Хадар
In South Africa he pursued his interest in astronomy, first measuring the parallax and proper motion of Alpha Centauri and Beta Centauri. He then became a prolific observer of variable stars, particularly those that were members of a binary system. He continued his observations for over 30 years, and pioneered the study of close binary systems. He published over 100 works on these topics. Between 1891 and 1920 he made over 250,000 observations of 98 variable stars.