What is the X in X-mas?
late 14c., from Latin Aurora, the Roman goddess of dawn, from PIE *ausus- "dawn," also the name of the Indo-European goddess of the dawn, from root *aus- "to shine," especially of the dawn (cf. Greek eos "dawn," auein "to dry, kindle;" Sanskrit usah, Lithuanian ausra "dawn;" Latin auster "south wind," usum "to burn;" Old English east "east").
Plural auroras or aurorae (ə-rôr'ē)
A brilliant display of bands or folds of variously colored light in the sky at night, especially in polar regions. Charged particles from the solar wind are channeled through the Earth's magnetic field into the polar regions. There the particles collide with atoms and molecules in the upper atmosphere, ionizing them and making them glow. Auroras are of greatest intensity and extent during periods of increased sunspot activity, when they often interfere with telecommunications on Earth. ◇ An aurora that occurs in southern latitudes is called an aurora australis (ô-strā'lĭs) or southern lights. When it occurs in northern latitudes it is called an aurora borealis (bôr'ē-āl'ĭs) or northern lights. See also magnetic storm.
["The Aurora Or-Parallel Prolog System", E. Lusk et al, Proc 3rd Intl Conf on Fifth Generation Comp Systems, pp. 819-830, ICOT, A-W 1988].