We all know about, and may even have seen a solar eclipse on Earth. A solar eclipse
is a beautiful and spectacular phenomenon where our Moon’s orbit takes it directly
between our viewpoint on Earth and the Sun. The moon, purely coincidentally, 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. The corona, normally too dim to see, extends for hundreds of thousands
of kilometres 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 tonnes of charged
particles, suspended in the Sun’s atmosphere by powerful magnetic fields. But is Earth
the only place in the solar system to experience such solar eclipses? Where can eclipses be
seen? I’m Alex McColgan, and you’re watching Astrum, and together we will look at the breath-taking
spectacle of eclipses in space. 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. The Moon casts a shadow on Earth about 250km in diameter, which
moves across the Earth as the Moon orbits. The Sun will only be totally obscured within
the diameter of this shadow, outside of that the Sun is only partially obscured from the
viewer’s perspective. This viewer is witnessing an annular or partial eclipse, also known
as a transit. These terms occultation and transit will be important for later in the
video. The shadow moves across the Earth extremely fast, at roughly 1km a second. 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
3,400km 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 penumbra
is the shadow where the Moon only partially obscures it. This part of the shadow is much
wider than the 250km 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 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 syzygy, a very cool word but not something you’ll need to remember for
this video, just thought you would find that interesting! So now you know the mechanics
of an eclipse, we can have a look at where other eclipses take place in the solar system!
Firstly, we’ll stick close to Earth. 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 Earth anyway.
That is because the Earth is four times as big in the Moon’s sky as the Moon is on
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 crescent shape (explain that flat earthers). But what
is different this way around is that unlike the Moon, the Earth has an atmosphere. This
means that when the moon is totally eclipsed, 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 colour is caused by Rayleigh Scattering, a topic
I’ve discussed in another video. Rayleigh 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 the 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. It is 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 DSCOVR satellite. Interestingly, this
is the side of the Moon you never see, as the Moon is tidally locked to the Earth 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. This satellite, unlike DSCOVR, orbits around
the Earth, which 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 system’s giant. The next time this will happen is on
the 11th Nov 2019, so if you get an opportunity, take a look! Otherwise you will have to wait
until 2032. 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 missed those two, chances are that you will never see it. These
transits happen in pairs, and then there is a 100 year gap until the next one, in other
words the next transit 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 HD 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 Deimos. They are both pretty small, Phobos is 22km across and
Deimos is only 13km across. They both orbit very close to the planet though, Phobos is
only 9,000km above the surface and Deimos is 23,000km, 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 Deimos. 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. Phobos’
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 Deimos, 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, Callisto and Europa. Interestingly, like we talked about before with the umbra
and penumbra, you can see that because Io is closest to the planet, its shadow is the
sharpest. Whereas you can see with Callisto, 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 Dione 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, Mimas, Dione, and Tethys orbiting Saturn. Once every 15 years, Saturn’s
rings and moons are aligned just right so that the Moon’s shadows stream across the
rings as well as on the planet. This video is a timelapse that lasted 9 ½ hours. Amazing.
Going a bit further out, we come to Neptune and its biggest moon Triton. Sadly we don’t
have video, but this image captured by Voyager 2 is gorgeous. Three days after passing by
Neptune, Voyager was able to capture the crescents 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 our 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 orbited in
front of the star from our perspective, the star dimmed before and after the planet obscured
it. With this information, we were able to count how many rings Uranus has. Did you also
know that transits of exoplanets in front of their stars are actually how we can detect
exoplanets? We have telescopes like Kepler that 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. And lastly, did you know
the Greeks were able to find out the distance of the Earth from the Sun using eclipses and
shadows? If you want to try this out for yourself, Brilliant.org offers a great course on detecting
exoplanets in faraway star systems, and also follows the same mathematical process Aristarchus
took when he discovered the distance of the Earth from the Sun. Astronomy doesn’t have
to be a far away and out of reach subject, as actively grappling with these puzzles will
really help you understand the science better, and Brilliant is a great teacher which will
ease you into the maths to make science that much more enjoyable for you. If you go to
brilliant.org/Astrum, you can sign up for free to have a sample of their courses, and
by using that link, the first 200 people will get 20% off the annual Brilliant premium subscription.
If you want to support my channel and also expand on your own knowledge, I highly recommend
this website. Thanks for watching! If you liked the video, the most important thing
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