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!
