As intriguing as space is, you don’t have to go far to find the most interesting, dynamic
and beautiful celestial object out there, the Earth. It has a surface ocean of water,
an oxygen rich atmosphere, and a comparatively powerful magnetic field. It also has a very
unique characteristic, it is the only place we know of that contains life, as well as
it being the place we call home. And yet, even though we live here, there is still so
much we have still to learn about our own planet. So, what makes this little blip in
the vastness of space so special, and what have we discovered about it so far?
I’m Alex McColgan, and you’re watching Astrum, and together we will delve through
everything you could want to know about Earth. Let’s start off by giving you some context
to this planet. You may have heard people call the Earth pear shaped, or even egg shaped.
I think that is a bit disingenuous, designed to make people think of a very odd-looking
Earth, although it is true that the Earth is not a perfect sphere. The Earth is wider
at the equator than at the poles, but only by about 45km. This is because of the rotation
of the planet, causing the planet to bulge at the equator. This means the point furthest
from the Earth’s centre of mass is not the top of Mount Everest, but rather the Chimborazo
volcano in Ecuador. On average, the Earth is 12,742km in diameter.
Earth has long been the talk of ancient scientists, philosophers and other as they tried to place
us in the universe we live in. Does the universe revolve around us? Do we orbit around the
Sun? And – dare I say it on Youtube – is the world flat or round? Although the idea
of a round Earth had been bandied around by Greek mathematicians since the 6 century BC,
it wasn’t until Eratosthenes around 240 BC that the circumference of the Earth was
estimated. The method he used was quite ingenious, he heard that during the summer solstice in
Syene, a city in southern Eygpt, the Sun was directly overhead, but in Alexandria, a city
in northern Egypt, the Sun still cast a shadow. Using the angles of the shadows, he was able
to work out the circumference of the Earth accurately to within 5-10%. The 16th century
brought about the heliocentric model, where we discovered through the help of Nicolaus
Copernicus, mathematical predictions by Johannes Kepler and observations by Galileo Galilei
that the Sun is the centre of the solar system, not the Earth.
Today, building on the legacy of these and many more brilliant people, we now know that
we are the third planet from the Sun, and one of four terrestrial or rocky planets.
Earth orbits on average 150,000,000 km from the Sun, 147,000,000 km at its closest point
and 152,000,000 km when its furthest away.
It takes one year to orbit… rather unsurprisingly.
But what is interesting is that our method of keeping time is maybe not as exact as you
may think. For instance, Earth actually takes 365.25 days to orbit the Sun once, which is
why every four years we have a leap year to account for the .25 extra days in our year.
A month is meant to match up with an orbit of the Moon around Earth, the words month
and moon being closely connected. But the moon takes roughly 28 days to orbit Earth
once, and our calendars do not reflect that! And in similar fashion to our years not being
exact, our days aren’t exactly 24 hours either. A sidereal day on Earth, or in other
words Earth’s rotation of 360° relative to fixed stars in the sky, is roughly 23 hours
and 56 mins. So where do those 4 minutes go every day? Well the answer is that we don’t
actually use a sidereal day to measure our time, we use a solar day, or
the point where the Sun goes full circle in our sky, and reaches the same place as the previous day. How is this
different from a sidereal day? Well, we use a solar day because the Earth isn’t stationary
and as it orbits, it moves slightly forward compared to the previous day, changing the
angles relative to the Sun. It needs those extra 4 minutes to have the Sun lined up exactly
overhead again each day, which is what gives us our exact 24-hour days. Although, it gets
even more complicated than that if you really want to delve into this, as due to the Earth’s
slightly elliptical orbit and axial tilt, true solar days can vary by up to 30 seconds
throughout the year. In order to counterbalance this, we use the mean solar day which averages
all these variations throughout the year together. Going further again, with tidal drag from
the moon slowing down Earth’s rotation, our days have actually become longer by about
2 milliseconds per century. This doesn’t make such a difference to us, but 500 million
years ago, models show that days were under 22 hours, and there were about 400 days in
a year! With all this combined, keeping time is probably more complicated than you might
have thought! The rotation of the Earth can be beautifully
observed from the ground through time lapses at night, with the fixed field of stars moving
across our view, the exact axis of rotation of the planet able to be seen from the points
the stars seemingly circle around. The orbit around the Sun combined with the axial tilt
of the Earth means that throughout the year, the Sun’s position at the same time every
day will be different. This can be seen in an analemma, a photo of combined shots taken
throughout a year. As you can see, the Sun moves in a figure of eight shape throughout
the course of a year. The north to south movement is due to the axial tilt of the Earth as it
orbits, something you’ve no doubt noticed with the change in seasons, with the Sun being
higher in the sky in summer than in winter. The variation in east to west is due to the
orbital eccentricity of Earth combined with something called the equation of time, which
is very basically what we talked about with the variations in a sidereal day, a true solar
day and a mean solar day. This graphical representation shows the combined effects of the equation
of time, the axial tilt of Earth and its orbital eccentricity.
Although we haven’t actually done
this on other worlds yet, if you were to make an analemma on another planet, each planet’s
analemma would look different. Here is a simulation of what it would look like on the other planets!
Again, the differences are due to the differences in the days, orbit and axial tilt of the planets.
