
Tomorrow is the shortest day of the year: 23 hours, 59 minutes, 38.7 secs - ColinWright
http://en.wikipedia.org/wiki/Solar_time#Apparent_solar_time
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T-hawk
The 23:59:38.7 value is the true solar day for September 16th, as compared to
the mean solar day which is defined to be 24 hours exactly when averaged over
the course of a year. The article for Equation of Time describes it in more
detail, with rather wordy explanations, I'll try to simplify.
[http://en.wikipedia.org/wiki/Equation_of_time](http://en.wikipedia.org/wiki/Equation_of_time)

It's a sine wave with two components. The larger comes from the Earth's
elliptical orbit around the Sun. When the Earth is near perihelion in January,
it moves faster and farther each day along its orbit, so needs an extra few
seconds for more rotation to bring your spot on the surface back under the
noon Sun. And the converse in July, where the slower orbital speed at aphelion
means each solar noon occurs a few seconds earlier.

The smaller component comes from the axial tilt and runs at twice the
frequency (two cycles per year). Near the solstices, the Sun's declination
(angle above the horizon at noon) remains nearly constant day to day, so all
of its daily motion is in the east-west direction. Near the equinoxes, the
Sun's apparent path is more oblique, so some of its apparent velocity occurs
in the north-south direction leaving its apparent east-west motion slower.
That shortens the solar day since the Earth has to rotate less far to the next
solar noon.

The slope of the equation reaches its negative maximum on Sept 16th, so that
is the shortest solar day. On this day, the Earth is roughly halfway from
aphelion to perihelion, so experiences its greatest orbital acceleration. And
it is near the autumnal equinox, where the Sun's apparent east-west motion
slows, additively with the same sign as the orbital component, thus creating
the global maximum.

It's worth noting all of this refers to "day" as noon to noon or midnight to
midnight, affecting daytime and nighttime equally. The phenomenon doesn't
affect the daylight/nighttime ratio or insolation or temperatures like the
seasonal cycle does.

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asmithmd1
I studied this stuff a few years ago when I made a sundial. I understood this
better - and now understand why that figure 8 symbol is printed in globes -
from learning about an analemma:

[http://en.wikipedia.org/wiki/Analemma](http://en.wikipedia.org/wiki/Analemma)

This is also why the earliest sunset in December 9th (Not the 21st) at 4:12PM

~~~
bostonpete
> December 9th at 4:12 PM

Where? Every location has its own time of sunset...

~~~
gamegoblin
Given the superlative, I assume that's the earliest at the earliest location
in the entire world (wherever that is, but it clears up the ambiguity of
multiple locations).

~~~
notatoad
there are many places in the north that will see the sun setting well before
4pm on december 9th.

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ed_blackburn
So. How do I compensate for this if I wish to convert a milliseconds since
epoch value to an accurate datetime type? Are these the concerns that smart
people who write DateTime types in frameworks deal with?

~~~
rmccue
You don't have to; solar days are not the same as "normal" days (defined by
TAI and used by UTC). This is why we have leap years, and leap seconds in UTC.

~~~
MichaelGG
Well you do sorta have to deal with it; leap seconds aren't fun, and are
actually quite stupid. Time's already complicated enough without adhering to a
few folks that care about "solar" time. Either kill them off, or move to leap
hours (which puts the next jump so far out, no one will care).

~~~
hkolk
Leap seconds are not accounted for in epoch-time. Basically, 23:59:59 and
23:59:60 have the same value (this also makes it easier by not having to
handle the 60 :) )

We disabled leap-seconds at work in our NTP servers, and just use the normal
mechanisms to compensate slowly.

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huherto
Ah ok, I think I get it. It is talking about a complete revolution of the
earth. It is not talking about a day as in day/night. And it is not really a
revolution, it is about the time it takes for the sun to be in the same
position from one day to another.

~~~
asmithmd1
"it is about the time it takes for the sun to be in the same position from one
day to another."

How else would you define a day?

~~~
jonesetc
Exactly a 360 degree rotation about it's axis.

