map view options, Marble has a Gazetteer of 12,000 cities (based on the http://www.worldgazetteer.com dataset); places of interest; plus a number of prominent mountains and volcanoes. The gazetteer has some basic information and links to the relevant Wikipedia entries. Due to the historical intentions of Marble, the places of interest have a strong bias towards historical points of interest. These points of interest start to turn Marble into a geographical encyclopedia. No one is every going to use Marble to actually find the wreck of the RMS Titanic, but thousands of school kids are going to use that reference to find where it is and as a jumping-off-point to Wikipedia and other online data sources.