text: noun as in concealment
  The answer: by watching the asteroid directly pass in front of a star, which is called occultation.
  An occultation of a planet is obviously still more interesting than that of a star.
  In any case, no displacement of a star by atmospheric refraction at occultation has been surely recorded.
  Its geographical position is pretty accurately laid down from occultation and other observations.
  This is the first occultation of a star or planet by the Moon observed and recorded in England.
  They had already proceeded together to the Cape of Good Hope on two occasions to study an eclipse and an occultation.
  From Roget's 21st Century Thesaurus, Third Edition Copyright © 2013 by the Philip Lief Group.