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The Unseen Journey: A Deep Dive into the Revolution of Earth We are all travelers on a magnificent, silent, and relentless voyage. Every s...
The Unseen Journey: A Deep Dive into the Revolution of Earth
We are all travelers on a magnificent, silent, and relentless voyage. Every second of every day, you are moving at a staggering velocity through the cold, vast vacuum of space. You are a passenger on a colossal, life-bearing vessel, a blue and white marble hurtling along a predetermined cosmic track. This journey, the very foundation of our concept of a year, is the revolution of our planet around the sun. It is a motion so profound and constant that we rarely pause to consider its breathtaking implications, its violent history of discovery, or the intricate celestial mechanics that govern it. This exploration is an invitation to look up from the daily grind and feel the motion, to understand the grand astronomical dance that dictates our seasons, sculpts our calendars, and ultimately, makes life on Earth possible. We will peel back the layers of this seemingly simple concept to reveal a universe of complexity, wonder, and profound consequence.
Before we embark on our year-long journey, it is crucial to
distinguish between the two primary motions of our planet. Often conflated,
they are vastly different in their mechanics and their effects. The first is
rotation. This is the Earth's spin on its own axis, an imaginary line running
from the North Pole to the South Pole. This rotation is what gives us day and
night. It takes approximately twenty-four hours for the Earth to complete one
full turn, presenting a different face to the sun at all times. As your side of
the planet spins into the sun's warm glow, you experience dawn and day. As it
spins away into the shadow of its own body, you experience dusk and night. This
is a personal, daily cycle.
Revolution, on the other hand, is a much grander, slower, and
more consequential motion. It is the Earth's orbital path around the sun. Our
planet does not sit still in space; it is in a perpetual free-fall towards the
sun, but with enough sideways velocity to constantly miss it. This delicate
balance between the sun's gravitational pull and our planet's own momentum
creates a stable, elliptical orbit. This journey is not measured in hours, but
in days, months, and ultimately, a single year. It takes the Earth roughly
three hundred and sixty-five and a quarter days to complete one full
revolution. This is the metronome that ticks away the seasons, the long, slow
breath of the planet that orchestrates the rhythms of life on a global scale.
While rotation dictates our waking and sleeping hours, revolution dictates the
very character of those hours, the warmth of the sun on our skin, the length of
the shadows we cast, and the migration of the birds in the sky. Our focus here
is this magnificent, year-long circumnavigation of our star.
The understanding that Earth revolves around the sun was not
self-evident. For millennia, humanity looked to the heavens and, with perfect
intuition, concluded that we were the center of everything. The sun rose in the
east and set in the west. The stars wheeled overhead in perfect, nightly
circles. It was obvious, it was felt, and it was codified into a model that
dominated Western thought for over a thousand years: the geocentric model. The
most sophisticated version of this model was crafted by the Alexandrian
astronomer Claudius Ptolemy in the second century AD. His system was a
masterpiece of complex mathematics, designed to explain the observed motions of
the planets. To account for the peculiar retrograde motion of planets like Mars
and Jupiter—where they appear to move backward in the sky for a time before
resuming their forward course—Ptolemy employed a system of epicycles, smaller
circles upon which planets moved, whose centers themselves moved along larger
circles called deferents, all centered on a stationary Earth. It was a
cumbersome but effective predictive tool. It worked, and it aligned with both
observation and philosophical doctrine, which placed humanity and Earth at the
pinnacle of creation.
The first serious crack in this geocentric edifice appeared in
the sixteenth century with a Polish cleric named Nicolaus Copernicus. A
cautious and meticulous man, Copernicus was not an observational astronomer in
the mold of Tycho Brahe, who would follow him. He was a theorist, a
mathematician who found the Ptolemaic system's complexity inelegant. He
proposed a radically simpler alternative in his book, "De revolutionibus
orbium coelestium" (On the Revolutions of the Heavenly Spheres), published
just before his death in 1543. Copernicus placed the sun, not the Earth, at the
center of the known universe. In his heliocentric model, the Earth was demoted
to just another planet, alongside Mercury, Venus, Mars, Jupiter, and Saturn,
all revolving around the sun. This single shift beautifully explained the
retrograde motion of the outer planets. It was an optical illusion, caused as
the faster-moving Earth on its inner orbit overtook and passed the
slower-moving outer planets, much like a faster car on a highway passing a
slower one makes the slower car appear to move backward relative to the distant
landscape.
