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Time
How has the way we measure time changed?
What is time?
Early Civilizations
Early civilizations could easily track days and months.
The most common way to track time was to observe the motion of objects in
space: sun earth and moon.
Days
Days were tracked from noon to noon.
24 hours
Months
Months were tracked from new moon to new moon.
29.5 days
Years
Early civilizations initially observed the sprouting of leaves on vegetation or
tracked it with the annual harvest.
These methods were inaccurate.
Using the stars and moon to track the year.
The position of stars and constellations appear to change as earth orbits the
sun.
So when the same constellation or the same star showed up in the same
place in reference to the horizon these civilizations new a year had past.
Some civilizations used lunar cycles occurred, while others based it on our
revolution around the sun.
Lunar calendar:
Tracks year based on cycles of the moon.
Leaves 11 days left in the year. It Takes 32 years to cycle back to the solar
calendar if not accounted for.
Often accounted for with an additional month added every few years.
If adjustments weren't made equinoxes and solstices and the start seasons
would be off.
Egyptian Solar calendar: first used 2781 BC
The first known solar calendar was created by the ancient Egyptians in 2781
BC.
There calendar tracked the movement of Sirius.
When Sirius lined up with a certain position on the horizon the new year started.
Egyptians noticed it took 365 days for Sirius to make it back to the same spot.
They divided their year up into 12 months with 30 days each and had 5 days
of festival.
Their calendar doesn't account for the extra quarter of a day each year so
their calendar would cycle every 1460 years
Julian calendar: 1st century BC
Scholars Commissioned by Julius Caesar to solve previous issues with previous
calendar.
First calendar to account for the extra 6 hours each year with a leap year.
365 days a year and every fourth year added a day.
They had to add an extra day every 130 years because it's not actually 6 hours
its 5 hours 48 min 46 sec.
Olmec And Maya
Olmec civilization independently comes up with a similar calendar and the
Maya perfect it around the 1st century BC.
The Maya, establishing that there are 365 days in the year, divide them into 18
months of 20 days.
Like the Egyptians (who have 12 months of 30 days), they complete the year by
adding 5 extra days at the end - days which are considered to be extremely
unlucky for any undertaking.
An unusual aspect of the Mayan system is the Calendar Round, a 52-year cycle
in which no two days have the same name.
Gregorian Calendar:
Same as Julian calendar 365 days with a leap year but there are a few
changes.
By the 16th century the seemingly minor error in the Julian calendar (estimating
the solar year to be 11 minutes and 14 seconds shorter than it actually is) has
accumulated to a ten-day discrepancy between the calendar and reality.
It is most noticeable on occasions such as the equinox, now occuring ten days
earlier than the correct calendar dates of March 21 and September 23.
Gregorian (cont)
Pope Gregory XIII employs Christopher Clavius, to find a solution.
Calculates the error to three days in 400 years
Clavius suggests an adjustment.they adjust for difference in time by making
another adjustment by every century year.
Century years (or those ending in '00') should only be leap years if divisible
by 400.
This eliminates three leap years in every four centuries and neatly solves the
problem.
The result, in the centuries since the reform, is that 1600 and 2000 are normal
leap years, but the intervening 1700, 1800 and 1900 do not include February 29.
Finally we had an accurate calendar!
Civilizations and Commerce Advance
Eventually there was a need for more accurate measures of time.
Sun dials, Water clocks, hour glasses, candles clocks, and sometimes these
clocks were combined to increase accuracy.
But these first clocks couldn't accurately measure minutes or seconds
First mechanical clock invented in 1200’s.
Atomic clock
As technology has advanced our need for a more accurate measure of time
was increased.
In 1949 the first atomic clock was made. This clock was accurate to the
millionths of a second.
In 2011 a newer more accurate model of the atomic clock was invented it is
accurate to 10's of quadrillionths of a second.
In 2014 the US invented an atomic clock that is three times more accurate than
the 2011 model.
Why do we need atomic clocks?
Required for modern technology: cell phones, lan lines, the internet, GPS,
and aviation systems.
In order to have these systems function we had to have clocks accurate to the
billionths of a second.
The more accurate the clock the more precisely these systems operate.
The 2014 atomic clock is accurate enough that it can detect variations in
Earth's gravity.
How does velocity affect time?
How does gravity affect time?