In the middle of the day, the sky starts
to darken. It's as if dusk has fallen
early. People look and notice something
is happening to the sun. A dark shadow
moves before it, [music] gradually
devouring every last trace of brightness
until our familiar lightbringer is only
a shimmering ghostly ring around a pitch
black orb.
A total eclipse is occurring.
This will soon be a reality. One is
coming to the continent of North America
in April 2024.
But if like me, you are one of the many
billions of people who won't get a
chance to see that particular solar
eclipse this April, you might be glad to
know that America is not the only place
they happen. And neither is Earth.
Have you ever wondered what a solar
eclipse looks like on another planet?
Wonder no more.
I'm Alex Mccoan and you're watching
Astramm. And today we're exploring solar
eclipses, but not just the ones that
happen here on this planet. Allow
yourself today to feast your eyes on the
actual images and even videos NASA has
taken of spectacular eclipses from
various places around the solar system.
Firstly, I'll quickly explain some of
the terminology to do with eclipses. A
total solar eclipse is when an object
moves in front of the sun, completely
obscuring it. also known as an
occultation of an object. This is the
typical kind of eclipse we see on Earth
when at certain points in Earth's orbit,
the orbit of the moon aligns with the
sun. There is something special about an
eclipse here on Earth. And I'm not just
saying that out of some Earth focused
pride. By a bizarre but highly fortunate
cosmic coincidence, the moon is the
right size and distance from us that its
angular diameter is almost identical in
size and shape to the angular diameter
of the sun in our sky.
This leaves for an impressive spectacle
where the corona of the sun or in other
words the sun's upper atmosphere creates
a ghostly aura around the moon.
This corona, normally too dim to see,
extends for hundreds of thousands of
kilome into space.
Looking closely around the edge of a
total solar eclipse, and you will also
see the silhouette of the moon's craters
along the outside, plus these reddish
wisps coming off from the sun. These are
prominences. Millions of tons of charged
particles suspended in the sun's
atmosphere by powerful magnetic fields.
During an eclipse here on Earth, the
moon casts a shadow about 250 km in
diameter, which moves across the Earth
as the moon orbits. In the case of the
eclipse happening this April, this
shadow will arrive at the Mexican West
coastline, then will make its way up
through the United States until it
passes into parts of Canada before
moving over the ocean once more. You can
see on this map the path the shadow will
take. If any of you happen to live in
its pathway or are close enough to make
the drive, you may want to try and see
this eclipse for yourself. The sun will
only be totally obscured within the
diameter of the shadow. Outside of that,
the sun is only partially obscured from
[music] the viewer's perspective. This
viewer is witnessing an annular or
partial eclipse, also known as a
transit. The shadow moves across the
Earth extremely fast at roughly 1 km a
second. [music]
Witnessed from a high altitude, it is a
majestic sight as the shadow shifts
across the landscape.
Satellites have also witnessed the
movement of this shadow. The shadow
isn't as sharp as you might expect, and
this is due to the angular diameter of
the sun and the moon and their distance
apart. The sun itself is huge, a
whopping 1.4 million km across. The moon
is much smaller at only 3,400 km across.
Now, this image isn't to scale, but it
shows visually why the shadow isn't
sharp. The umbra is the shadow where the
sun is completely obscured and the pen
umbra is the shadow where the moon only
partially obscures it. This part of the
shadow is much wider than the 250 km
wide umbra shadow.
So why doesn't the moon create eclipses
every month when it orbits in front of
the sun? Well, this is because the
moon's orbit is not in line with the
earth's orbit around the sun. This means
there are only a couple of times per
year when the alignment is right. This
alignment of three celestial objects is
known as a scissy. A very cool word, but
not something you'll need to remember
for this video. I just thought you would
find that interesting. If you live in
the UK like me, sadly you won't get to
see much of the eclipse this April
unless you live at the furthest west
parts of the country as the sun will be
dipping below the horizon just as it
begins. Maybe we will get to see some
devil's horns though. Still a
spectacular sight indeed. But the Earth
is not the only place to experience
solar eclipses and we have the images
and videos to prove it.
Let's explore eclipses of our closest
celestial neighbor, the moon, because it
would make sense that if the moon can
occult the earth, then surely the earth
can occult the moon. And the answer is
yes. But it's not the shadow that's the
really visually appealing part of this
from the earth anyway. This is because
the earth is four times as big in the
moon's sky as the moon is on [music]
earth's. So when the earth fully
obscures the sun, the whole moon is in
the umbra.
At first, the shadow of the earth
creates a cresant shape. Explain that
flatearthers. But what is different this
way around [music] is that unlike the
moon, the earth has an atmosphere. This
means that when the moon is totally
eclipsed, [music] the Earth's atmosphere
refracts the sun's light around the
planet, gently illuminating the moon in
a reddish hue. This makes for a
beautiful but almost spooky view. The
color is caused by really scattering, a
topic I've discussed in another video.
Scattering is the same process that
makes our sky blue and our sunsets red.
