Most people take it as a given fact that our
Earth is orbiting the Sun. But if you were sat down and asked exactly
how we know the world is round without the use of highly specialised technology, you
may be at a loss! For instance, you have to be above 40,000
feet to begin to notice the curvature of the Earth, which is probably higher than most
of you have flown. How do we actually know the Earth is round
and that we’re not the centre of the universe? Even today, there are people who passionately
believe in a flat earth model, where the earth is a disk and the Sun simply orbits overhead. Throughout history, the geocentric model was
often thought of as the correct model, where the Earth is the centre of the universe and
everything orbits around it. So how have some civilizations thousands of
years ago come up with a heliocentric model – a round Earth orbiting around the sun? Without space agencies photos, how would you
prove it? Is there a way to experience this first hand
from down here on Earth? So let’s go into a bit more detail about
the heliocentric model. The Earth is round, and has a rough ball shape,
the mass of which causes gravity. The moon is in orbit around it. This system in turn is part of the solar system,
a large number of planetary objects in orbit around the sun. The sun in turn is part of the Milky Way galaxy,
one of billions upon billions in the known universe. But what are the tell-tale signs of our planet’s
place in the universe? In this video I want to explain a few relatively
simple observations we can make from Earth. The first experiment is the size of the sun. If you are to measure the angular diameter
or apparent size of the Sun, you would find it to be roughly 30 arcminutes, or 0.5 degrees
in the sky. It doesn’t matter if it is morning; noon
or evening, the angular diameter of the Sun stays roughly the same. Now this works with the heliocentric model
as the Sun stays roughly the same distance from the Earth, deviating only about 3% throughout
its year. With the flat Earth theory, the distance from
the Sun will vary greatly throughout the day. During midday, the Sun would be closest and
during sunset and sunrise the sun would be furthest away. As we know, the further an object is, the
smaller it appears, for example a straight road going off into the horizon. But in reality, the Sun actually even appears
slightly larger during sunset and sunrise due to the refraction of the earth’s atmosphere,
rather than smaller even though it should be over 2 times further away than at noon. The fact the Sun stays the same size in the
sky testifies that the Sun is a fairly constant distance away from the Earth. The next frame of reference we can use to
show that we are in orbit around the Sun is the planets. We can view the planets’ orbit from Earth,
and depending on their size and position, we can work out our own orbit. For example, at certain points in our year,
Mars appears to goes backwards on itself, it doesn’t follow a straight line in the
sky. This shows it orbits the Sun at a further
point than Earth, and that Earth orbits quicker. If the geocentric model was correct, all the
planets would orbit at a constant speed, not deviating from their course, which does not
happen. Another proof Mars orbits further away than
Earth is that is you were to use a telescope, Mars would always show its day side, no matter
what time of the year it is. Venus on the other hand orbits between us
and the Sun, and because of this it has phases like the moon, and the bright side of Venus
always faces the direction of the Sun. Measuring its position in the sky, we see
that its year is quicker than Earth’s, and we can see that it moves around the Sun because
its angular diameter and its phases change throughout the year. When it’s a full disk shape, it is behind
the Sun, and also smallest in our sky. When it enters a new phase, or is almost completely
unlit, it’s because it is nearest to us and its day side is facing the opposite direction
towards the Sun. Still looking up into the sky, on cloudless
nights we see some reference points that are pretty much stationary, the stars. Or are they? In relation to each other, they don’t really move, only on astronomical timescales.
But if you were to make a time lapse of high exposure photos throughout the night, you
would find the all the stars move in the direction the Earth is rotating on its axis. And how do we know the axis it’s rotating
on? Making a time lapse aimed at the North Star,
or Polaris, reveals the centre point. The same can be done at the south. Depending on the hemisphere you’re standing
on also reveals different stars. Standing anywhere on the Southern Hemisphere
for example means you can never see the North Star, and from where I am on the North Hemisphere
means I can’t see the Southern Cross. The fact only certain stars are visible to
you is proof of a round Earth, and the movement of the stars is proof of its rotation. My final point is the 4 months of day at both
poles. It is well documented that there is midnight
Sun at both the north pole and the south pole. The north pole midnight sun would make sense
in a flat earth sense, but not midnight sun at the south pole. The sun would somehow have to shine all over
Antarctica at the same time, which if it couldn’t do. With the round Earth, Summer in the northern
hemisphere with the axial tilt of the Earth means the north pole is constantly in sunlight,
the same with the south pole during its summer. This theory can be tested simply by going
to the north and south pole during their respective summers and seeing for yourself. Alternatively, you can check on weather websites
to see the sunset and sunrise and when they happen throughout the world. The fact we have seasons if further proof
of the heliocentric model. Every 6 months, the direction of Earth’s
axial tilt is pointing the opposite way, meaning that sometimes the northern hemisphere is
more exposed to the Sun and sometimes the southern hemisphere. Just talking about stars again, these seasons
explain why these stars aren’t on a continuous loop every night. For example, if I was to look at a clear sky
at 8pm at night in February, the constellation Orion would still be below the horizon. Looking at it in August at 8pm, it is now
visible in the sky. Thankfully though, with the huge advances
in technology in today’s world, we can simply observe what the Earth looks like from space
using the ISS livefeed and other satellites and probes. Remarkably, you can even see the ISS as it
orbits over your home, and a few photographers have been able to capture photos of it in
surprising detail. This makes life easier for us, but takes away
some of the profound appreciation you can get discovering these things for yourself. I challenge you to try a few of these things,
go out and look at the planets, look at the stars, and don’t look at the Sun (without
proper protection anyway!) Well thanks for watching! I hope you learned some new things today. Did you enjoy it? If so, give it a like and pass this video
on to any friends who may like it too! If you haven’t already, don’t forget to
subscribe as I have a lot more videos like these and there will be more on the way. And with that, I will see you next time!