Despite its elegance, the Copernican model was not immediately
accepted. It faced several hurdles. First, it flew in the face of millennia of
tradition and religious doctrine. Second, it was not initially any more
accurate at predicting planetary positions than the Ptolemaic model. Copernicus
had clung to the ancient Greek belief that celestial orbits must be perfect
circles, so he still had to employ a few small epicycles to make his model fit
the observational data. The revolution of ideas, like the revolution of the
Earth, would require more time, more data, and more brilliant minds.
The next giant step was taken by a Danish nobleman with a
passion for the stars and a golden nose. Tycho Brahe, who lived in the late
sixteenth century, was arguably the greatest pre-telescopic observational
astronomer. For decades, from his magnificent observatory, Uraniborg, he
meticulously charted the positions of the planets and stars with an accuracy
that had never been achieved before. He amassed a treasure trove of data, a
comprehensive map of the heavens as it was then known. Tycho himself did not fully
embrace the Copernican model; he developed a hybrid system where the sun
orbited the Earth, but all the other planets orbited the sun. However, his true
legacy was his data. In his final years, he hired a brilliant but cantankerous
young German mathematician named Johannes Kepler to be his assistant. When
Tycho died unexpectedly in 1601, Kepler inherited this priceless collection of
observations.
For years, Kepler toiled over Tycho's data, particularly the
detailed records of Mars's orbit. He was driven by a conviction that the
universe was built on simple, harmonious geometric principles. He tried to fit
the orbit of Mars into a perfect circle, as Copernicus and the ancients had
done, but the data simply would not comply. There was a small, persistent, and
infuriating discrepancy between the predicted circular orbit and the actual
observed position of Mars. After years of struggle, Kepler had a revolutionary
breakthrough. He abandoned the sacred cow of the perfect circle. He discovered
that the orbit of Mars, and indeed the orbit of all planets, was not a circle
but an ellipse. This was the first of his three laws of planetary motion, a
discovery that would shatter the old astronomical order and lay the groundwork
for modern physics. His second law stated that a planet sweeps out equal areas
in equal times, meaning a planet moves faster when it is closer to the sun and
slower when it is farther away. His third law established a precise
mathematical relationship between a planet's orbital period and its average
distance from the sun. Kepler had accurately described the how of
planetary motion with stunning precision, but he did not know the why.
What force was pushing and pulling the planets in their elegant elliptical
paths?
The answer came in the next generation from the mind of an
English genius, Isaac Newton. In his monumental work, "Philosophiæ
Naturalis Principia Mathematica," published in 1687, Newton presented a
unified theory of motion and gravity that explained everything from the fall of
an apple to the orbit of the moon. His Law of Universal Gravitation proposed
that every object in the universe attracts every other object with a force that
is proportional to their masses and inversely proportional to the square of the
distance between them. The sun, with its immense mass, exerts a powerful
gravitational pull on the Earth. This pull is constantly trying to drag our
planet into the sun's fiery heart. But the Earth also has its own momentum from
its formation, a sideways velocity. The Earth's revolution is not a motion of a
planet being pulled around the sun, but rather a planet continuously falling towards
the sun, but moving sideways so quickly that it constantly misses. Imagine
firing a cannonball from a tall mountain. If you use a small charge, it falls
to Earth nearby. If you use a larger charge, it travels farther before curving
down. If you could fire it with an incredible amount of force, it would travel
so fast that as it fell, the curve of the Earth would drop away beneath it. The
cannonball would be in orbit, perpetually falling but never hitting the ground.
This is precisely what is happening to our planet. Newton's laws provided the
physical explanation for Kepler's descriptive laws, cementing the heliocentric
model and revealing the elegant, universal mechanics that govern the revolution
of our Earth.
With the historical and physical foundations laid, we can now
examine the specifics of our planet's journey. The Earth's orbit is not a
perfect circle, as Kepler so brilliantly deduced. It is an ellipse, a shape
that resembles a slightly squashed circle. An ellipse has two central points,
or foci. In the case of Earth's orbit, the sun is not at the center of the
ellipse, but sits at one of these two foci. This off-center position has
profound implications for our journey.
The degree to which an ellipse is stretched out from a perfect
circle is called its eccentricity. A perfect circle has an eccentricity of
zero. A long, thin, cigar-shaped ellipse has an eccentricity approaching one.
