Do you have a best friend who you meet approximately every day and is there for you 24/7365?
Sorry, that is not good enough!
If you have a friend truly had your back, they would be there for you 24.0000006/7/365.2421891.
Yes, This is what we call a Day.
George Washington was born on February 22nd, 1732 at least that is what we’re told, however, his family Bible says he was born on February 11th, 1731/2, so which is it, mister, I cannot tell a lie!
Don’t even ask about 1752 in Russia 1752 looked pretty normal but check out what the British Empire was up to that year nothing really out of the ordinary except ember the second was followed by the 14th were eleven days just deleted where’d they go what happened then?
it is time to question time and how the earth moves to reconcile both of these things has led to some pretty strange things.
Tumbling through the universe
We all love time-lapse videos of the stars moving across the sky, but really, we are the ones who are tumbling through the universe on a giant wet rock vehicle called earth with a windshield called the sky as viewed from above the North Pole we spin counter-clockwise west chases east.
I always remember this by thinking of the US as a weird Mane headed animal with Texas and Florida legs running forward but we don’t just spin we also revolve around the Sun on a plane tilted 23.4 degrees relative to our spin it’s kind of nauseating at this scale but from this perspective, you can see that the Sun rising and setting is just the earth pointing you towards and then away from the Sun this motion causes your sunrise your noon the moment when the Sun is highest in your sky before your Sunset to more closely investigate this movement let’s talk about meridians.
Meridians In our Day
You are on one at this very moment your meridian is just aligned from where you are right now straight towards the North and South Poles it’s a line of longitude as opposed to the horizontal lines that lay flat when north or south is up that we call flat attune or latitude.
The Sun is highest in the sky to you your noon when your Meridian has pointed right at the Sun a cool thing happens at this moment all shadows around you point directly towards one of Earth’s poles unless you’re on the subsolar point, the subsolar point is the point on Earth’s surface directly below the Sun it’s always somewhere you can check its current location online, on the subsolar point shadows fall straight down so they can easily disappear twice a year the subsolar point crosses over Hawaii the only place in the US where it hits land and when it does it is called Lahaina noon.
Meaning cool Sun straight vertical objects look unnatural during this brief time like they don’t belong as if they were photoshopped in without regard for reality in Honolulu a sculpture by Isamu Noguchi called sky gate casts a twisted shadow all day every day except during Lahaina noon when its shadow is a perfect circle.
You may not live in a place where the Sun ever appears directly overhead but once every Earth rotation the subsolar point falls somewhere on your Meridian making it noon for you the technical name for this noon for you is local apparent solar noon.
The clocks on your wrist and the clock on your phone doesn’t tell you your local apparent solar time because long ago we realized that if every meridian had its own time a person just a few kilometers away seeing different shadows than you did would disagree with you on what time it was so towns adopted their own time now later on this trick was standardized and time zones as we know them today came about but that’s not all we didn’t like about shadow based sundial time to explore deeper we have to begin by asking what’s a day?
What is a day?
I mean obviously, it’s just the time it takes the earth to turn around once right but according to what everything else in space is moving in some way to the universe doesn’t include a convenient sheet of graph paper at Absolute rest we can trace paths on the best we can do on that front is to look at very far away stars so far away like distant features of the landscape out the window of a moving car they barely move as Earth does now to them a Meridian on earth completes a trip around about once every twenty-three point nine hours this is called a side aerial.
dayside aerial means about the Stars even though the sidereal day seems pretty clear it’s not what our calendars and clocks are based on because there’s a nearer star whose position relative to us has a bigger effect on our lives the Sun looking down on the north pole at Earth’s counter-clockwise spin the earth also moves counter-clockwise around the Sun after a sidereal day the earth has moved a bit along its orbit so some more rotation is required for the same meridian to point back towards the Sun again this longer definition of one rotation is what the modern calendar and clock is based on it is called the solar day.
But here’s the thing exactly how long the earth has to rotate to complete a solar day changes day to day our clocks are just based on the average amount of time this takes so throughout the year they fall ahead and behind the Sun this is a solar graph a picture of the sun’s path across the sky every single day.
If our clocks told us local apparent solar time if you took a picture of the sky every day at noon you should get a line of Suns but this is what happens over a year it will appear as though your clock is running slow and then fast and then slow again and then fast again this problem was known since at least ancient times even if its cause was it to reconcile the two the equation of time was constructed in this sense equation means to reconcile the equation of time.
Equation of time
Was applied to what a clock said in order to compute the real-time.
The solar time a sundial would show now some fancy clocks called equation clocks were made that would do this for you but eventually, we gave up and just said no the real-time isn’t what the Sun says it’s what our inventions say now this transition was a big one it was humanity growing up it was like the first time you realize you’re stronger than your parents we realized our timepieces were more regular and turned our backs on the timepieces nature had.
But what causes this disagreement in the first place?
As it turns out the answer revolves around revolving the way the Earth revolves around the Sun if the Equator faced the Sun all the time and the earth always orbited at the same speed the subsolar point would just stay right there on the equator throughout the year and the amount of extra time spent rotating the earth needed to do to finish a solar day would always be the same but those two things aren’t the case first of all the Earth’s orbit is slightly elliptical so its speed varies throughout the year when it’s moving around the Sun faster around the beginning of January.
The solar day
The amount of extra turning time needed to complete the solar day is longer than when it’s further away from the Sun and moving more slowly there’s more because the earth is tilted the subsolar point is dragged throughout the year in a circle around Earth that’s not the equator so it changes directly moving northeast then leveling out and going southeast before leveling out and going northeast again during times of the year.
When the subsolar point is being dragged by Earth’s orbit mostly east it gains against Earth’s spin faster more time is required for the day to finish now by coincidence we are alive at a time when both of these phenomena lengthen and shorten days at roughly the same time so they add up making September 18th almost a minute shorter than the longest day of the year December 22nd for northern hemisphere Ian’s December has the shortest periods of daylight but the whole solar day from sunrise to sunrise is for everyone on earth the longest of the year on December 22nd people in the North just spent most of it in darkness.