A History of Our Knowledge of the Solar System

Astronomy is the study of the planets, stars,
and everything else in space, and is one of the oldest natural sciences in existence.
From ancient civilizations simply looking up at the night sky, to modern astronomers
using a telescope capable of seeing 15 billion lightyears away, astronomy has always been
an important (and fascinating) part of human history. In its earlier form, astronomy was closely
tied to mythology. Before there were any scientific explanations for the mysterious moving objects
in the sky, it was attributed to the divine. But ancient astronomers paved the way for
future generations, who were able to build upon the collective knowledge of their ancestors. The word “planet” comes from the ancient
Greeks. They used the term “wandering star” to refer to anything in the sky that moved
relative to the seemingly ‘fixed’ stars. As well as the 5 planets visible to the naked
eye, this definition also included both the Sun and the Moon. It also excluded Earth itself,
which was thought by most to be the centre of the universe… up until as recently as
500 years ago. Of course, aside from the Greeks, other prominent
civilizations were responsible for their own innovations and discoveries; the Babylonians
were perhaps the first to track the movement of planets, while Chinese observations of
Jupiter directly formed the structure of the Chinese calendar, among other things. One of the most fundamental questions faced
by ancient astronomers was determining the shape of our planet. Now you may have heard
somewhere that Christopher Columbus discovered that the Earth was round. Thankfully, these
days, most people know that this is a myth. Long before the time of Columbus, all educated
people knew that the Earth was round… I guess some things never change… In fact, the earliest known suggestion of
the Earth being round was from more than two thousand years ago, by the Greek philosopher
and mathematician, Pythagoras… y’know, the triangles guy. He didn’t actually have
any evidence or logical reasons, though, he just thought that a sphere was the “most
perfect” shape and therefore that’s what the Earth is most likely to be. The first to offer any kind of evidence of
a spherical Earth was Aristotle in 350 BC. He noticed that during a lunar eclipse, when
the Earth casts a shadow on the Moon, the shadow is curved. As well as this, he noticed
that the stars appear differently depending on the observer’s location on Earth, writing
that some stars that are visible from Egypt are not visible from Europe. Another Greek by the name of Eratosthenes,
went one step further, and actually managed to measure the circumference of the Earth,
with just some simple geometry. He had heard of a place called Syene, where the Sun shone
directly down a well at noon on the summer solstice, casting no shadow whatsoever. This
is due to its location almost exactly on the Tropic of Cancer, so the Sun would be perfectly
overhead. Eratosthenes measured the angle of a shadow in Alexandria at about 7 degrees,
meaning that the distance between the two cities represented about 1/50 the circumference
of the Earth. The distance was known to be 5,000 stadia, so 250,000 for the whole circumference.
Since the experiment was done using an ancient unit of measurement no longer in use, the
‘stadion’, which could be converted to modern units in several different ways, the
exact result of his experiment is unknown. But it was certainly impressive for 2000 years
ago. Some other ancient discoveries included: calculating
the distance to the Moon using a technique called “parallax”, which is still used
even to this day. It was also worked out that the reason the Moon and other planets shine
is from reflected sunlight. Most of the work done in ancient times has now been lost, but
much of it is known to us today because of a man called Claudius Ptolemy. Some time around AD 150, he completed the
“Almagest”. This was an extended documentation of the collective astronomical knowledge from
the prior few centuries. It was considered one of the most important scientific works
of all time. Despite how influential it was, unfortunately it did firmly cement the ‘geocentric’
model of the universe among the scientific community. This model puts the Earth as a
stationary object at the centre, with the Sun and other planets revolving around it.
He even came up with his own model, which involved “epicycles”. This helped explain
some anomalies in the planets’ movement that weren’t consistent with the current
model. Skipping forward a little to the Middle Ages,
which was actually a comparatively quiet time for European astronomy… too busy killing
each other, probably. Conversely though, this time was considered a golden age for Indian
astronomy. One of the most important figures was Aryabhata,
circa 500, who was way ahead of his time. He suggested that the Earth rotates around
its axis every day, asserting that it was the Earth that was moving, not the stars…
but this idea didn’t catch on at this time. He also managed to determine the length of
a sidereal year, accurate to 3 minutes from modern calculations. Another Indian, was Bhāskara II, from the
12th century. His extensive work on both mathematics and astronomy was divided into 4 parts, with
several chapters outlining a multitude of data on the Sun, Moon, planets, and stars. Indian astronomy was a big influence to the
Islamic World, who experienced a golden age around this time. Muslim scholars were possibly