The Earth's orbit is only very slightly elliptical. Its eccentricity is a mere
0.0167. This means that visually, the orbit is almost indistinguishable from a
circle. However, this small deviation is significant. Because the sun is at one
focus of the ellipse, the Earth's distance from the sun is not constant
throughout the year. There are two special points in our orbit that mark the
extremes of this distance.
The point in the Earth's orbit where it is closest to the sun
is called perihelion. This occurs in early January, during the winter in the
Northern Hemisphere. At perihelion, the Earth is approximately 147.1 million
kilometers (about 91.4 million miles) away from the sun. The point where the
Earth is farthest from the sun is called aphelion. This occurs in early July,
during the Northern Hemisphere's summer. At aphelion, the distance to the sun
increases to about 152.1 million kilometers (about 94.5 million miles). This
difference of about five million kilometers may seem vast, but given the
enormous scale of our orbit, it represents only about a three percent variation
in the solar energy we receive.
This variation in distance directly affects the Earth's
orbital speed, as described by Kepler's second law. The sun's gravitational
pull is stronger when the Earth is closer, so our planet must move faster to
maintain its orbit and not be pulled in. Consequently, the Earth travels at its
fastest speed at perihelion, reaching about 30.3 kilometers per second (about
18.8 miles per second). At aphelion, when the sun's pull is weaker, the Earth
slows down, moving at its most leisurely pace of about 29.3 kilometers per
second (about 18.2 miles per second). This means that the Northern Hemisphere
winter, when Earth is closest to the sun and moving fastest, is actually
slightly shorter than its summer. The time it takes for the Earth to move from
the autumnal equinox to the vernal (spring) equinox is roughly seven days
shorter than the time it takes to move from the vernal equinox back to the
autumnal equinox. Our planet's journey is not one of constant speed, but a
constantly changing velocity, a cosmic dance of acceleration and deceleration
dictated by our distance from our star.
The Great Deception: Why Distance from the Sun Does Not Cause
the Seasons
The fact that the Earth is closest to the sun in January and
farthest in July immediately shatters one of the most common and persistent
misconceptions about our planet's motion: that the seasons are caused by the
Earth's distance from the sun. If this were true, the entire planet would
experience summer at the same time, in January when we are at perihelion, and
winter in July at aphelion. But we know this is not the case. When it is winter
in New York, it is summer in Sydney. The true cause of the seasons is far more
fundamental to our planet's very being: its axial tilt.
The Earth does not spin on an axis that is perfectly
perpendicular to its orbital plane, the flat disk of its revolution around the
sun. Instead, the Earth is tilted on its side. The axis of rotation is angled
at approximately 23.5 degrees relative to the plane of its orbit. Imagine a
spinning top that is not perfectly upright but leaning over to one side. The
Earth is like that, and crucially, as it revolves around the sun, this tilt
remains pointed in the same direction in space, always oriented towards the
North Star, Polaris. It is this fixed, unyielding tilt, combined with our
revolution, that creates the seasons.
The seasons are all about the directness and duration of
sunlight. The amount of solar energy a particular location receives depends on
two key factors: the angle at which the sun's rays strike the Earth and the
length of time the sun is above the horizon. When the sun's rays hit the Earth
directly, or at a high angle, the energy is concentrated over a smaller area,
leading to more intense heating. When the rays strike at a low angle, they are
spread out over a much larger area, diluting their energy and resulting in less
heating.
As the Earth makes its year-long journey, this 23.5-degree
tilt causes different parts of the planet to be oriented more directly towards
the sun at different times of the year. Let's trace this journey through the
four key astronomical markers of the seasons.
The journey begins in the Northern Hemisphere with the June
solstice, which occurs around June 21st. At this point in the Earth's orbit,
the Northern Hemisphere is tilted most directly towards the sun. The
sun's rays strike the Northern Hemisphere at their most direct angle of the
year. The sun appears high in the sky, its path across the heavens is long, and
the days are at their maximum length. North of the Arctic Circle, the sun does
not set at all, resulting in the phenomenon of the "midnight sun."
This concentrated, prolonged solar energy brings summer to the Northern
Hemisphere. Conversely, the Southern Hemisphere is tilted away from the
sun. The sun's rays arrive at a low, oblique angle, spreading their energy
thinly. The sun's path across the sky is low and short, the days are brief, and
the nights are long. This is the winter solstice for the Southern Hemisphere.
