171221期:冬至的太阳与银河

冬至的太阳与银河

让我们张开双臂迎接冬至——北半球冬季的第一天和南半球夏季的首日的到来。在天文学里,冬至是季节转换的节点(二分二至点之一),是根据在地球所见的太阳每年在黄道上的位置确定的。

在2017年的冬至时分,太阳于协调世界时(UTC)16:28抵达它在赤道最南的位置-23.5度,而它在天球赤经则为18时。此时太阳处于人马座中,方位上很靠近银河系的中心。

事实上,若你幸运地看到了位于背景昏暗的恒星和星云前方的冬至日的太阳(很难,尤其是在白天),你所见到的景观很可能类似于上图这幅拼接的全景影像。为了拼接出这张影像,需要先用多张摄于纳米比亚幽暗天空的壮丽银河组合成全景的底图,然后在2015年12月21日对太阳进行快照,通过数码叠加,最后制成这幅代表今日北半球冬至,靠近银河系中心的明亮太阳图景。

译:银河说翻译组 华夫饼喝酸奶

Solstice Sun and Milky Way
Composite Image Credit & Copyright: Stefan Seip (TWAN)

Explanation: Welcome to December’s solstice, first day of winter in the north and summer for the southern hemisphere. Astronomical markers of the seasons, solstice and equinox dates are based on the Sun’s place in its annual journey along the ecliptic, through planet Earth’s sky. At this solstice, the Sun reaches its maximum southern declination of -23.5 degrees today at 16:28 UTC, while its right ascension coordinate on the celestial sphere is 18 hours. That puts the Sun in the constellation Sagittarius in a direction near the center of our Milky Way galaxy. In fact, if you could see today’s Solstice Sun against faint background stars and nebulae (that’s really hard to do, especially in the daytime …) your view might look something like this composited panorama. To make it, images of our fair galaxy were taken under dark Namibian night skies, then stitched together in a panoramic view. From a snapshot made on December 21, 2015, the Sun was digitally overlayed as a brilliant star at today’s northern winter solstice position, close to the center of the Milky Way.

 

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