the first to really question the Ptolemaic model, and even geocentrism in general. One of the most important suggestions, by
a Persian astronomer, was that the planets and stars are subject to the same laws of
physics that we are here on Earth. This may not seem like a big deal, but at the time,
thinking that things in the sky could follow the same rules as things on Earth was a very
different approach – especially when there were centuries of religious ideas caught up
in the way people understood the so-called “heavenly bodies”, as they were still called
in academic circles. After the Middle Ages, massive strides of
progress were made, in what became known as the Copernican Revolution, started by the
works of Polish astronomer, Nicolaus Coperincus. This began with the assertion that, contrary
to popular belief, the Earth was not the centre of the universe. Copernicus was by no means
the first to suggest that it wasn’t, in fact it was suggested way back in 280 BC by
Greek astronomer Aristarchus of Samos. However, he was the driving force that eventually led
to the incredibly important paradigm shift in the scientific community, from a geocentric
model, to a heliocentric model – that is, with the Sun in the middle. Shortly after Copernicus came a huge turning
point in astronomy – the invention of the telescope. Up until this point, all observations
were limited to just what could be seen with the naked eye. Liking what he saw, Italian
astronomer Galileo wanted to get in on the action, and made his own in the following
year. In 1610, he discovered 4 moons of Jupiter,
which today are known as the “Galilean moons”: Io, Europa, Ganymede, and Callisto. He originally
described them as “stars wandering about Jupiter”. This was the first time that objects
were discovered that could definitively be shown to not revolve around the Earth, giving
further credibility to the heliocentric model. One of Galileo’s lesser-known contributions
to astronomy, was observing the phases of Venus. The planet has phases, similar to that
of the Moon, based on the relative positions of the Sun, Earth, and Venus. When the Sun
is directly between the two planets, Venus is fully illuminated from Earth’s perspective.
Under the geocentric model, there is no scenario in which the Sun could possibly be between
Earth and Venus. In the age of the telescope, the geocentric model quickly began to fall
apart. Unfortunately for Galileo, there were some
who clutched onto the geocentric model, despite the mounting evidence against it: the Catholic
Church. Galileo’s claims were said to contradict the bible, and he was tried for heresy. He
was forced to recant his statements under threat of torture, and was placed under house
arrest until his death in 1642. Another influential astronomer of this time,
was Johannes Kepler. He greatly improved our understanding of the Solar System with his
3 laws of planetary motion. The 1st law basically states that planets
don’t revolve in perfect circles, but rather, ellipses. And the Sun is not directly in the
middle, but at one of two focal points. The 2nd law states that sections of this ellipse
with equal areas will take equal time for the planet to move across. So a planet does
not always orbit with a constant velocity. The 3rd law shows that a planet’s distance
from the Sun is proportional to its orbital period, with a simple formula of P square
=A cubed. This scientific revolution is often said to
have come to end with none other than Sir Isaac Newton and his law of universal gravitation.
This showed that the reason the planets orbit the Sun is the same reason why an apple falls
from a tree: gravity. It’s the same force of nature, both here on Earth, and out in
space. Even after the invention of the telescope,
it would still take more than 170 years for the first ever ‘new’ planet to be found.
The discovery came from British astronomer William Hershel, who actually thought he had
discovered a comet. He originally wanted to name it Georgium Sidus, after King George
III, but this wasn’t too popular outside of Great Britain. Some external name suggestions
were “Hershel”, “Neptune”, and the name we now know it as today: Uranus, the
latinised version of the Greek deity, “Ouranus”. The man who suggested this was a German astronomer
called Johann Elert Bode. He was best known for the Titius-Bode Law, a now-disproved hypothesis
which predicted planetary distances. With the exception of the large gap between Mars
and Jupiter, the law’s predicted distances lined up pretty much with all the planets,
including the newly discovered one. So this led to a thorough search between Mars and
Jupiter, as several astronomers attempted to find the supposedly “missing” planet. In 1801 it was thought that this task had
been achieved. At the exact distance as Bode’s law suggested, the missing planet was found…
Ceres. The following year yet another new planet was found in the same vicinity, Pallas.
Shortly after, a third, Juno, and shortly after that, a fourth, Vesta. Astronomers became
uneasy about just naming more and more new planets. However, for several decades there
were no new discoveries made, and for nearly half a century, astronomy textbooks listed
the 11 planets. A new celestial classification had been suggested
by William Hershel in 1802: “asteroid”, Greek for “star-like”. This suggestion
was almost universally adopted in 1845, when a 5th object was found, and all discovered
