The Earth's axis points at the North Star.
And the heavens have been revolving around
that star since the dawn of time - except
they haven't.
When the Egyptians built the pyramids, the
North Star wasn't our current North Star Polaris.
No, it was Thuban.
Why did it change?
It's because the Earth's axis is moving.
This motion was first discovered by Hipparchus.
He was looking at the positions of the stars
and comparing them with their positions a
century earlier at the same time of year.
He found that the stars were moving at least
one degree every century.
What could be causing this motion?
When you're riding in a car and the car turns,
you feel an acceleration.
In the same way that you experience an acceleration
when a car turns, there is also an acceleration
as the world turns.
This acceleration works against the force
of gravity, so you actually weigh less at
the equator.
And you weigh more when you're at the North
Pole, one percent more.
This also causes the earth to bulge.
The earth is 26 miles wider at the equator.
Because the earth is not a perfect sphere,
the gravitational force of the Sun isn't perfectly
balanced.
One side of the earth gets tugged just a little
bit more and this causes the Earth's axis
to spin completing one revolution every 26,000
years.
This also affects the length of the year.
Now you can measure the length of a year several
different ways.
One way is to look at the seasons.
This is called the tropical year.
Another way is to look at the positions of
the stars.
This is called the sidereal year.
In January, the Sun is in the constellation
Capricorn which means children born in January
have Capricorn as their astrological sign.
Over the course of a year, the Sun moves through
all the signs of the zodiac.
Once the earth is back in its original location,
we have completed one sidereal year.
But the tropical year doesn't depend on the
Earth's position.
It depends on the Earth's axis.
When the North Pole faces away from the Sun,
we in the northern hemisphere experience the
winter solstice, the shortest day of the year.
If you measure the length of time between
two winter solstices, you get a tropical year.
So what's different?
Well, if the Earth's axis is moving, it could
line up with the Sun before the earth has
returned to its original location.
This is what happens,
although the effect is small.
The axis only moves about one-hundredth of
a degree and a tropical year is only 20 minutes
shorter than a sidereal year.
How did this affect the 
development of the calendar?
Let's go back to the Romans.
You may remember Julius Caesar as the man
who conquered the Gauls, overthrew the Roman
Republic, and named the month of July after
himself.
But in his free time, he cleaned up the Roman
calendar which was a total mess!
Before Caesar, the calendar lasted between
355 days and 378 days.
Caesar put it at 365 days
with a leap day every four years.
This was the Western calendar until
Pope Gregory XIII said that's not good enough.
He removed some of the leap days to better
match the length of a tropical year.
The Gregorian calendar wasn't adopted in Russia
until 1918 and as a result the Russians showed
up at the 1908 Olympics 12 days late.
Why did Pope Gregory base
our calendar off the tropical year?
What if he had used the sidereal year?
Imagine it's the year 15,000 AD and Americans
are celebrating Christmas in the middle of
summer and that is what a sidereal calendar
would mean.
Now a tropical calendar gets the seasons right,
but it puts the earth of the opposite side
of the Sun in the year 15,000.
And so Geminis will be born in December, Cancers
in January, Leos in February, Virgos in March.
Everything will be backwards!
And as we know from astrology, the positions
of the Sun and planets at the moment you were
born predicts all future events in your life.
Here is a chart showing you the positions
of the planets when I was born and as you
can see I am destined for greatness.
Don't believe me?
Just look at my horoscope today.
Today, you will make a YouTube video.
Some people will think that you love astrology,
while others will think you're been sarcastic.
But everyone will love your video.
For more astrological videos - I mean astronomical
videos - please click to subscribe.
