00:07In the middle of the day, the sky starts to
darken. It’s as if dusk has fallen early.
00:15People look, and notice something is happening
to the Sun. A dark shadow moves before it,
00:22gradually devouring every last trace of
brightness until our familiar light-bringer
00:27is only a shimmering, ghostly ring around a
pitch-black orb. A total eclipse is occurring.
00:37This will soon be a reality. One is coming to
the continent of North America in April 2024.
00:46But if, like me, you are one
of the many billions of people
00:50who won’t get a chance to see that
particular solar eclipse this April,
00:55you might be glad to know that America
is not the only place they happen…
01:00And neither is Earth. Have you ever
wondered what a solar eclipse looks
01:06like on another planet? Wonder no more.
I’m Alex McColgan, and you’re watching
01:15Astrum. And today we’re exploring solar
eclipses – but not just the ones that
01:21happen here on this planet. Allow yourself today
to feast your eyes on the actual images and even
01:28videos NASA has taken of spectacular eclipses
from various places around the solar system.
01:40Firstly, I’ll quickly explain some of the
terminology to do with eclipses. A total
01:46solar eclipse is when an object moves in
front of the Sun, completely obscuring it,
01:51also known as an occultation of an object. This
is the typical kind of eclipse we see on Earth,
01:58when at certain points in Earth’s orbit;
the orbit of the Moon aligns with the Sun.
02:04There is something special about an eclipse
here on Earth, and I’m not just saying that
02:09out of some Earth-focused pride. By a bizarre
but highly fortunate cosmic coincidence, the
02:17Moon is the right size and distance from us that
its angular diameter is almost identical in size
02:23and shape to the angular diameter of the Sun in
our sky. This leaves for an impressive spectacle,
02:31where the corona of the Sun, or in other words
the Sun’s upper atmosphere, creates a ghostly
02:37aura around the Moon. The corona, normally too
dim to see, extends for hundreds of thousands
02:45of kilometres into space. Looking closely
around the edge of a total solar eclipse,
02:52and you will also see the silhouette of
the Moon’s craters along the outside,
02:57plus these reddish wisps coming off from
the Sun. These are prominences, millions of
03:04tonnes of charged particles, suspended in the
Sun’s atmosphere by powerful magnetic fields.
03:11During an eclipse here on Earth, the Moon
casts a shadow about 250km in diameter,
03:17which moves across the Earth as the Moon orbits.
In the case of the eclipse happening this April,
03:24this shadow will arrive at the Mexican west
coastline, then will make its way up through
03:29the United States until it passes into parts
of Canada before moving over the ocean once
03:34more. You can see on this map the path the
shadow will take; if any of you happen to
03:39live in its pathway or close enough to make the
drive, you may want to try and see the eclipse
03:46for yourself. The Sun will only be totally
obscured within the diameter of this shadow,
03:52outside of that the Sun is only partially
obscured from the viewer’s perspective.
03:56This viewer is witnessing an annular or
partial eclipse, also known as a transit.
04:03The shadow moves across the Earth extremely
fast, at roughly 1km a second. Witnessed from
04:10a high altitude, it is a majestic sight as the
shadow shifts across the landscape. Satellites
04:16have also witnessed the movement of this shadow.
The shadow isn’t as sharp as you might expect,
04:22and this is due to the angular diameter of
the Sun and the Moon, and their distance
04:26apart. The Sun itself is huge, a whopping 1.4
million km across. The Moon is much smaller,
04:35at only 3,400km across. Now this image isn’t to
scale, but it shows visually why the shadow isn’t
04:43sharp. The umbra is the shadow where the Sun
is completely obscured, and the penumbra is the
04:50shadow where the Moon only partially obscures
it. This part of the shadow is much wider than
04:55the 250km wide umbra shadow. So why doesn’t the
Moon create eclipses every month when it orbits
05:04in front of the Sun? Well this is because the
Moon’s orbit is not in line with Earth’s orbit
05:09around the Sun. This means there are only a
couple of times per year when the alignment is
05:15right. This alignment of three celestial objects
is known as a syzygy, a very cool word but not
05:22something you’ll need to remember for this video,
just thought you would find that interesting!
05:27If you live in the UK like me, sadly you
won’t get to see much of the eclipse this
05:32April unless you live at the furthest West parts
of the country, as the Sun will be dipping below
05:38the horizon just as it begins. Maybe we
will get to see some devil’s horns though,
05:44still a spectacular sight indeed. But the Earth is
not the only place to experience solar eclipses,
05:52and we have the images and videos to prove it.
