2020 March 18

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Anticrepuscular Rays over Florida
Image Credit & Copyright:
Bryan Goff

Explanation: What’s happening behind those clouds? Although the scene may appear somehow supernatural, nothing more unusual is occurring than a Sun setting on the other side of the sky. Pictured here are anticrepuscular rays. To understand them, start by picturing common crepuscular rays that are seen any time that sunlight pours though scattered clouds. Now although sunlight indeed travels along straight lines, the projections of these lines onto the spherical sky are great circles. Therefore, the crepuscular rays from a setting (or rising) sun will appear to re-converge on the other side of the sky. At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. Featured here is a particularly striking display of anticrepuscular rays photographed in 2016 over Dry Tortugas National Park in Florida, USA.

Tomorrow’s picture: open space

Bryan Goff

说明: 这片云的后方到底发生了什么事?虽然这幅景观看来有些超自然,不过它只是落日在那端天空造成的效应而已。这种现象称为反云隙光。要了解这种现象,让我们从白画时,阳光穿过云隙所形成的平常云隙光开始说吧。虽然这些透过云隙的阳光的确是沿着直线前进,不过当它们投射到球状的天空后,就成为一个个圆。也因此,来自落日或旭日的云隙光,视觉上会在反方向重聚起来。而这种重聚在和太阳相隔180度反日点的光线,就称为反云隙光。这幅特别精采的反云隙光景观,是在2016年摄于美国·佛罗里达州·海龟国家公园的上空。

明日的图片: open space

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