It is probably a false claim (myth). This claim is widely used in official literature, but a number of people have disputed it. For example, Pikes peak is a highly qualified contender. Search google for "diablo kilimanjaro view".
I used to live at the base of this mountain. The view is indeed fabulous, but the claim is from a plaque in the gift shop at the summit. Guy who made the claim had a hotel on the mountain long ago he was promoting. 3900' mountains have limited views!
I think there is a quantitative way of determining such a measure. On a digital elevation model, simply trace straight rays at all (reasonable) azimuths and elevations from the summit of interest to obtain a map like this: Half Dome.
Of course, this approach ignores haze and other atmospheric events (curving rays), and also ignores the earth's curvature (easy to correct for) but you get a pretty good idea of what's going on.
Morgan, very nice exercise. You can calculate visibility areas with a standard GIS software like ArcInfo. However, the crux of the problem is obtaining enough digital elevation model data to cover all visible areas. This is very difficult as the visible areas can be 200+ miles in radius, which means a high resolution dataset of the area will be huge. Using a smaller dataset you'll always miss a few distant peaks.
Yes, these are endemic plants on Mt Diablo; the names of quite a few native plants carry the name of the mountain.
I've heard the claim before that Diablo has one of the country's farther ranging views. Hadn't heard the bit about it being only 2nd to Kiliminjaro. How is that determined, or from which sources did you get that (not disputing your claim, merely curious)?
Unless there's another Mt Diablo nearby, this mountain is in the news this week as the home of the thought-to-be-extinct
Mount Diablo Buckwheat
As of July 2nd, 2006, the Summit Trail was closed at the South Gate Entrance due to landslides. There was a barrier about 200 or 300 yards from the trailhead. I'll try again later this summer, but I'll call the Park beforehand.
The Reason for viewing the largest Land Mass is not just Mt. Diablo's elevation, but the distance to other peaks that might block the view. ie Northerly on a clear day we can see Mt. Shasta about 180 miles, north of Redding,CA, easterly to the Sierras about 100 miles, and westerly to the Pacific Ocean. Mt. Kilimanjero is far taller, but has other mountains blocking it's distant views, so the Claim for Mt. Diablo is probably correct.
entrance fee is now $10