If I understand you correctly, you are advocating for defining the second such that its length varies, but its connection to a day does not? And you would also introduce locality to the definition?
edit: Or perhaps you meant to tune variably, and not redefine the second. That sounds more sensible, but not as practical.
Well, there are several definitions of "day", (86.4 kiloseconds, calendar day, stellar day, sidereal day, and solar day, for five of the more common) and of all the definitions I am aware of, only 86.4 kiloseconds is constant.("Defined by nature" does not, itself, seem to be well-defined in a way which would distinguish most of them from the second.)
Having each day be 3 minutes longer/shorter than the previous one is pretty crazy, even if it's mitigated by the fact that 1 Venusian day is ~243 Earth days.
That really makes me wonder how timekeeping would work for civilizations that evolved on planets with variable day length. Earth has variable periods of sunlight depending on latitude, but Earth's day length itself is relatively constant.
Why are you basing things off of seconds? The day is a constant defined by nature, I would much prefer a system based around the day. A second is actually pretty arbitrary.
> Counting time up to 1.0 days is a lot more intuitive than counting up to 8.64 myriaseconds. The second is a unit of time which isn't tied to anything intuitive (actually, it's entirely arbitrary). If we're going to shake things up, let's at least use days instead, yeah?
No. The length of a day is not constant. The size of a second is defined based on the length of the day in 1900.
"The key variable that this new information sheds light on is ?T, the difference between time measured according to the Earth's rotation and time independent of the Earth's rotation. Thus, variations in ?T represent variations in the actual length of a day on Earth."
The X.509 RFC and it's relevant substandards define a day to be 86400 seconds long. Not longer, not shorter. There is no half a day tolerance in there, only tolerance of less than a second (which LE exceeded by hitting an entire second).
> but is it not the case that the length of the synodic day varies throughout the year, on account of Kepler's 2nd. law?
I don't see the relevance; the length of the sidereal day isn't constant either.
Variation in the length of the synodic day is the reason a day may contain other than 86400 seconds. If days are not of constant length, you could vary the length of a second on a day-to-day basis, or you could define the length of a "reference day" and then define the second as a convenient fraction of that. We have taken the second approach (though the first was used historically).
But we don't care about the vibration of cesium; if that were to change, we would adjust by changing the definition of a second, not by accepting that seconds were now of a different duration than before. Thus, the fact that cesium is referenced in an "official" definition of the duration of a second is meaningless. The officialness of that definition is illusory; in reality, seconds continue to be defined as a convenient fraction of an average day.
There simply isn't a good way to convert between labels for specific periods of time and units for measuring the passing of time. The problem is that language and common usage don't make much of a distinction between the two. If you ask people how long a day is, they'll tell you 24 hours. Even Google will tell you that, but it isn't true. A day is usually 24 hours. DST and leap seconds can both change the length of a day though. So a day isn't a unit for measuring time at all. It's a name for a certain period of time. Leap seconds can even mess with terms that seem bulletproof like "hour" or "minute". When there's a leap second, one minute of the year becomes 61 seconds, and the corresponding hour becomes 3601 seconds.
The only unit of time you can reliably work with is the second (and metric derivations thereof, like millisecond).
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