It had been noted that there were small systematic differences in the times measured by different astronomers, and these were first systematically studied by German astronomer Friedrich Bessel (1782–1846).
Märgati, et erinevate astronoomide mõõdetud ajavahemikud erinesid vähesel määral, kuid süstemaatiliselt ning esimene, kes neid erinevusi põhjalikult uuris, oli Saksa astronoom Friedrich Bessel (1782–1846).