Why does the sun sometimes partially or even completely disappear from the sky for a few
minutes?
Let’s start with the basics.
Earth, our home planet, is locked in an elliptical path around the sun.
This elliptical path is known as the orbit of our planet.
Earth travels around the sun along that orbit and completes one revolution in approximately
365 days and 6 hours.
As you may have noticed, this is roughly the length of one year.
So, does that mean Earth is at exactly the same spot in its orbit every year on your
birthday?
Well, since it takes the same amount of time every year to complete one revolution around
the sun, it’s only fair to assume that on your birthday, Earth will be in the exact
same spot every year.
We have actually answered that question in another video that you can watch by clicking
on the 'i' button at the top right corner of the screen.
Just as Earth moves around the sun in an elliptical orbit every year, our natural satellite—the
moon—also revolves around the Earth.
It takes about 27.322 days for the moon to complete one revolution around our planet.
So, Earth revolves around the sun, and the moon revolves around Earth.
Pretty neat, right?
Due to all the inter-related motion happening between these three celestial bodies, several
physical phenomena occur.
When the new moon, during its revolution around the Earth, moves between the sun and the Earth,
it blocks out the sun’s rays, which casts a shadow on parts of the planet.
This is called an eclipse of the sun or a solar eclipse.
In other words, a solar eclipse happens when the moon passes in a direct line between the
sun and the Earth.
Now, you may be thinking that the moon is infinitesimally small compared to the sun,
so why is it ever able to cover part of or sometimes even the entire sun?
Although the moon is indeed about 400 times smaller than the sun, it also happens to be
around 400 times closer to us than the sun.
This is why both the moon and the sun appear to be the same size in our sky.
The moon is very small compared to the sun and to Earth, which is why the moon’s shadow
isn’t big enough to engulf our entire planet.
Therefore, the moon’s shadow is always limited to a certain area.
This area changes even DURING an eclipse, as both the celestial bodies are in constant
motion with respect to one another.
When the moon eclipses the sun, it casts two types of shadows on Earth: the first is the
Umbra, which is a small and very dark shadow.
If you’re in a place on Earth where Umbra is cast, the entire central portion of the
sun will be blocked out from your perspective.
The second type of shadow is known as the penumbra; it is a larger and relatively ‘lighter’
shadow than the umbra.
If the penumbra passes over you, then only a small part of the sun will be blocked out
from your point of view.
Depending on the type of shadow the moon casts on Earth, solar eclipses are broadly classified
into four types:
The first is a total solar eclipse.
This is the most spectacular of all eclipses because, during a total solar eclipse, the
entire sun is completely blocked out by the moon.
This can only happen when the moon is near perigee, the point of the moon’s orbit when
it is closest to the Earth.
You can only see a total solar eclipse if you’re standing within the umbral shadow.
Since the Earth keeps rotating, the umbral shadow doesn’t stay at one point on Earth—it
keeps moving too!
The imaginary line created by the umbral shadow is known as the path of totality.
If you’re in any of the places through which this imaginary line passes, you can see when
the sun gets completely blocked out by the moon.
Next up is a partial solar eclipse.
This type of eclipse is observed when the moon covers only a part of the sun and casts
a penumbra on Earth.
Since it only covers a part of the sun, this phenomena is known as a partial solar eclipse.
When the moon passes through the center of the sun, but its disk isn’t big enough to
cover the entire disk of the sun, then an annular solar eclipse occurs.
When this happens, the sun’s outer edges remain visible, making it look like a shiny,
fiery ring in the sky.
Unlike a total eclipse, during an annular eclipse, the moon doesn’t completely block
out the sun.
The fourth type of eclipse is the rarest of all; when the moon’s position between the
sun and the Earth is so delicately balanced that Earth’s curvature plays a role, we
call it a hybrid solar eclipse.
During this type of eclipse, some parts of the Earth witness an annular solar eclipse,
while other parts see a total solar eclipse.
For that reason, this type of eclipse is also known as an annular-total solar eclipse.
Since the moon continually revolves around the Earth, it’s only natural that it sometimes
comes between the sun and the Earth, while at other times it goes BEHIND the Earth, so
that the Earth comes between the moon and the sun.
When the latter happens, we see a lunar eclipse.
The moon completes one revolution around our planet in approximately 27 days and rotates
at the same rate as it revolves around the Earth; however, since the Earth rotates on
its axis and also moves in its own orbit around the sun, from our perspective, the moon appears
to orbit us every 29 days.
Earth casts two types of shadows on the moon—umbra and penumbra—when it is oriented between
the sun and the moon.
Lunar eclipses can be divided into three types, the first being a total lunar eclipse.
The most picturesque and dramatic of all, a total lunar eclipse happens when the moon,
Earth and the sun are perfectly aligned so that the moon falls under the umbral shadow
of our planet.
During a total eclipse, Earth completely blocks any sunlight from reaching the moon.
Due to this, the moon loses its characteristic whitish-grey color and becomes a ‘sunset
red’.
This happens due to refraction from the Earth’s atmosphere.
The shorter, blue wavelengths of sunlight are scattered outward by the atmosphere, whie
the longer red wavelengths are refracted inward toward the moon, giing it a reddish tinge
during a total lunar eclipse.
Next is a partial lunar eclipse, which happens when the moon, Earth and sun align in such
a way that only PART of the moon passes through the umbral shadow cast by the Earth.
During a partial eclipse, you can see Earth’s shadow covering a small part of the moon’s
surface.
Then there’s a penumbral lunar eclipse.
As the name suggests, you can see this type of eclipse when the moon passes through Earth’s
penumbral shadow.
This celestial event is so subtle that many of us don’t even notice it visually, as
the moon appears only slightly darker than its usual hue.
So, unless you are ACTIVELY LOOKING to observe the penumbral eclipse, it can be very easy
to miss.
You can watch a lunar eclipse if you’re on the nighttime-side of Earth, and for those
who were wondering, it’s safe to watch a lunar eclipse with the naked eye.
In contrast, it’s highly dangerous to watch a solar eclipse with the naked eye.
Observing a solar eclipse without appropriate precautions can potentially cause permanent
eye injuries.
The light from the sun that reaches the Earth consists of a broad range of radiation, from
ultraviolet radiation at wavelengths longer than 290 nanometers all the way to radio waves
existing in the meter range.
The tissues in our eyes transmit a considerable portion of this radiation to the back of the
eye—the light-sensitive retina.
This can result in damage to the light-sensitive rod and cone cells in the eye.
Basically, just make sure that you’re wearing appropriate eye gear when you observe this
spectacular cosmic phenomenon.
Solar and lunar eclipses bring millions of stargazers out into the streets every year;
they unite people in a remarkable cosmic way.
Given that the same celestial events used to strike fear in the hearts of people and
were considered bad omens, humanity has certainly come a long way when it comes to understanding
the forces of nature!