The Earth’s rotational axis is roughly 23.5° to the plane of the solar system. This axial
tilt is what gives Earth such varied seasons. Interestingly, the changes in temperature
between seasons are not mainly due to the closer proximity of a point on Earth during
that hemisphere’s summer, but rather it is due to the amount of sunlight hitting that
point. Being an extra few thousand kilometres closer to the Sun doesn’t play such a large
role when the distance between the Earth and the Sun is 150,000,000km. If it did, we would
find that the Earth’s perihelion, or the closest Earth gets to the Sun in its orbit
would make massive difference, because at the perihelion we are 5 million kilometres
closer to the Sun than at the aphelion, or the furthest part of Earth’s orbit. If you
still think it does make a big difference, could you take a guess at when the Earth last
reached perihelion? It was actually only the 2nd January 2019! What really makes a big
difference to the temperature between seasons is the surface area the sunlight hits. During
summer, with the Sun overhead, a point will be exposed to a much more concentrated amount
of sunlight. During winter, with the sun low in the sky, the same amount of sunlight spreads
out over a much larger surface area. Also, during summer, days are longer, allowing more
sunlight to reach the surface compared to winter.
As sunlight hits the ground or ocean, heat is released into the atmosphere. Earth has
a reasonably dense and compact atmosphere, although it has a low overall mass in comparison
to places like Venus and Titan. It consists of mainly 78% nitrogen, 21% oxygen and 1%
argon, with trace amounts of other gases like carbon dioxide. Water vapour is also present
in the atmosphere, but it can vary wildly from only 0.01% to 4%, but it averages at
1%. It’s this water vapour that makes clouds in the atmosphere. O₃ is also present in
a special layer in the atmosphere called the Ozone layer. This Ozone absorbs a lot of the
harmful ultraviolet radiation from the Sun, which helps permit life on land. Without it,
Earth would be a dangerous place to go outside for lifeforms like us. The atmosphere in fact
serves life in many different functions, it provides useful gases like oxygen and CO₂,
causes small meteors to burn up before striking the surface, and even moderates temperatures
around the globe. Without the atmosphere retaining some of the heat from the Sun, the Earth would
be a far chillier -18°c on average compared to the actual 15°c average now.
The atmosphere also drives one of the most essential systems to us, the water cycle.
As the Sun warms the Earth and its oceans, some heat is released into the atmospheric
gases near the surface, causing them to rise. Caught up in these particles can be water
vapour, which rise, cool and condense on dust particles in the air to form clouds. These
water droplets can travel considerable distances before eventually falling in the form of rain.
The water is then transported back to the oceans through rivers, which completes the
cycle. Not only does the water erode the surface, it is also vital to the life that lives on
the surface, from plants, to bacteria, to animals.
Another hugely important system on Earth that I’ve mentioned already is the Earth’s
hydrosphere, predominantly its oceans. The prevalence of life on land is hugely reliant
on the oceans of Earth. The oceans on Earth are vast, making up 1/4400 of Earth’s total
mass. To give you a sense of the scale of Earth’s oceans, if the Earth’s surface
was completely smooth, the entire planet would be covered in an ocean almost 3km deep. Luckily
for us, that is not the case, and the Earth’s surface is highly variable. This is due in
part to quite a unique feature of Earth, its plate tectonics.
You see, Earth has a comparatively thin crust, underneath which is a hot and active mantle.
We sometimes see the effects of this mantle flowing underneath us as the plates on the
crust bump, scrape and pull apart from one another, forming volcanoes, earthquakes, mountains
and trenches. Using an exaggerated 3D map of the elevation of Earth, you can see clearly
mountain ranges and deep trenches that have formed thanks to plate tectonics. As a result
of this, you may think that plate tectonics are dangerous to life on Earth, and perhaps
to individuals in the wrong place at the wrong time it could be, but this renewal of the
surface keeps the Earth fertile and fresh. Underneath the mantle is the planet’s core,
consisting of what is thought to be metals iron and nickel. Earth is the densest planet
in the Solar System at 5.5g/cm³, and this is due mainly to the core. It is very big
compared to other planets, the outer core reaching roughly 5,000km in diameter. The
inner core is thought to be solid, but also has extreme pressures of 360 GPa and it reaches
6,000°c in temperature. The core is also where the magnetic field
of Earth originates. As the core moves, it converts kinetic energy into electrical energy,
generating a massive magnetic field around the planet, kind of like the magnetic field
around a giant dipole magnet. This magnetic field also serves life on the ground, as it
diverts solar winds - or highly energised particles from the Sun - around the planet
and also to its poles, causing beautiful aurora. The most common colour for auroras on Earth
is green due to the solar wind’s interaction with atomic oxygen in the atmosphere.
There’s one more very unique aspect about Earth that I want to cover in this video,
and that is its Moon, also known as Luna. Although many other planets have moons, Earth
has the largest moon relative to the size of the planet. The result of this a spectacular
sight in the sky on a clear night, a huge celestial object that is so close we can make
out details on its surface with our naked eye. But having something so massive so close
to us has a very real effect on our planet too. The Moon influences our ocean tides through
its own gravity, which pulls at the water in our oceans, causing a bulge as the planet
rotates. This bulge even exists in the Earth’s crust, although to a less noticeable degree.
However, these tidal forces could be the reason Earth has such active plate tectonics.
Having done all the research for this video, it kind of blows my mind to think of how many
factors had to be right for life to have formed and developed on Earth. But as a result of
all these factors aligning, we are now part of a species with a population approaching
8 billion people, a species with the intelligence to question our own existence and to find
our place in this universe of ours. It also amazes me to see the beauty of this planet.
Thanks to space exploration, we get a view of our home that people would have only dreamed
of only 100 years ago. Will anything else in space ever be as beautiful and welcoming
as this our home planet? I’d be interested to hear your thoughts about this and anything
else you got from this video in the comments! Well that was it! The final video in the “Our
Solar System’s Planets” series. If watching my weekly videos isn't enough
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