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Whenever one celestial body passes in front of another,
the closer object will usually block any light
coming from the object behind it, creating an eclipse.
For instance, we see a solar eclipse whenever the Moon
passes in front of the Sun and casts its shadow over Earth.
But every once in a while, it’s not that simple.
Rather than being totally blotted out, there’s something we can still see.
So today we’re going to talk about when an eclipse… isn’t exactly an eclipse.
Aside from solar eclipses, the other type of eclipses
we see from Earth are lunar eclipses.
We get one every few years, any time the Earth passes
directly between the Sun and Moon.
But unlike a total solar eclipse, which completely blots out the Sun, a lunar eclipse
generally doesn’t blot out the Moon.
The Moon just darkens and turns red.
And it’s not like the Earth’s shadow isn’t big enough.
The darkest part of its shadow, known as the umbra,
completely engulfs the Moon during one of these events.
But some light gets through anyway.
And it’s all because of Earth’s atmosphere.
As sunlight shines past Earth, particles in the atmosphere scatter that light.
But they don’t scatter all sunlight equally.
Shorter, bluer wavelengths scatter more easily,
so they veer off into space.
Meanwhile, redder light, which comes in longer wavelengths,
can travel more directly through the atmosphere.
But even that red light doesn’t travel in a perfectly straight line.
The ring of atmospheric gases surrounding the Earth acts like a lens that
bends light inward, into the Earth’s shadow, giving the Moon a red glow.
The exact shade of red actually depends on air quality on a given day.
Fires, dust storms, and volcanic eruptions can all toss up ash or dust
that will block some light, darkening an eclipsed Moon.
So, the Moon gives us the most obvious example of an eclipse that…
doesn’t completely eclipse.
But it’s not the only one.
Throughout our galaxy, most stars are in binary systems,
meaning they’re not lone stars like our Sun.
Instead, they’re gravitationally bound to another star,
and the two orbit around a shared center of mass.
If we’re looking at one of these binary systems edge-on,
we’ll see an eclipse each time one of the stars crosses in front of the other.
Now, you won’t notice anything if you’re just looking up with the naked eye,
but a telescope observing the system over time will pick up periodic dips
in brightness each time one star crosses in front of another.
Most of the time.
In a 2014 study, astronomers from the University of Washington
announced the discovery of an eclipsing binary that
periodically brightened instead of dimming.
It was the first time anyone had ever seen something like it,
but it actually wasn’t a complete surprise.
In 1973, an astronomer in Geneva predicted that certain binary systems
could produce a brightening pattern instead of a dimming one.
He calculated that this could happen if one of the stars in the systems
was a super-dense object like a white dwarf or neutron star.
In that case, each time the compact object passed in front of its companion,
its gravity would bend spacetime, creating a gravitational lens
that would bend the passing starlight inward.
And from our point of view, the system would appear magnified,
exactly like if we were looking at it through a magnifying lens.
For decades after this phenomenon was predicted,
no one ever observed anything like it.
But when the authors of the 2014 study discovered an eclipsing binary that
periodically brightened, they had a pretty good idea what they were looking at.
In this case, the binary is made up of a Sun-like star and a white dwarf,
which is one of the densest objects in the universe.
Its gravity dramatically warps spacetime, so every time it passes in front
of its companion, it creates a lens that magnifies the Sun-like star.
When it comes to gravitational lenses, one of the few objects that
warps spacetime more than a white dwarf is a black hole.
And that can have some pretty wild consequences.
Like, when a black hole stands in front of a light source,
you might think that not only would light be eclipsed by the black hole,
it would also be swallowed and lost forever over the black hole’s horizon.
But actually, since these objects warp space so much,
the light coming from behind them doesn’t travel in a straight line.
The black hole creates an extreme gravitational lens that bends the light
that should be in its shadow and brings it into plain sight.
In 2021, astronomers actually observed this phenomenon
for the first time… by accident.
They were originally trying to study the source of some X-ray flares that were
shooting out of a black hole and reflecting off the disk of gas swirling around it.
But one researcher noticed that their telescopes were
picking up some extra flashes of light.
And these flashes had a slightly different wavelength and appeared slightly later,
as the main flares were fading.
The team figured out that they were looking at a light echo:
light that was bouncing off the part of the disk directly behind the black hole.
Since it was traveling backward first, before bouncing forward,
it arrived with a slight delay, just like an ordinary sound echo.
So, even though this light was heading straight toward the black hole,
which was directly between it and Earth, in the end, it didn’t get eclipsed.
What’s amazing about all this is that, overall, eclipses seem pretty simple.
You can model most of them with a couple pieces of fruit and a flashlight.
But even these seemingly simple events come with twists.
And they remind us that no matter how well we think we know the universe,
there are always surprises.
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