Two types of <a href="https://en.wikipedia.org/wiki/Aurora_(astronomy)" class="extiw" title="en:Aurora (astronomy)">auroral rays</a>.<br><small>Illustration from 1911 Encyclopædia Britannica, article <a href="https://en.wikisource.org/wiki/1911_Encyclop%C3%A6dia_Britannica/Aurora_Polaris" class="extiw" title="s:1911 Encyclopædia Britannica/Aurora Polaris">Aurora Polaris</a>.</small>
Encyclopædia Britannica, 1911
2013-12-07 15:07:05
<a href="https://en.wikipedia.org/wiki/Encyclop%C3%A6dia_Britannica_Eleventh_Edition" class="extiw" title="en:Encyclopædia Britannica Eleventh Edition">Encyclopædia Britannica Eleventh Edition, Vol. 2, Plate I.</a> <a href="//commons.wikimedia.org/w/index.php?title=File:EB1911_-_Volume_02.djvu&amp;page=977" class="image"><img alt="EB1911 - Volume 02.djvu" src="https://upload.wikimedia.org/wikipedia/commons/thumb/4/49/EB1911_-_Volume_02.djvu/page977-100px-EB1911_-_Volume_02.djvu.jpg" decoding="async" width="100" height="157" srcset="https://upload.wikimedia.org/wikipedia/commons/thumb/4/49/EB1911_-_Volume_02.djvu/page977-150px-EB1911_-_Volume_02.djvu.jpg 1.5x, https://upload.wikimedia.org/wikipedia/commons/thumb/4/49/EB1911_-_Volume_02.djvu/page977-200px-EB1911_-_Volume_02.djvu.jpg 2x" data-file-width="1920" data-file-height="1932"></a>