镶蓝带的红月

2021年12月01日

A Blue-Banded Blood Moon
Image Credit: Angel Yu

Explanation: What causes a blue band to cross the Moon during a lunar eclipse? The blue band is real but usually quite hard to see. The featured HDR image of last week’s lunar eclipse, however — taken from Yancheng, China — has been digitally processed to equalize the Moon’s brightness and exaggerate the colors. The gray color of the bottom right is the Moon’s natural color, directly illuminated by sunlight. The upper left part of the Moon is not directly lit by the Sun since it is being eclipsed — it in the Earth’s shadow. It is faintly lit, though, by sunlight that has passed deep through Earth’s atmosphere. This part of the Moon is red — and called a blood Moon — for the same reason that Earth’s sunsets are red: because air scatters away more blue light than red. The unusual blue band is different — its color is created by sunlight that has passed high through Earth’s atmosphere, where red light is better absorbed by ozone than blue. A total eclipse of the Sun will occur tomorrow but, unfortunately, totality be visible only near the Earth’s South Pole.

Almost Hyperspace: Random APOD Generator

Tomorrow’s picture: small galaxy, local group

镶蓝带的红月
影像提供: Angel Yu（安格尔．于）

说明: 在月食期间，为何月亮会镶着一道蓝带？这道蓝带是真的，只是非常难分辨。这幅在上星期月食期间、摄于中国盐城市的高动态范围影像，数位拉平月亮明暗区的亮度差异，并夸大月面的色泽，才让这道蓝带得以显现出来。不过，影像右下方受阳光直射的月盘，其泛灰的色泽是月亮的真实自然色彩。左上的月盘在月食期间，则进入地球的暗影里，因此没受阳光直接照射。当时淡淡照耀它的，是深入地球大气层的红化阳光。因此，此区的月面色泽泛红，故有血月之称；其成因与泛红的落日雷同，都是因为阳光中的红光受到散射移除的比率高于蓝光之故。然而月盘不寻常蓝带的成因不同，因为照亮它的，是穿过地球高层大气的蓝化阳光，源自臭氧层对红光的吸收高于蓝光。明天将再发生日全食，不过很遗憾的，只有在地球南极附近才得见。

几乎超空间: 随机APOD生成器

明日的图片: small galaxy, local group