Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

UltravioletEarth from an Observatory on the Moon

Image Credit: G. Carruthers (NRL) et al., Far UV Camera, Apollo 16, NASA

Explanation:Which planet is this? Earth. The featured false color picture shows how theEarth shines in ultraviolet (UV) light. The image is historic because it wastaken from the surface of the Moon by humanity's first lunar observatory.(Another is operating now.) Although very little UV light is transmittedthrough the Earth's atmosphere, what sunlight does make it through might causea sunburn. The part of the Earth facing the Sun reflects much UV light, butperhaps more interesting is the side facing away from the Sun. Here bands of UVemission are the result of auroras and are caused by charged particles expelledby the Sun. Other planets showing auroras in the UV include Mars, Saturn,Jupiter, and Uranus.

月球天文臺在紫外波段拍攝的地球

解釋：這是哪顆行星？地球。這幅特征偽彩色照片顯示，地球如何在紫外（UV）波段發(fā)光。這幅影像是歷史性的，因為它是人類的第一個月球天文臺從月球表面拍攝的。（另一個目前正在運行中。）雖然只有非常少量的紫外光通過地球的大氣層，否則陽光確實可能造成灼傷。地球面對太陽的部分反射很多紫外光，但也許更有趣的是背對太陽的一面。這里，紫外輻射帶是極光的結果，是太陽拋射的帶電粒子造成的。其它行星，包括火星、土星、木星和天王星在紫外波段也有極光。