泰德火山的三角形影子
影像来源与版权:Juan Carlos Casado (TWAN)

说明:这座火山的影子,怎么看起来是三角形的?泰德火山实际的形状,和它三角形影子所暗示的有别,并非是完美的金字塔形。此外,三角形的影子不是泰德火山所特有,而是站立于巨峰或火山峰顶时,会经常见到的景象。关键的因素之一,是日落(或日出)时,观测者是顺着向地平线伸展的长长影子远眺。即使巨大的火山是个完美的立方体,影子也会是长方形的,而且影子向远方的地平线伸展时,会逐渐缩减成锥形,就如同平行的火车铁轨会聚在远处一样。这张壮丽的主题影像,前景有西班牙加那利群岛特内里费岛的Pico Viejo火山口。接近满相的月亮,则是摄于月全食过后不久。
翻译:“每日一天文图”成大物理分站

A Triangular Shadow of a Large Volcano
Image Credit & Copyright: Juan Carlos Casado (TWAN)

Explanation: Why does the shadow of this volcano look like a triangle? The Mount Teide volcano itself does not have the strictly pyramidal shape that its geometric shadow might suggest. The triangle shadow phenomena is not unique to the Mt. Teide, though, and is commonly seen from the tops of other large mountains and volcanoes. A key reason for the strange dark shape is that the observer is looking down the long corridor of a sunset (or sunrise) shadow that extends to the horizon. Even if the huge volcano were a perfect cube and the resulting shadow were a long rectangular box, that box would appear to taper off at its top as its shadow extended far into the distance, just as parallel train tracks do. The featured spectacular image shows Pico Viejo crater in the foreground, located on Tenerife in the Canary Islands of Spain. The nearly full moon is seen nearby shortly after its total lunar eclipse.
Astronomy Picture of the Day 06/09/2019