first light vs predawn

first light

noun

  • The time when light first appears in the morning; dawn, especially the start of dawn. 

  • The first use of a telescope or other instrument to capture an astronomical image. 

predawn

noun

  • The period immediately preceding dawn. 

adj

  • Before dawn. 

How often have the words first light and predawn occurred in a corpus of books? (source: Google Ngram Viewer )