This image is beautiful in that you can
see the different wavelengths of light
being scattered through Earth's
atmosphere from deep red from this side
through to blue on this side. From the
moon's perspective, none of the sun
would be visible during a total eclipse,
but the atmosphere on Earth would be
illuminated. So, you would see a ring
around it. This is an actual view of a
lunar eclipse on the moon by one of the
JAXA probes in 2009. [music]
It's quite the awe inspiring sight. The
Earth and Moon aren't just getting in
the way of each other either. Here is
the Earth eclipsing the Apollo 12
spacecraft in 1969 while it was on its
way back home. And here's the moon
getting in the way of the Earth from the
perspective of the Discover satellite.
Interestingly, this is the side of the
moon you never see as the moon is tidily
locked to the Earth, [music] which means
the same face is always looking towards
Earth. From this perspective, the moon
looks very foreign, but it is indeed a
real video of our only natural satellite
transiting the Earth.
From another satellite's perspective,
but this time looking at the sun with
the SDO satellite, the moon often makes
an appearance. The position of this
satellite as it orbits the Earth means
the moon can block the sun occasionally.
And here's the moon again. This time
from the perspective of one of the
stereo satellites.
The moon isn't the only thing that
orbits between us and the sun. Mercury
often transits across the sun. A tiny
minnow compared to the solar systems
giant. The next time this will happen is
on the 13th of November, 2032. So, a
little while away. You might want to put
it in your calendars for now.
Venus also orbits between us and the
sun. And as it is much closer to us and
bigger than Mercury, its silhouette
appears much larger. Its last transit
was in 2004 and 2012. But sadly, if you
miss those two, chances are that you
will never see it. These transits happen
in pairs. And then there is a 100year
gap until the next one. In other words,
the next transit [music] will be in
2117.
This happens for the same reason the
moon doesn't eclipse us every month. The
orbits just don't often align. Still, we
have highdefinition videos of the last
one, and it is quite the sight to
behold.
Moving to another planet now, we can go
to Mars, which has plenty of unmanned
robotics either in orbit or on the
surface.
Mars also has two moons, Phobos and
Deamos. They are both pretty small.
Phobos is 22 km across and Damos is only
13 km across. They both orbit very close
to the planet though. Phobos is only
9,000 km above the surface and Damos is
23,000 km, which means although tiny,
you can still easily see them from the
surface of Mars, especially Phobos.
The Curiosity rover was able to capture
a moment where incredibly Phobos
eclipsed Damos. This video is captured
in real time and shows the size
differences of the moons in the Martian
sky. And this is not all the Curiosity
rover captured. It was also able to see
a transit of Phobos in front of the sun.
Due to the distance of Phobos to Mars,
it moves across the sky fairly quickly,
only taking about 7 hours to orbit once.
This means that this video you are
watching is in real time, and these
solar eclipses on Mars don't last for
more than about 30 seconds.
The surrounding ground does get
noticeably darker during an eclipse by
Phobos, as can be seen from the rover's
perspective, but it can also be seen
from space. Forbus' shadow here can be
seen by the Viking 1 orbiter and also
here more recently by the Mars Global
Surveyor. The Opportunity and Spirit
rovers have also seen a transit of
Damos, but it appears much smaller, just
a dot passing in front of the sun. It
doesn't cause a noticeable decrease in
brightness.
Mars is pretty impressive, but that's
not all the solar system has to offer.
Have a look at this video captured by
the Hubble Space Telescope looking at
Jupiter.
Jupiter has four large moons, three of
which at certain points can transit the
planet at the same time, leaving three
big shadows. The moons in question in
this video are Io, Kalisto, and Europa.
Interestingly, like we talked about
before with the Umbra and Penumbra, you
can see that because Io is the closest
to the planet, its shadow is the
sharpest. Whereas you can see with
Kalisto, the furthest away of these
three moons, the penumbra is much
larger, causing a blurry shadow.
And in this video, Hubble spies the
occultation of Ganymede, the largest
moon of Jupiter.
Cassini saw some incredible transits and
occultations of Saturn's many moons.
Here is one of Epimetheus passing in
front of Titan with Dioni coming in from
the side. This little white dot coming
in from the left just under the ring is
in fact a bright background star.
And this Hubble view is magnificent.
Here are Enceladus, Mimis, Dioni, and
Tetis orbiting Saturn.
Once every 15 years, Saturn's rings and
moons are aligned just right so that the
moon shadows stream across the rings as
well as on the planet. This video is a
time lapse that lasted 9 and 1/2 hours.
Amazing.
Going a bit further out, we come to
Neptune and its biggest moon, Triton.
Sadly, we don't have a video, but this
image captured by Voyager 2 is gorgeous.
3 days after passing by Neptune, Voyager
was able to capture the presence of
Neptune and Triton before Neptune
slipped in front of Voyager's view of
Triton.
Going further out again, we come to the
furthest celestial object explored in
the solar system, Pluto.
As New Horizons whizzed by Pluto in
2015, it turned its camera back towards
Pluto to capture the dwarf planet
totally eclipsing the sun. What it saw
was dazzling. The sunlight streaming
through and illuminating the atmosphere
and its haze layers with the ridges and
mountains on Pluto's surface highlighted
by the stark contrast of Pluto's night
side.