objects between Mars and Jupiter were re-classified. These 5 would be the first asteroids in what
would shortly thereafter be called the Asteroid Belt, which today is known to have possibly
millions of asteroids. Just 1 year later, and an actual new planet
was discovered, Neptune. It was discovered by German astronomer Johann Gottfried Galle,
but its position had been hypothesised in the years previously by two other astronomers,
due to irregularities in the orbit of Uranus. It was this discovery that was the nail in
the coffin for the Titius-Bode Law, as Neptune was nowhere near where it should have been.
The discovery of Pluto 84 years later would further confirm this, and ironically led to
a similar dispute within the scientific community – but more on that later. The 20th century was an exciting time for
astronomy due to the rapid technological advancements that were made. In the aftermath of WWII,
the United States and the Soviet Union got ahold of German V-2 rocket technology, which
the US used to take the first ever photograph of the Earth from space in 1946. As the Cold War tensions between the two superpowers
grew, an intense cosmic rivalry emerged, known as the “Space Race”. This competition
for space dominance began in 1957 when the Soviets launched Sputnik I into orbit, becoming
the first man-made satellite in space. The USSR had several more iconic firsts during
the 50s and 60s: the first animal in space, the first man in space, the first woman in
space. Despite all the firsts that were achieved
by the Soviets, the US ultimately managed one of the most iconic moments in not only
astronomy, but in human history… [“It’s one small step for man, one giant
leap for mankind”] Neil Armstong became the first human to set
foot on the Moon. This moment in 1969 was the de facto end of the Space Race, not because
this was an achievement that was insurmountable, it’s just that the Soviet economy began
to decline in the 1970s, leaving them unable to compete. Despite a slower start, it was
clear that the US had now taken over technologically. From 1972 onward, the superpowers shifted
from competition to cooperation, beginning with the Apollo-Soyuz Test Project. Since the Space Race, technology has continued
to improve, allowing us to see into the furthest depths of the Solar System, and far beyond.
There have been several major milestones in the last few decades, such as the Hubble Telescope,
the International Space Station, and the Mars Rovers, just to name a few. In 2006, for the first time, the word “planet”
was given an official definition; with 3 criteria from the IAU. The new definition excluded
Pluto, which has since been referred to as a “dwarf planet”. This was done because
astronomers had been finding lots of trans-Neptunian objects similar to Pluto, creating the same
kind of difficulties that scientists had faced over a hundred years before. These objects,
along with Pluto, and several thousand more discovered since, are collectively referred
to as the Kuiper Belt. Of course, the Solar System doesn’t end
here. In fact, anything that orbits the Sun can be said to be part of the Solar System,
and its gravitational field could potentially reach 2 lightyears away! The theorized Oort
Cloud is a spherical cloud made up of billions, or maybe trillions of small icy objects and
is believed to be the limit of the Solar System. Estimates put its expanse up to 100,000 astronomical
units. This is nearly half-way to the nearest star, Proxima Centauri. There is so much to
the Solar System that we have still yet to explore. Even returning closer to Earth, we still don’t
know everything. Today we know of the 8 planets, but many of us grew up learning about 9. In
recent years there have been several astronomers searching for another 9th planet, which may
well be discovered some day. Or perhaps something much bigger, such as finding signs of extraterrestrial
life; with recent discoveries of water on other planets, it might not be all that far-fetched. As we continue to explore outwards, our knowledge
of the Solar System will improve and evolve, hopefully breaking new boundaries and hitting
new milestones. As time goes on, our understanding of the Solar System will be shaped by discoveries
and innovations of the future, for many years to come. Space exploration has impacted our lives here
on Earth in ways that you might not be aware of. Many technologies we take for granted
these days, like the GPS in our phones, was originally a NASA technology. Looking at just
how important NASA has been in shaping the world we live in, is a great documentary I
watched called “A World With NASA”, which is available on CuriosityStream. The documentary
shows how much further behind we would be, technologically, if it hadn’t been for NASA. You can start watching thousands of educational
documentaries now, by signing up to CuriosityStream, which costs just 2.99 a month. And my viewers
you can even get a 30-day free trial free at CuriousityStream.com/wonderwhy. And right
now is a great time to sign up, because with your subscription, you’ll also get access
to Nebula – a streaming service for independent creators, featuring some channels you’ve
probably heard of, like GCP Grey, Kurzgesagt, and Wendover Productions. Nebula has a lot
of original, exclusive content, such as the Logistics of D-Day by Real Engineering. Again, that’s curiositystream.com/wonderwhy
for thousands of hours of educational content. Thank you so much for watching, and I’ll
see you next time.


