On Sat, 13 Mar 1999, Jack Aubert wrote: > But isn't it literally wrong? As long as there are 60 minutes in an hour, > how can 24 of them vary at all? > "Les dur?es de vingt-quatre heures" egalent toujours 24*60 minutes...non? > N'importe le p?riode de l'ann?e. > I think it's just a case of unclear writing. >
Perhaps the writing is unclear, and I believe the difference they mention is more related to EoT than to the length of the day, BUT... It *is* basically not wrong! The length of a Solar Day does indeed vary throughout the year. The "hour" was (initially) based upon 1/24 of a Mean Solar Day. "Mean" as in "Average". As the Earth's orbit is elliptical, when it is nearer the Sun - perihelion, roughly in Northern Hemisphere Winter - it's angular velocity around the Sun is greater than at aphelion. This makes the effective length of the Solar Day shorter. That is, the time, measured in some constant units like atomic-clock-seconds, from one point when the Sun is directly South of the observer (on the meridian) to the next such point is some seconds shorter then what it would be six months later. The Mean time between such crossings is 86,400 seconds, but the actual time can be as much as 15 seconds more or less. Dave
