These strange spherical boulders on the Mendocino Coast in northern California are revealed only during low tide. Resembling bowling balls, they are actually called concretions and were formed as the softer surrounding sandstone was eroded by the Pacific Ocean over millions of years. There are at least four conditions that need to be satisfied to make this photo possible. First, the right time of year – the galactic core of the Milky Way is only visible during the summer season (April to September). Second is the phase of the Moon – around new moon is best so that the light from the Moon doesn’t wash out the Milky Way. Third, the tide needs to be low otherwise the boulders are completely submerged. Finally, the weather – during the summer, this area is often engulfed in fog resulting in poor visibility. One Sunday night all these factors were favourable so I decided to make the eight-hour round trip from San Francisco Bay where I live to capture this image.
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