ABSTRACT
This chapter considers four different timescales: sidereal, solar, dynamic, and atomic, the three first scales being associated with astronomical observations. The year can be defined in several different ways. The chapter discusses the nonuniformities of sidereal and Universal times caused by the nonuniformities of the diurnal rotation. The instant of the passage of the true Sun in meridian is affected by nutation, by the two terms of the annual aberration, and by the diurnal aberration. The sidereal day is confused with the interval of time between two culminations in the upper meridian of an equatorial star. The sidereal year is the interval of time between two passages of the sun over an ecliptic star devoid of proper motion. The agreement in longitude was very good in the second part of the 1700s and in the first part of the 1800s; it gradually deteriorated before 1750 and after 1850.