Occultations and transits may be
breathtakingly beautiful, but are they
actually useful to us scientifically?
Well, did you know that Uranus was
discovered to have rings because of an
occultation of a background star? As the
planet passed in front of the star from
our perspective, the star dimmed before
and after the planet obscured it. With
this information, we are able to count
how many rings Uranus has. On top of
that, the transits of exoplanets in
front of their stars are actually how we
can detect exoplanets. Telescopes like
Kepler and TESS measure the brightness
of stars in the sky. If a star dims, it
could be because one of its planets just
passed in front of it. If the star
continually dims in a pattern, for
instance, once every 100 days, then we
know that a planet orbits that star and
takes 100 days to do so. Using this
method, space agencies like NASA have
discovered over 3,000 exoplanets, more
than all other methods of exoplanet
detection combined. It can even help
scientists calculate the size of the
planet by measuring how much the star
dimmed or the composition of that
planet's atmosphere by looking at the
spectra of the light that passes through
from the star to us.
It might be some time before we see a
solar eclipse again. After the one in
April, the next eclipse [music] won't
happen in America until 2044.
Here in the UK, the next one won't occur
until 2090.
If I live to see it, I will be a very
old man. But I'm amazed as I see all of
the images of eclipses that take place
throughout the solar system. There's a
special beauty to each one of them. A
fleeting moment where one celestial body
brushes lightly against another, even if
it's only through their shadows. rather
than harbingers of doom. These moments
fill me with awe and remind me how
connected the universe is. Even across
thousands, millions, or even billions of
kilometers of space, we can notice a
planet's passing. And if you're in the
right place at the right time, oh, what
wonders you can see.
You may have heard many news stories
about all the thousands of exoplanets
that have been discovered using the
Kepler telescope.
As of the 1st of July 2018, Kepler has
confirmed the existence of 3,797
planets in 2,841
star systems with 632 systems having
more than one planet. But are any of
these planets habitable? What are the
chances of life being found outside of
Earth? Are we alone in this universe? Or
can life be more prevalent than we
think? And if there is life, where can
it be found? I'm Alex Mccoan and you're
watching Astramm. And together we will
explore known exoplanets in the Milky
Way galaxy to see if any of them have
the potential to harbor life.
Now, we've not been able to actually
image exoplanets in any kind of detail.
In fact, this is the clearest real image
we have of an exoplanet taken by ESO's
very large telescope. Which may make you
question, if this is the best image of
an exoplanet we have, how can we
discover exoplanets? And how do we know
life could be on one? To answer the
first question, we have to look at how
Kepler worked. Kepler is a space probe
which constantly monitored about 150,000
stars in a fixed field of view using its
camera. The field of view focuses on a
patch of sky near the constellation
signis. This is what Kepler can see. The
data it collects is sent to Earth and
analyzed to see if any stars dim
periodically.
You see, the concept is if a star's
planet passes in front of Kepler's view,
the star will dim. If it dims, for
instance, once every 100 days, we can
confirm that it is a planet and it takes
100 days to orbit.
Kepler is really good at finding
exoplanets.
Before Kepler came into operation, these
were the exoplanets we knew about. As
you can see, most of them are many times
the size of Jupiter.
Since Kepler came into operation, we
have discovered and confirmed the
existence of thousands of exoplanets
with thousands more still unconfirmed.
Remember, these are planets which have
been discovered in only this patch of
sky. There is still a lot more out
there.
Kepler unfortunately is no longer
functioning how it used to as some of
the reaction wheels inside it are
broken. However, the good news is that
there is a new exoplanet finding
spacecraft called TESS, which just came
into operation a couple of weeks ago,
which will cover an area in the sky 400
times larger than the Kepler mission. It
is expected that during its mission, it
will be able to find more than 20,000
exoplanets.
In order to determine details about an
exoplanet, the distance from us to its
star needs to be worked out using
complicated maths.
For stars within 400 light years away
from us, we can use trigonometry and the
orbit of the Earth to create a
difference in angles.
Beyond that, there is no direct
measurement. So, the best current method
is by comparing a stars color spectrum
to its brightness. The color spectrum of
a star corresponds to what type of star
it is. Once the color spectrum is known,
scientists then know how bright the star
should be.
Comparing the apparent brightness or the
apparent magnitude to the actual
brightness, the absolute magnitude of
the star reveals how far away it is.
This method is proven as scientists have
done this test on stars that are within
400 lightyears and the results produce
similar distances in both tests. Once
the distance to the star has been
determined, the amount the star dims can
be used to see how big the planet is,
its distance from the star, and [music]
the exoplanet's mass based on the orbit
of the exoplanet. Using other telescopes
like Hubble, ESO and eventually the
James Web and the W first telescopes.
