I suppose a web-based standalone version of Google Sky was inevitable, once the Google Maps API supported it, and now it’s here. Highlights include infrared, microwave and historical-map layers with opacity controls and a series of image collections from space telescopes linked from the bottom. (I should note, as I often do, that the controls don’t work perfectly in Safari, but worked fine in Firefox.)
I have some quibbles, some more serious than others.
While it’s simple to use, it does have its limitations: it can be frustrating trying to have a simple look around, and can be very hard to orient yourself at higher zooms (though it is nice to have your cursor’s declination and right ascension shown at all times, it would be more helpful to know, for example, what constellation you’re in). I still find the patchwork imagery a little less than satisfying, though I imagine it’s not easy to bridge between a high-resolution photo of, say, the Carina Nebula and lower-resolution imagery of the surrounding starfield. Standalone planetarium software like Stellarium is easier to navigate and the sky is more seamless, but at the cost of the integrated stellar imagery. The sky tours are a bit of a letdown: Backyard Astronomy is just the Messier objects, which are Northern-Hemisphere-only; it’d be nice to see some of the other, best-of-the-NGC catalogues out there. I was also disappointed that the Solar System only shows one planet at a time.
But, as Google has shown, everything they do is a work in progress. Having Google Sky on the Web is a natural progression, and I’m hopeful that it will improve. And, of course, it will support mashups, which may well make all the difference.