Secular variation of the obliquity of the ecliptic.

This paper summarizes observational determinations of the secular variation of the obliquity of the ecliptic. The solar and planetary determinations show a discordance between observation and theory amounting to -0.3 arcsec/century, but determinations from lunar occultations agree with the theoretical variation. It is shown that incomplete reduction of solar, Mercury, and Venus observations to the fundamental-star reference system may be the cause of the apparent observed correction to the secular rate of the obliquity.