noun
mean solar day
Technical term in astronomy and timekeeping; refers to the average length of a solar day over a year.
平均太陽日は24時間と定義される。
The mean solar day is defined as 24 hours.
太陽日 (solar day) is the actual time between two successive solar noons, which varies slightly throughout the year. 平均太陽日 is the averaged, constant value used for civil timekeeping.
Compound of 平均 (mean, average) and 太陽日 (solar day). A calque from Western astronomical terminology.