天文學名詞的解釋不完整,根據英國及美國聯合出版的 Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Astronomical Almanac 中 (第206頁) 從地球觀測的行星現象計算定義如下:
The times of conjunction and opposition of the planets are those at which the difference between the apparent geocentric longitudes of the planet and the Sun is 0° and 180° respectively. In order to avoid calculating of the geocentric longitude of each planet, the heliocentric geometric longitudes may be used as tabulated, and a small correction applied to correct for the effect of light-time.