These exoplanets can be studied to find
out their composition particularly of
their atmospheres. The way this is done
is again from the spectra of the
exoplanet's light. To give you an
example of how this is done, imagine
white light shooting through a prism,
producing what is actually a blend of
colors [music] spanning from violet to
red. Light from a star shooting through
an atmosphere produces a similar effect,
except certain bands of light are not
present. This indicates there is a
certain gas in the atmosphere that is
absorbing the light in that wavelength,
not allowing it to pass through. The
dips in this image shows what Earth's
spectrum looks like as sunlight passes
through the atmosphere.
The dips show that oxygen is present as
well as water vapor, carbon dioxide, and
methane. These gases all absorb the
sun's light at these wavelengths.
Looking at a section of wavelengths and
comparing them with other planets in the
solar system, sulfur compounds can
clearly be seen on Venus and methane on
Neptune is apparent.
This means that as we study exoplanets
in detail and determine their spectra,
we can search for atmospheres that
resemble our own. If it does, then the
chances are that it could be a habitable
world and also that it may already
harbor life. Inhabited planets could
have telltale signs of life like smog
and pollution which would be seen in the
planet spectrum.
So, have any exoplanets like these been
found? Well, out of the thousands of
exoplanets that have been discovered, 16
of them are thought to be rocky planets
and sit in the Goldilock zone or the
habitable zone of their respective
stars.
Let's just remind ourselves of the
ingredients needed for life as we know
it.
We believe that liquid water needs to be
present. Liquid water is essential
because biochemical reactions can take
place in water.
Water is also an excellent solvent that
easily dissolves and carries nutrients
and other compounds in and out of cells.
Life forms on Earth are made primarily
of water. In fact, our human bodies are
more than 60% water.
Life also needs sufficient protection
from cosmic and solar radiation which
can break down and damage cells.
On Earth, this protection comes from our
magnetic field.
There also needs to be essential
chemicals found in the ground on Earth
and an energy source which for us is the
sun.
So the Goldilock zone is where assuming
other conditions are right liquid water
could theoretically pool on the surface.
This zone is always different depending
on the parent star and how big and hot
it is.
Looking at our own solar system, Venus
might just be in the Goldilock zone as
well as Earth and Mars.
We already know that only one in three
planets in our own solar systems
Goldilock zone can have liquid water.
So, just being in the right place is not
always enough.
The type of star is also important. Our
star, although seemingly active on the
surface, is actually quite stable
compared to a lot of other types of
stars. The sun will likely be 10 billion
years old before it burns out.
On the other hand, the hottest types of
stars will only last for millions of
years in comparison. It is thought that
this is not enough time for life to form
around it. Certainly not animal life
that can communicate as we understand
it.
So, we have a lot of filters we can now
use to narrow down our search for
habitable worlds. Out of the original
3,797
confirmed planets, 16 are terrestrial
planets that orbit within the Goldilock
zone of their stars. However, the
habitable zone of some stars means the
planets are close enough to be tidily
locked, meaning only one face of the
planet sees its star.
This proximity to the star also means
the planet [music] is exposed to a lot
of solar radiation, sometimes thousands
of times more than we are exposed to on
Earth. In other words, out of those 16,
only four are likely candidates to be
Earthlike. Although, it is worth
mentioning that exoplanets in systems
like the Trappist one system could still
be habitable or have life even if all
the planets are tightly locked. But the
Trappist system is worthy of its own
video, so I won't expand on it here.
These four exoplanets are Lighton B,
Kepler 62F, Kepler 186F, and Kepler
442B.
Realistically speaking though, we don't
know much about them other than their
size and mass. Our current technology
isn't accurate enough to determine the
composition of their atmospheres.
One of the James Webb Space Telescope's
missions is to study known exoplanets in
greater detail, which is exciting as in
combination with the finding power of
the test telescope I mentioned earlier,
this next decade could be full of
discoveries.
But by using the Kepler data as a fairly
decent sample size, can we estimate how
many habitable worlds there could be in
our galaxy alone?
Ethan Seagull did a very interesting
article, which I will link to in the
description, where out of those 150,000
stars, the chances of seeing anything at
all using the transit method is very
small. Less than 1% for a planet
orbiting the distance of Mercury, let
alone Neptune, which has a 0.01%
chance of detection. This is due to the
way planets tend to all orbit along a
plane. And if it's not lined up, we
won't see anything.
Also, smaller planets are much harder to
detect. Look how hard it is to see an
Earth-sized planet compared to a
Jupiterized planet.
Taking all these things into account,
Ethan Seagull placed a low estimate of
6.4 billion planets in their stars
habitable zone in our galaxy, the Milky
Way.
This means at least one of them has to
have life too surely. Well, we really
don't know how prevalent life is at all.
And until we do some serious research
and improve our technology, we have no
way of knowing yet.
Some people claim that all factors
considered, the chances of humans being
able to develop and live on Earth was 10
to the power of 10,123
to 1. In other words, extremely
unlikely. How many factors actually are
needed to be right for life to exist? Is
it all over the place or purely a freak
coincidence here? Building on what we
know already means the future is going
to be very exciting.