Let’s explore eclipses of our closest celestial
06:03neighbour – the Moon. Because it would make
sense that if the Moon can occult the Earth,
06:09then surely the Earth can occult the Moon. And
the answer is yes, but it’s not the shadow that’s
06:17the really visually appealing part of this, from
Earth anyway. That is because the Earth is four
06:23times as big in the Moon’s sky as the Moon is on
Earth’s, so when the Earth fully obscures the Sun,
06:29the whole Moon is in the umbra. At first
the shadow of the Earth creates a crescent
06:35shape (explain that flat earthers). But what is
different this way around is that unlike the Moon,
06:42the Earth has an atmosphere. This means that
when the moon is totally eclipsed, the Earth’s
06:48atmosphere refracts the Sun’s light around the
planet, gently illuminating the Moon in a reddish
06:53hue. This makes for a beautiful but almost spooky
view. The colour is caused by Rayleigh Scattering,
07:02a topic I’ve discussed in another video. Rayleigh
Scattering is the same process that makes our
07:07sky blue and our sunsets red. This image is
beautiful in that you can see the different
07:14wavelengths of light being scattered through the
Earth’s atmosphere, from deep red from this side
07:20through to blue on this side. From the moon’s
perspective, none of the Sun would be visible
07:25during a total eclipse, but the atmosphere on
Earth would be illuminated so you would see a
07:31ring around it. This is an actual view of a lunar
eclipse on the Moon by one of the JAXA probes
07:37in 2009. It is quite the awe inspiring sight!
The Earth and Moon aren’t just getting in the way
07:45of each other either. Here is the Earth eclipsing
the Apollo 12 spacecraft in 1969 while it was on
07:52its way back home. And here’s the Moon getting
in the way of the Earth from the perspective of
07:57the DSCOVR satellite. Interestingly, this
is the side of the Moon you never see,
08:03as the Moon is tidally locked to the Earth which
means the same face is always looking towards
08:08Earth. From this perspective, the Moon looks very
foreign, but it is indeed a real video of our only
08:15natural satellite transiting the Earth. From
another satellite’s perspective, but this time
08:22looking at the Sun with the SDO satellite, the
moon often makes an appearance. The position of
08:28this satellite as it orbits the Earth means the
Moon can block the Sun occasionally. And here’s
08:35the Moon again, this time from the perspective
of one of the STEREO satellites. The Moon isn’t
08:43the only thing that orbits between us and the
Sun. Mercury often transits across the Sun,
08:49a tiny minnow compared to the solar system’s
giant. The next time this will happen is on
08:54the 13th Nov 2032, so a little while away. You
might want to put it in your calendars for now.
09:02Venus also orbits between us and the Sun, and as
it is much closer to us and bigger than Mercury,
09:09its silhouette appears much larger. Its last
transit was in 2004 and 2012, but sadly if you
09:17missed those two, chances are that you will
never see it. These transits happen in pairs,
09:23and then there is a 100 year gap until the next
one, in other words the next transit will be in
09:292117. This happens for the same reason the Moon
doesn’t eclipse us every month; the orbits just
09:36don’t often align. Still, we have HD videos of
the last one, and it is quite the sight to behold.
09:49Moving to another planet now, we can go to Mars
which has plenty of unmanned robotics either in
09:55orbit or on the surface. Mars also has two moons,
Phobos and Deimos. They are both pretty small,
10:04Phobos is 22km across and Deimos is only
13km across. They both orbit very close to
10:12the planet though, Phobos is only 9,000km
above the surface and Deimos is 23,000km,
10:19which means although tiny, you can still
easily see them from the surface of Mars,
10:25especially Phobos. The Curiosity rover was
able to capture a moment where incredibly,
10:31Phobos eclipsed Deimos. This video is captured
in real time and shows the size differences of
10:38the moons in the Martian sky. And this is not
all the Curiosity Rover captured; it was also
10:45able to see a transit of Phobos in front of the
Sun. Due to the distance of Phobos to Mars, it
10:52moves across the sky fairly quickly, only taking
about 7 hours to orbit once. This means that this
10:59video you are watching is in real time, and these
solar eclipses on Mars don’t last for more than
11:05about 30 seconds. The surrounding ground does get
noticeably darker during an eclipse by Phobos as
11:13can be seen from the rover’s perspective, but it
can also be seen from space. Phobos’ shadow here
11:19can be seen by the Viking 1 orbiter, and also
here more recently by the Mars Global Surveyor.
11:25The Opportunity and Spirit rovers have also seen
a transit of Deimos, but it appears much smaller,
11:32just a dot passing in front of the Sun. It
doesn’t cause a noticeable decrease in brightness.
