大般涅槃經集解 22.23

22.23

案僧亮曰：答慧星問也。謂佛不並興，若辟支出世，真佛滅度。人生倒見，謂不祥也。僧宗曰：黑月者，以脩羅手障，令不見也。又言水精白銀為月，以水精邊故，如黑月也。

Sengliang states: This answers the question about comets. It means that buddhas do not appear simultaneously: if a pratyekabuddha appears in the world, the true Buddha has entered extinction. People produce inverted views and consider it inauspicious. Sengzong states: As for 'the dark moon', it is because an asura's hand obstructs it, causing it not to be seen. Also, it is said that the moon is made of crystal and silver; because of the crystal side, it is like the dark moon.