For almost all of human history, the
night sky has guided us across
continents, illuminating our path
forward. The stars are as entwined with
our civilization as are the land and the
sea. But that sky is changing. While the
stars, nebula, and supernova remnants
are as brilliant as ever, few of us will
ever get the chance to see them as they
once were, even though all it would take
to do so is a flick of a switch.
I'm Alex Mccoan and you're watching
Astramm. Join me today as we explore how
the night sky has changed in our own
lifetimes and uncover the drastic
effects that artificial light, both
visible and invisible, has had on us on
crucial ecosystems and on important
scientific research. We will see how a
well-intentioned green revolution made
it worse. How we've put artificial
constellations of our own into orbit.
And how even the natural constellations
themselves are changing. That's right.
We're going to see how our sky will
change in 1,000, 1 million, [music] and
even billions of years into the future.
As it is with anything that changes
gradually over time, we rarely realize
what is happening. But every once in a
while, that gradual change can be laid
bare, as it was during the 1994
earthquake in Los Angeles. A large part
of the city's electrical grid was
knocked out in the early hours of
January 17th, revealing the effects of
decades of increased light pollution in
an [music] instant.
So shocked were the residents by what
they saw as they evacuated their homes,
that some looked up in awe of what had
always been there, hidden by the city
lights. One even phoned the observatory
to ask what the giant silvery cloud was
that they were seeing. Sadly, even the
sky when you first looked the stars as a
child is not how it is now. From 2011 to
2022, the average light pollution
increased by 10% a year, meaning it has
more than doubled over that time.
[music] So, let's travel to a dark sky
location on a clear night, far away from
the sky glow of towns and cities, and
look up to understand what we've lost
before we learn how we might get it
back.
There is a useful measure used to rate
the quality of the night sky by eye
called the bortal scale which ranges
from 1 to 9. In a city under the
brightest of skies, you may only be able
to count 100 stars. This is a nine on
the bottle scale. But as you travel away
from the lights, the sky reveals more
until it becomes nearly unrecognizable.
As you drive out to the surrounding
urban area, the night sky darkens to
Bortal 7. Here, on a clear night, you
might get to see a hint of the Orion
Nebula, visible only as a diffused
looking star. Clouds under these skies
will be very well lit. Once you're out
past the city limits into [music] the
rural countryside, you're now under
Bortal 3 skies, and much more is
revealed. Messia objects 15, 4, 5, and
22, all star clusters, [music] are now
naked eye objects. Andromeda is clearly
visible. And if you look carefully,
you'll glimpse the Triangulum Galaxy.
These two galaxies at 2.5 to 2.7 million
lightyears away are the furthest objects
we can see with the naked eye. And what
about that giant silvery cloud they saw
in Los Angeles? That's our Milky Way, a
diffuse band that [music] arcs across
the sky which contains over 100 billion
stars.
Finally, after a few hours of driving,
you're now under Bortal one skies. A
true dark sky site with a pristine view.
When we set out from the city, you could
only see a 100 stars. Here you will see
thousands. It may even take you some
time to notice the constellations that
you thought you knew, lost as they are
in the awesome detail of this dark sky.
If you're lucky enough to live in the
southern hemisphere, you'll be looking
towards the galactic center. And you may
even notice that it casts shadows onto
the ground. My friends, in the northern
hemisphere, no need to be envious. Our
fortunes will be reversed as we orbit to
the opposite side of the galaxy in a
mere 120 million years. Elements of the
beautiful Orion complex are visible in
both hemispheres. A 1,200 light-year
wide star forming region containing the
horse head flame and Orion nebula,
though binoculars are strongly advised.
The closest object that is fully
revealed in such a dark sky produces no
light of its own, interplanetary dust.
This dust is illuminated by the sun and
forms a band known as the zodiacal
light, which spans from the horizon to
horizon, featuring a central focus
called the gagenshine,
appearing directly opposite the sun's
position over Virgo. Sadly, those of us
who live in cities never experience a
genuine night sky at all and are
effectively stuck in an artificial
twilight. This can have unexpected
effects on us as well as the night sky.
So, how do a few light bulbs block out
an entire galaxy? In a word, sky glow.
Any light that is emitted upwards will
pass through the atmosphere where it can
be scattered in any direction. The light
that is reflected back towards the
ground to our eyes or telescopes is seen
as sky glow which has the effect of
drastically reducing the signal to noise
ratio of incoming light from space
affecting our ability to see it and to
study it.
You may have experienced something
similar on a dark, foggy drive. Perhaps
in an attempt to see more of the road,
you've turned on your full beams. The
slight upward angle of these lights
reflects light up, illuminating the fog
in front of you, which has the opposite
effect of what you intended. It bleaches
the field of view and obscures the road
ahead. To make things worse, it's not
just that the sky is now brighter.
[music]
Tragically, our eyes are now also less
able to see in the dark when we go
outside. Human eyes have the incredible
natural ability to detect brightness
across a full nine orders of magnitude.
