The next step brings us to the wonderful variable ο, or Mira, whose changes have been watched for three centuries, the first observer of the variability of the star having been David Fabricius

in 1596. Not only is the range of variability very great, but the period is remarkably irregular. In the time of Hevelius, Mira was once invisible for four years. When brightest, the star is of about the second magnitude, and when faintest, of the ninth