An equinoctial hour is one of the 24 equal parts of the full day (which includes daytime and nighttime). Its length, unlike the temporal hour, does not vary with the season, but is constant. The measurement of the full day with equinoctial hours of equal length was first used about 2,400 years ago in Babylonia to make astronomical observations comparable regardless of the season. Our present hour is an equinoctial hour, freed only from its seasonal variation and from the small error due to some uniform Earth rotation, and realized by modern technical means (atomic clock, satellite and VLBI-Astrometry). When the temporal hour was used, the daytime and nighttime, whose lengths vary greatly throughout the year, were each divided into 12 hours. This corresponded to the earlier sentiment and custom of not grouping the night with the daytime. The name equinoctial hours refers to the fact that the temporal hours of the daytime (daylight hours) and those of the night are of equal length at each of the equinoxes.
---
This article is an automated import of the Wikipedia article "Equinoctial hours", available at https://en.wikipedia.org/wiki/Equinoctial_hours . Text is licensed under CC BY-SA 4.0.