We are able to do this because we have
two types of vision. The first is known
as phototopic vision and it's [music]
the one you're using now. It's our
daytime color vision recorded by our
eyes cone cells. The second is scotopic
vision. An ultra sensitive monochrome
night vision recorded by our eyes rod
cells. It is this scotopic vision that
allows us to see the faintest details in
the night sky.
Modern lighting is reducing our access
to scotopic vision because to the human
eye all light is not equal. In the past
10 to 15 years, a movement towards more
energyefficient LED lights has shifted
the spectrum of light pollution from the
orange hues of the sodium lamps to the
blue green frequencies of LEDs, which we
are much more sensitive to. These blue
green frequencies slow down dark
adaptation or worse can fully reverse it
in minutes. which is why you'll often
see astronomers and nighttime pilots
using red lights or red goggles to
ensure scotopic vision is maintained. To
access full scotopic vision, we require
a near absence of short frequency light
for at least 30 minutes and it can
continue to improve for up to 2 hours.
So, you may be wondering, if we are so
sensitive to the blue frequencies of
typical LED lights, why can't we use
less of it and reduce overall light
pollution? And you'd be right, we could.
The reality is though that we don't.
LEDs are so efficient compared to sodium
and filament lights that we save energy
and money even when they run much
brighter. So, that's what we often do.
We run them brighter because we all feel
safer in a well-lit area, even if we are
statistically no more likely to be the
victim of crime. Our eyes also contain
retinal cells that are not involved with
seeing and instead help to regulate our
body clock. These cells are particularly
sensitive to shorter wavelengths, too.
So artificial light and LEDs in
particular don't just affect our ability
to see the night sky. They affect us in
other more severe ways, too. The
disruption of our circadian rhythms by
light disrupts the production of several
key hormones, including melatonin.
Incredibly, this can affect our body
through a dramatic cascade that can lead
to metabolic and mood disorders and most
severely cancer.
While for us humans there is at least
some upside in extending the length of
the day artificially, for the rest of
nature there is almost nothing but
downside.
Lights interfere with the circadian
rhythms of wildlife too, disrupting how
and when they sleep, migrate, reproduce,
hunt, and evolve. Almost every species
studied to date is negatively affected
by lighting at the level of both the
individual animals and their
populations. Scientists have studied
fishes, birds, amphibians, insects,
invertebrates, ecosystems hundreds of
meters down below the surface of the
sea, and even trees. Deciduous trees,
which regrow their leaves each year, can
be tricked into maintaining their
foliage all year round, causing stress
that can eventually kill the tree. There
are simple things we can do that manage
that rare combination of saving us money
and improving our health. The best, of
course, is to turn off unwanted lights,
particularly outdoor lighting and
uplighting. When installing new or
replacing new lights, look to install
low power LEDs with low color
temperature of, say, 3,000 Kelvin
instead of 5,000 Kelvin. Welldesigned
lighting will have shielding to focus
the light to where it needs to be and
minimize excess light shone into the
atmosphere.
But away from groundbased artificial
lights, we are now facing a significant
increase of light pollution coming to us
from space.
Since 2018, SP X has launched nearly
7,000 satellites into space. Oh, and one
Tesla Roadster. As it stands, SpaceX
have twice as many active satellites in
low Earth orbit as every other company
and country on the Earth combined.
I've seen some hockey stickshaped graphs
in my time, but this one is
stratospheric.
The vast majority of these new
satellites have gone into low Earth
orbit at an altitude of only 550 km. The
United Nations Office for Outer Space
Affairs have speculated that 100,000
more satellites could be launched in the
next decade.
There are no regulations that limit the
number of satellites that can be
launched and many agreements and
understandings around reducing the
potential for light pollution are
informal. These satellites in low Earth
orbit are particularly bad for light
pollution, contributing to skylow and
producing trails in amateur photography
and research images that can render them
unusable.
Astrophotography can require long
exposures of several minutes and is
likely that these exposures will capture
the trails of satellites in that time.
This is an image of the Horsehead Nebula
and there are three trails in this image
and even I myself have seen it in my own
shot of the Orion Nebula.
We can easily see the effect satellites
have on visual astronomy. We can even
watch them trail across the sky in real
time. But these man-made constellations
that deliver GPS, mobile communications,
and satellite internet all operate in
the radio spectrum, too, spanning
meahertz and gigahertz frequencies,
which can potentially interfere with
scientific observations. Radio signals
easily pass through galaxies, solar
systems, interstellar gas clouds, and
Earth's atmosphere, revealing secrets of
our universe that would otherwise be
hidden. All the more incredible then
that the radio signals from my Wi-Fi
router can barely make it through a few
stud walls. But radioastronomy was key
in establishing the shape and rotation
of the Milky Way, discovering quazars
and pulsars and detecting [music]
organic molecular species like cyanide,
ammonia, water, and amino acids in
interstellar molecular gas clouds. It
was a radio telescope in Ohio that
received the famous WOW signal in 1977.
It was detected at 1,420
MHz, a value believed to be the
frequency of choice that an alien
civilization would attempt to
communicate over. And it is protected
from commercial use to keep it quiet. It
is though still debated whether the WOW
signal was terrestrial interference or
an extraterrestrial phenomenon.
