Sighting Half Dome from Mt. Diablo
It took four attempts spanning three months, but it was worth it: finally, I sighted Yosemite’s mighty Half Dome from the summit of Contra Costa County’s Mt. Diablo.
Friend David Beckemeyer and I made the trip up last week, on February 5. This was two days after a good Bay Area rain, which cleared the atmosphere of dust. Half Dome is 130 some odd miles from Mt. Diablo, so the air has to be clean, clear, and cloud free. Morning is the best time to attempt the view: by midafternoon, clouds start rolling in over the Yosemite Valley, obscuring the view. In addition to all that, winter is the best time to make the attempt, when the air is less likely to contain large amounts of water vapor.
There are a few simple ways to get the bearing to Half Dome from Mt. Diablo. You can go up into the beacon lighthouse and use the compass rose there to find due east. Half Dome is about 93 degrees off true north, meaning it’s just about due east of Mt. Diablo. If you’re using a magnetic compass, subtract about 15 degrees from its reading to account for magnetic declination in California. Either way, look for Half Dome sitting high on top the ridge, where the land meets the sky. Good binoculars (8×42 or 8×50) are required no matter how you sight. You cannot see Half Dome without optics. See below.
Yet a third way to sight Half Dome: Clifton Court forebay reservoir also lies along the line from Mt. Diablo to Half Dome. The reservoir is the large rectangular body of water half way between where you stand and the distant horizon. On an east-west line bisecting Clifton Court reservoir, follow all the way up to the horizon. Half Dome is lurking near that bisector where it meets the sky. David ginned up a map showing that line.
We stayed up top for about an hour, taking in the tremendous panoramic view Mt. Diablo affords, and showing Half Dome to the few passersby who wanted to know what we were up to, what with our binoculars, compass, and telescope. Mt. Diablo boasts one of the best so-called viewsheds in the world. The view of the Sierra Nevada, San Francisco proper, the Farallon Islands (60 miles distant) on a clear day, and the entire Bay is sight to behold.
You have to know where to look for Half Dome, to be sure. Even with good optics, you’re not really sure what size of object you’re looking for. As for sighting Half Dome with the naked eye, let alone viewing it, I’d say that’s practically impossible: at 130 miles and an estimated 2000 feet across, it takes up less than 0.2 degree field of view. Granted, there is some approximating going on here, but even if Half Dome were a mile across, you’re still talking less than half a degree field of view.
Finally, as an added treat, here is the Half Dome webcam, which refeshes every half minute or so. During the winter, you have a bird’s eye view of the hourly change in weather condistions that are common in the park. What a sight.
[tags]half dome,mt. diablo,yosemite[/tags]
