In previous versions of Celestia, the only source of illumination was the sun (or suns, in multiple star systems.) But, light reflected from the surface of a planet can also be a significant. If you look at images of the International Space Station, you'll notice that the Earth-facing portion is lit by a bluish light reflected from the Earth's oceans. Recent images from the Cassini mission show the moons of Saturn with their Saturn-facing hemispheres illuminated by the faint glow of Saturnshine. You can now observe these effects in the latest version of Celestia.
Much work has been done to add globular clusters to Celestia. Each globular cluster is drawn realistically in three dimensions using a statistical model that accounts for such features as overall cluster diameter, the core diameter, and the distribution of red giants. Brightness across a globular cluster is modeled by the King profile. The new globular cluster catalog contains all 150 globular clusters of the Milky Way, with a measured radius, core radius, and King concentration for each object. The names assigned to each cluster are compatible with the scheme used in the SIMBAD astronomical database.
New sky grids
Celestia's sky grid feature has been completely rewritten and greatly improved. The spacing of lines in the grid will now adapt to the field of view, making it a much better tool for orienting yourself at high zoom levels. Coordinate labels are conveniently located at the edges of the view window. Celestia supports grids for several different coordinate systems, including equatorial, horizontal (altitude/azimuth), galactic, and ecliptic.
Updated star catalogs
The main star catalog used in Celestia has been regenerated from an updated version of the Hipparcos catalog, Floor van Leeuwen's new reduction of raw data from the Hipparcos mission. The basic catalog has been supplemented with over 1900 measured stellar radii from the CHARM2 survey, so a significant fraction of Celestia's stars now have more reliable figures given for their sizes. The orbits of binary stars were updated, and the names of stars in multiple systems have been changed to a SIMBAD compatible syntax. The new extrasolar planet catalog includes the dozens of new worlds discovered since the last Celestia release.
A new animated cursor in 1.6.0 makes Celestia easier to use. The animation effect directs your eye more quickly to the selected item. An arrow at border of the view window points to the selection when it is offscreen. And, the cursor changes color when the selection is hidden by a foreground object, helping you locate the the selection in three dimensions.
While the shapes of most large objects in the solar system are closely approximated by flattened spheres, there are some exotic worlds that are noticeably elongated along one axis. The most extreme example of this is the dwarf planet Haumea (aka 2003 EL 61), which looks more like an American football than a familiarly spherical planet. Now that Celestia supports triaxial ellipsoid bodies, you can see the true appearance of such worlds.
Different orbit colors may be assigned for each object in a solar system. This can help you quickly distinguish the orbits of individual planets. Or, you can highlight the orbits of a group of related objects.
Since 2006, the International Astronomical Union has recognized a new class of objects called dwarf planets. To date, there are five bodies in our Solar System that have been assigned dwarf planet status. Version 1.6.0 brings Celestia up to date with the IAU decision and adds a dwarf planet class. You can visit all five recognized dwarf planets and their satellites.
New reference marks
Reference marks augment the 3D view with additional information that can help you understand the locations and relationships between solar system objects. Celestia 1.6.0 adds a longitude/latitude grid to aid in locating surface features on planets. Another new reference mark is the terminator, the boundary between the day and night sides of a world.
Updated Cassini mission
The primary mission of the Cassini spacecraft lasted through mid-2008. In April 2008, an two-year extended mission was approved. The new version of Celestia includes the extended mission trajectory, so that you can follow Cassini around Saturn through 2010. The Huygens probe separation sequence has been improved using new features in version 1.6.0—watch Huygens separate smoothly from the main Cassini spacecraft and rendezvous with Titan three weeks later.
Updated ISS model
The new Celestia package includes a much more detailed model of the International Space Station, complete with all the modules added through 2008. With the new planetshine feature in 1.6.0, the updated ISS looks spectacular in low Earth orbit.