But while we are talking about how
communications interfere with astronomy,
astronomy can also interfere with
communications. In 1978, Arno Penszius
and Robert Wilson were awarded the Nobel
Prize in physics for discovering that
the interference in their antenna wasn't
pigeons. It was the cosmic microwave
background. the cooled remnants of the
universe's first light, the detection of
this low-level hum in their Bell Labs
antenna, and understanding its cause,
all but prove the Big Bang hypothesis.
In another happy accident, the radio
waves emitted by the black hole at the
center of our galaxy, Sagittarius A
star, were detected because of its
interference with trans oceanianic phone
calls.
I love these two stories as it
highlights humanity at its best, at its
most inventive, and at its most curious.
Pausing in the hustle of developing new
technologies to remark, "Huh, that's
odd." Led to two incredible discoveries.
So much has been discovered in the radio
wave spectrum, yet there are deeper
mysteries that cannot go unsolved.
That's why the square kilometer array
observatory is being built in radio
quiet zones. The most ambitious radio
observatory project of all time. I don't
say that lightly. It is. Its stated aims
are to probe the cosmic dawn and the
emergence of the first stars and
galaxies to challenge Einstein's general
theory by indirectly observing
gravitational waves. to seek the origin
of life, to look for aliens, and to
understand how our galaxy and universe
are changing due to pressures like
entropy, gravity, and dark energy. But
what does that change look like to us
from Earth?
If we look up to the stars through a
polluted sky, space appears stagnant and
it can be easier to believe that we are
instead on a Truman Show like film set
with fixed stage lights for stars than
on a rock in motion through our galaxy.
But under a dark sky, nothing feels
still as we get a sense of our voyage
through space and time. Earth is
orbiting the black hole at the center of
our galaxy at 828,000 [music]
kmh.
We are heading away from the galactic
plane and hurtling in towards the
center. Our perspective and every object
we can see is moving. In this video by
issa from data captured by the Gaia
telescope, we can see how our night sky
really is changing. In 1,000 years time,
only the keenest astronomers would
notice a difference from today's
constellations, though we may be lucky
enough to witness Beetlejuice going
supernova. But in around 10,000 years,
we will have a new north star. Polaris
will have moved away from the celestial
pole and it will have been replaced by
the brighter Vega.
And let's take a closer look at the
constellation of Orion and the stars
around it. They will change dramatically
over the next 450,000 years. Orion's
sword will shatter and his bow will
disintegrate.
Zooming out to the entire night sky,
let's watch our galaxy evolve over 4.5
million years. Orion is in the bottom
right.
What we can't know with this data,
though, are how many more stars will go
supernova and how many more will be
born. We'll just have to wait and see.
And that's going to be a heck of a lot
easier if you would just turn off those
up lights.
But over the next few billion years, the
sky will begin to look so unrecognizable
that you would be forgiven for thinking
we were in a different galaxy altogether
because we would be. Andromeda currently
takes up 3° of our sky, about six moon
widths. It is accelerating towards us
and is expected to reach us in 5 billion
years by which point it will be the sky.
Galaxies are so diffuse that Andromeda
and the Milky Way would pass through
each other almost without impact. But
it's what we can't see that is all
around us that will seal our fate.
Gravity. Over the next 5 to 10 billion
years, our two galaxies will yo-yo and
rip each other apart with a huge burst
of star formation from the mixing gases,
turning two spiral galaxies into one
elliptical blob. Unfortunately, you
would be unlikely to see this from Earth
because of the extreme natural light
pollution caused by the sky literally
being on fire. Our son will have become
a red giant.
In 1 trillion years, the last star will
be born. There is nothing but decay left
for the universe. The galaxies we can
see outside of our extended galaxy will
become faint and dim as they move beyond
our observable universe. In 120 trillion
years, all stars will be extinguished
and the night sky will fade to black.
As is so often the case in our complex
world, there are unexpected outcomes
from our behaviors. Our night sky is
changing in many ways. It has been
obscured, but it is not lost. Towns,
cities, and governments all over the
world are beginning to understand the
issues and are making changes in
ordinances and in law to protect our
skies and the wildlife underneath it.
So, if this video has inspired you to
visit a dark sky or reduce your impact,
head to darks sky.org to find out more
and to lightmap.info
if you're curious about the light
pollution in your area.
on this world of ours. We take a lot of
celestial things for granted.
For example, the fact that when there
are no clouds in the sky, sky is blue.
We have the moon
and the sun, which are both about the
size of a pee when held at arms length.
These things are all very predictable.
But what if you were standing on a
different planet or even a different
moon? How would the sky appear then?
Let's start at the beginning. This is
Mercury, the closest planet to the sun.
With no atmosphere and no moon, at
night, you'd see the whole Milky Way in
pristine detail. Venus, the Earth, and
even our moon would also be very bright
in the sky. During the day, on the other
hand, the sun is the centerpiece of the
sky. It can sometimes appear over 3.2
times bigger in the sky than on Earth
and over 10 times brighter. Mercury is
in fact very dark gray. But because of
the brightness of the sun, it appears to
us like it's white.