11:39Mars is pretty impressive, but that’s not all
the solar system has to offer. Have a look at
11:45this video captured by the Hubble Space telescope,
looking at Jupiter. Jupiter has four large moons,
11:53three of which at certain points can transit
the planet at the same time, leaving three big
11:59shadows. The moons in question in this video
are Io, Callisto and Europa. Interestingly,
12:07like we talked about before with the umbra
and penumbra, you can see that because Io
12:12is closest to the planet, its shadow is the
sharpest. Whereas you can see with Callisto,
12:18the furthest away of these three moons, the
penumbra is much larger, causing a blurry shadow.
12:26And in this video, Hubble spies the occultation
of Ganymede, the largest moon of Jupiter.
12:33Cassini saw some incredible transits and
occultations of Saturn’s many moons. Here is
12:39one of Epimetheus passing in front of Titan, with
Dione coming in from the side. This little white
12:45dot coming in from the left just under the ring is
in fact a bright background star. And this Hubble
12:53view is magnificent, here are Enceladus, Mimas,
Dione, and Tethys orbiting Saturn. Once every 15
13:03years, Saturn’s rings and moons are aligned just
right so that the Moon’s shadows stream across
13:09the rings as well as on the planet. This video
is a timelapse that lasted 9 ½ hours. Amazing.
13:20Going a bit further out, we come to Neptune
and its biggest moon Triton. Sadly we don’t
13:26have video, but this image captured by
Voyager 2 is gorgeous. Three days after
13:33passing by Neptune, Voyager was able
to capture the crescents of Neptune
13:37and Triton before Neptune slipped in
front of Voyager’s view of Triton.
13:42Going further out again, we come to the furthest
celestial object explored in our Solar System,
13:47Pluto. As New Horizons whizzed by Pluto in
2015, it turned its camera back towards Pluto
13:55to capture the dwarf planet totally eclipsing
the Sun. What it saw was dazzling, the
14:02sunlight streaming through and illuminating the
atmosphere and its haze layers, with the ridges
14:07and mountains on Pluto’s surface highlighted
by the stark contrast of Pluto’s night side.
14:19Occultations and transits may be breathtakingly
beautiful, but are they actually useful to us
14:26scientifically? Well did you know that Uranus was
discovered to have rings because of an occultation
14:33of a background star? As the planet orbited
in front of the star from our perspective,
14:39the star dimmed before and after the
planet obscured it. With this information,
14:44we were able to count how many rings Uranus has.
14:48On top of that, the transits of exoplanets
in front of their stars are actually how we
14:53can detect exoplanets. Telescopes like
Kepler and TESS measure the brightness
14:59of stars in the sky. If a star dims, it could
be because one of its planets just passed in
15:06front of it. If the star continually dims in
a pattern, for instance once every 100 days,
15:13then we know that a planet orbits that star
and takes 100 days to do so. Using this method,
15:20space agencies like NASA have discovered over
three thousand exoplanets; more than all other
15:26methods of exoplanet detection combined. It
can even help scientists calculate the size
15:31of the planet by measuring how much the star
dimmed, or the composition of that planet’s
15:36atmosphere by looking at the spectra of the
light that passes through from the star to us.
15:47It might be some time before we see a solar
eclipse again. After the one in April,
15:53the next eclipse won’t happen in America until
2044. Here in the UK the next one won’t occur
16:00until 2090 – if I live to see it, I will be a
very old man. But I’m amazed as I see all of the
16:08images of eclipses that take place throughout
our solar system. There’s a special beauty to
16:13each one of them – a fleeting moment where one
celestial body brushes lightly against another,
16:19even if it’s only through their shadows. Rather
than harbingers of doom, these moments fill me
16:26with awe, and remind me of how connected
the universe is. Even across thousands,
16:32millions, or even billions of kilometres
of space, we can notice a planet’s passing.
16:39And if you’re in the right place at the
right time, oh what wonders you can see.
16:49Thanks for watching. And as a reminder, if you do
want to see a solar eclipse, you should never look
16:57at the Sun directly, even through a camera lens.
Even when eclipsed by the Moon, the Sun emits
17:06enough infrared, ultraviolet and intense visible
light to seriously damage your eyes. Fortunately,
17:13the sponsor of today’s video VisiSolar has that
side of things covered. VisiSolar lenses are made
17:19using 2 mil scratch-resistant Silver Polymer lens
material with an optical density of 5 or greater.
17:27This allows them to block out 100% of infrared and
ultraviolet light, and 99.999% of intense visible
17:35light, which is really quite impressive. I find
it fascinating that you need that much protection;
17:42it really highlights how powerful the Sun is, even
when eclipsed. It’s not something you want to mess
17:48around with, so it’s a good thing VisiSolar
is recognised ISO-compliant by the American
17:53Astronomical Society. If you or your family are
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the Smartphone Photo Solar Lens that lets you take
18:05photos of the eclipse to keep for the memories?
If you click on my link in the description below
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