  1. Asad Jogiat

    March 5, 2020 at 1:48 am

    Idk if anyone’s said this but ur voice is perfect for those sleep/calm videos

  2. Leo Staley

    March 5, 2020 at 2:21 am

    Ancient chinese did not know the earth was a sphere. the jesuit priests in the age of exploration revealed it to them.

  3. Leo Staley

    March 5, 2020 at 2:25 am

    It would be better to describe Galileo's vocation as lens maker. he decided to aim his lenses upwards, and boom, revolution.

  4. Leo Staley

    March 5, 2020 at 2:27 am

    Ouranos" is the latinized version of the greek name Ouranos". Uranus is an Anglicization of that. The greek is Οὐρανός

  5. Shalommen Shalom

    March 5, 2020 at 3:23 am

    You forgotten Indians ??? They Setup the 9 planets In every temples & The Draw Mandalas , movement of Earth & other planets around the Sun .

  6. MrFresh3991

    March 5, 2020 at 4:13 am

    Well that ancient sumerian cilinder depicting also 9 spheres of different dimension arround a star makes me always wonder thou……

  7. E Jr

    March 5, 2020 at 9:57 am

    Dear WonderWhy:

    Are there any books on astronomy and the universe you could recommend?
    I only have Carl Sagen's: COSMOS book.

    -Thank you, love you're content. 🙂

  8. PasserMontanus

    March 5, 2020 at 12:27 pm

    Do I hear "Frozen Star" in the background?

  9. gavin kemp

    March 5, 2020 at 7:08 pm

    7:42 you would observe phases of venus if venus was between the earth and the sun which is what was suggested.

  10. Slav Emperor

    March 5, 2020 at 9:58 pm

    Eratosthenes of Cyrene (276 BC – 195/194 BC), best known for being the first person to calculate the circumference of the Earth, which he did by comparing angles of the mid-day Sun at two places, a known North-South distance apart. His calculation was remarkably accurate. He was also the first to calculate the tilt of the Earth's axis, again with remarkable accuracy. Additionally, he may have accurately calculated the distance from the Earth to the Sun and invented the leap day. He created the first global projection of the world, incorporating parallels and meridians based on the available geographic knowledge of his era.

  11. Dark sky's

    March 5, 2020 at 11:03 pm

    kerbal space program music lol

  12. Fredoric1001

    March 5, 2020 at 11:08 pm

    isn't this the "space song" from Kerbal Space Program ?

  13. fantasyexctasy

    March 6, 2020 at 12:14 pm

    Those guys "looking" through a telescope in the first minute look like they've never used a telescope before

  14. Rohan Sharma

    March 6, 2020 at 3:40 pm

    Any Astronomy pros that can help me out with a conundrum? Why are we able to see Venus at night?

    It is the brightest star in the night sky, yet the side of earth that is at nighttime is facing away from the sun, and since both Mercury and Venus orbit closer to the Sun then Earth, they should always be facing the "day side" of Earth and not be visible at night. So why do we still see both planets? But specifically Venus considering its brightness and unique place in cultural contexts.

    I am even more confused now after this factoid @ 7:37 in the video, as it says that Venus is fully illuminated when the sun is between the earth and venus, and this irregularity was part of why they thought the geocentric model didn't work. My understanding of why we can see Venus at night is that light bends around the earth to our eyes, which is why we see it at night. How can it bend that much, and why does it allow us to see Venus but not the Sun at night? And in the diagram @ 7:37 , could it be theoretically possible that the light or image of Venus bends around the sun to make it visible on earth.

    Any help or clarification is much appreciated!

  15. NOGI48

    March 6, 2020 at 8:54 pm

    Yet we still don't know anything.

  16. Alex Grey

    March 7, 2020 at 5:19 am

    At 6:57 i kinda doubt this clip is real. Given that Jupiter is always further away from sun than earth, we should not be able to see jupiter's night side. Same with mars and other planets that are further from the sun than earth.

  17. Nata1014

    March 7, 2020 at 5:36 am

    Too many puns have been made about Uranus and it’s kinda getting to big.

  18. Honeybunny /ハニーバニー

    March 7, 2020 at 5:45 am

    If you do space WonderWhy, Shy can’t you do human body next? Like how knowledge of what we thought was inside our body and discovery.

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