What about Mars? How does it compared to
us? Well, we have a planet that from the
surface appears relatively Earthlike.
There is an atmosphere and the sun
appears about 5/8 the size on Earth,
which produces the equivalent brightness
of a slightly cloudy afternoon here.
During the day, the sky is orange to
scarlet in color, and sunsets and
sunrises produce these blues and
purples. It's quite the opposite of
Earth.
And during the night, stars are visible
in the sky, as well as both of Mars's
moons, which appear quite small. Moving
on to the outer solar system, now things
are going to get even more spectacular.
Now, these planets don't have solid
surfaces to land on, which means we're
going to look from their moon's
perspectives. And starting with Jupiter,
we're going to land on the surface of
its closest big moon, Io.
None of Jupiter's moons have anything
more than traces of an atmosphere. So,
the sky of Io itself would appear quite
black. The dominant feature of the sky
would of course be Jupiter, which would
appear a massive 38 times bigger in Io's
sky than our moon in our sky. It would
also be exceptionally bright compared to
our moonlight, even dimly illuminated
the moon during its nighttime.
Even so, the sun is 27 times dimmer than
on Earth and only a fifth of the size.
Another spectacular phenomenon that
occurs quite frequently are total solar
eclipses viewed from the moon's
perspective. The largest four moons of
Jupiter called the Galilean moons are
big enough to totally eclipse the sun.
And due to their orbital resonances,
three can cast their shadow on Jupiter
at the same time. Now unbelievably this
is a real image captured by the Hubble
Space Telescope demonstrating this
point. Already we are experiencing
drastic differences in the skies of
objects in our very own solar system.
But let's take it one step further and
go to the very last planet.
Neptune is the very edge of deep space.
At certain points of its year, it can be
even further away from the sun than
Pluto. And the only thing found further
out than Neptune are icy rocks. The sun
is only a 30th of the size in Neptune's
sky, and Neptune receives roughly
1,000th of the sunlight Earth does.
Our last visit for today will be on
Triton, the largest of Neptune's moons.
On the horizon, you will see its pale,
thin atmosphere illuminated by the sun.
Neptune itself is 16 times bigger in the
sky of Triton than how our moon would
appear, but is 250 times dimmer.
For us, the sun is crucial for survival.
On Triton, the temperature is -230°
centigrade, far too cold for us to
survive. But to me, it's hugely
interesting to know how different skies
can look even within our own solar
system. I've only scratched the surface
today of different views from different
planets or moons, comets, and even
asteroids. Each have their unique
stories to tell. Stories which we
ourselves can discover.
Look into the night sky on a clear night
and you'll see thousands of stars
spanning across your view.
These stars are a beautiful reminder of
the cosmos we live in.
But curiously, when you look at the many
different images released by space
agencies of objects in space, the
background looks mysteriously void of
anything.
Why is that? Surely with nothing in the
way like an atmosphere, we should be
able to see all the stars that would be
in that view.
In fact, even Apollo astronauts reported
not being able to see any stars on the
moon where no atmosphere was in the way.
So, what's up with that?
Well, this is all to do with how eyes
and cameras work.
When we look at a bright object, our
iris shrinks in order to block out some
of the light.
This is what happened with the Apollo
astronauts. Even though there was no
atmosphere above them, the reflection of
the sun's light from the moon's surface
made their eyes adjust to as if it were
day.
One astronaut reported only being able
to see some of the brightest stars when
he was in the shadow of the lunar
module, looking directly up with his
protective visor raised.
I imagine though, if astronauts ever
land on the night side of the moon and
have their eyes adjust the near pitch
black conditions, they would get a
stunning view of the stars in our
galaxy.
When a camera on board a spacecraft
images a planet, the shutter speed, or
the amount of light a shutter allows
into the camera before it closes again,
is set for the equivalent of daytime
levels. Because what we are usually
looking at is the day side of the
object.
This shutter speed is too quick for
background stars to be picked up.
But every so often, we do want to image
the night side of the object, which
means we must increase the exposure time
to allow more light into the camera.
When this happens, suddenly the
background becomes full of stars.
This is a time-lapse of raw images taken
by the Cassini spacecraft. As you can
see, Cassini takes a lot of different
images with different exposures. In some
images there are no background stars
visible and in others there are.
But this leads onto another interesting
point about the images space agencies
release to the public.
You may have seen some of these stunning
images of Saturn before, but you may not
realize that this isn't one image, but
it's rather a mosaic of several images
stitched together.
In such mosaics, you definitely won't
see background stars because the
sections of black around the image won't
have been added to the mosaic.
Lots of spacecraft take images this way.
The reason being it can dramatically
increase the resolution of an image and
reduce the amount of data that has to be
transmitted back.
Bandwidth come in at quite a premium
where data speed can be reduced to 2
[music] kilobytes pers at the farthest
distances.
So, there you go. Now you know why you
don't tend to see stars in the
background of space images.
A massive thank you to our astronauts on
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it was still made possible thanks to the
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