Edit: Which 1 minute after posting I realize is the same thing, so forget that
half-baked response and instead look at these different ideas of the measure
of a day:

[http://en.wikipedia.org/wiki/Day#Astronomy](http://en.wikipedia.org/wiki/Day#Astronomy)

~~~
sixbrx
Your first answer wasn't wrong, it just defines a "siderial day", using other
stars as a reference point instead of the sun. After rotating 360 deg with
respect to the background stars (which we assumed fixed for our purposes), the
earth then has to rotate a few minutes more to get the sun back in the same
position.

~~~
Roboprog
And thus "right ascension" is measured in minutes (and hours). Astronomers
don't use degrees to measure "longitude" (right ascension) on the celestial
sphere.

Face North (at night) and look right. Everything on that side ascends. In
astronomy, East-West distances are measured by how long it takes things to
rotate past a fixed observational direction.

Every night, a slightly different chunk of sky will also be directly overhead
at midnight, due to the difference observed earlier between solar (sun-
relative) day length and sidereal (star-relative) day length.

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kghose
At which latitude? I hear those polar days can drag on a bit more
[http://en.wikipedia.org/wiki/Polar_day](http://en.wikipedia.org/wiki/Polar_day).

~~~
ColinWright
You put a marker in the ground that then becomes your point of reference, so
it works even if the Sun doesn't set.

~~~
btilly
But not if it doesn't rise.

But saying that the longest day happened during the night may be technically
true on the South Pole, but still sounds odd.

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jmt7les
Thank god because the next day is GTA V.

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pestaa
Please no! Even the default 24 hours is too little for spending time with my
girlfriend, attending class, doing work, thinking about the side project,
learning Haskell, planning my fiction book...

And for one day my sleep deprivation is also increased by 21.3 seconds.

~~~
Samuel_Michon
_What is noprocrast?_

It's a way to help you prevent yourself from spending too much time on
News.YC. If you turn it on you'll only be allowed to visit the site for
maxvisit minutes at a time, with gaps of minaway minutes in between. The
defaults are 20 and 180, which would let you view the site for 20 minutes at a
time, and then not allow you back in for 3 hours. You can override noprocrast
if you want, in which case your visit clock starts over at zero.

~~~
vidarh
The problem I've found with things like noprocrast and assorted browser
extensions is finding the exact right balance between making it enough of a
hassle to get me to go away and do something more productive most of the time,
but not intrusive enough that I'll end up disabling it permanently when it
blocks me when I really want to have some downtime and "deserve it".

Noprocrast simply doesn't work for me for that reason.

What's worked best for me is the "Delayed Gratification" extension for Chrome,
that delays certain websites for a specific amount of time. I found about 30
seconds is sufficient for me to go be able to convince myself to go back to
work most of the time when I find myself just randomly going to a site like
Reddit or HN, yet short enough that I can wait out the delay without being too
tempted to disable it when I really want to.

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wut42
The head of the wikipedia table says it's for 1998 ?

~~~
ColinWright
It also says:

    
    
        > These lengths will change slightly in a few years
        > and significantly in thousands of years.
    

I guess the values won't have changed much in 15 years.

~~~
McUsr
No, but it changes with some seconds every year.
[http://en.wikipedia.org/wiki/Terrestrial_Time](http://en.wikipedia.org/wiki/Terrestrial_Time)

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kenrick
Ha its my moms birthday tomorrow

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djscram
That's good news for those of us waiting for the GTA V release at Midnight
tomorrow.

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brudgers
This is the sort of thing which encourages people who exult science to act as
if those who don't are stupid. Where is the word "sidereal" in the headline?
Leaving it out is part and parcel of scientism's self assertion as the one
true belief.

That's not what people ordinarily mean by "day" \- the boxes on a calendar are
all the same size. It's not even what people ordinarily mean by "shortest day"
\- i.e. the day with the least amount of daylight.

Not only is it a fact of little or no practical utility, it is nearly
impossible to verify by reasonable means and therefore for the vast majority
of those who assert it to be true it is a revelatory truth accepted on faith.
Only those with full notebooks, big telescopes and accurate chronometers have
a chance of verifying the hypothesis. Otherwise, it is just part of a canon.

~~~
tlholaday
News flash: all gods are imaginary.