Three months later, around September 22nd, the Earth reaches
the autumnal equinox. At this point, the Earth has moved a quarter of the way
around its orbit from the June solstice. The tilt of the axis is now sideways
relative to the sun. Neither hemisphere is tilted towards or away from our
star. The sun's rays are focused directly on the equator. The result is a
moment of balance. Day and night are of nearly equal length all over the
planet. The sun rises due east and sets due west. This marks the beginning of
autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
The journey continues to the December solstice, around
December 21st. Now the situation is the reverse of June. The Earth is on the
opposite side of the sun, and the Southern Hemisphere is tilted most directly towards
the sun. It receives the most direct sunlight and experiences its longest days,
ushering in its summer. The Northern Hemisphere is now tilted farthest away
from the sun, receiving weak, indirect sunlight and experiencing its shortest
days and the depths of winter. South of the Antarctic Circle, the sun stays
above the horizon for twenty-four hours a day.
Finally, around March 20th, the Earth arrives at the vernal,
or spring, equinox. Like the autumnal equinox, the planet's tilt is sideways to
the sun. The sun's rays are again directly over the equator, and day and night
are of roughly equal length worldwide. This marks the beginning of spring in
the Northern Hemisphere and autumn in the Southern Hemisphere.
And so the cycle continues, driven not by our distance from
the sun, but by the unchanging tilt of our world as it revolves. The seasons
are a testament to the beautiful interplay between two of Earth's motions: its
fixed axial rotation and its grand orbital revolution. It is a global mechanism
of heat distribution, a planetary-scale pump that moves solar energy from the
equator towards the poles, driving weather patterns, ocean currents, and the
very pulse of life.
The revolution of the Earth is the ultimate timekeeper. It is
the physical basis for the year, the most fundamental unit of time for
organizing human society, agriculture, and culture. But our planet's orbital
period does not fit neatly into our system of whole-number days. As mentioned,
a complete revolution takes approximately 365.25 days. This extra quarter of a
day is the bane of calendar-makers throughout history. Ignoring it would cause
our calendar to drift out of sync with the seasons. After just four years, the
calendar would be a full day behind. After a hundred years, the vernal equinox,
which should fall around March 20th, would occur in late April. The seasons
would slowly migrate through the months, throwing agricultural cycles and
religious festivals into chaos.
The first major attempt to solve this problem was the Julian
calendar, introduced by Julius Caesar in 46 BC. On the advice of the astronomer
Sosigenes of Alexandria, Caesar instituted a 365-day year with a leap day added
every four years. This created an average year length of 365.25 days, which was
remarkably close to the actual solar year. The Julian calendar was a massive
improvement and served the Western world for over 1,600 years.
However, it was not perfect. The actual solar year, known as
the tropical year (the time between successive vernal equinoxes), is slightly
shorter than 365.25 days. It is about 365.2422 days long. This tiny difference
of about 11 minutes per year may seem insignificant, but over centuries, it
adds up. By the sixteenth century, the Julian calendar had drifted by about ten
days from the solar year. The vernal equinox was occurring around March 10th
instead of March 21st. This was a serious problem for the Catholic Church, as
the date of Easter is calculated based on the vernal equinox. Pope Gregory XIII
decided to correct this drift.
In 1582, he introduced the Gregorian calendar, which is the
calendar most of the world uses today. The Gregorian reform had two parts.
First, it corrected the accumulated drift by simply skipping ten days. The day
after Thursday, October 4, 1582, was declared to be Friday, October 15, 1582.
Second, it introduced a more precise rule for leap years to prevent future
drift. The new rule stated that a year is a leap year if it is divisible by
four, except for years that are divisible by 100, unless they are
also divisible by 400. So, the years 1600 and 2000 were leap years because they
are divisible by 400. But the years 1700, 1800, and 1900 were not leap years.
This rule creates an average year length of 365.2425 days, which is an
incredibly close match to the 365.2422-day tropical year. The Gregorian
calendar will only be off by one day every several thousand years. Our modern
calendar is thus a sophisticated monument to our understanding of the Earth's
revolution, a human-made system designed to mirror the celestial clock as
closely as possible.
As the Earth revolves around the sun, the sun appears from our
perspective to move against the backdrop of the distant, "fixed"
stars. This apparent path is called the ecliptic. The ancient civilizations,
particularly the Babylonians, noticed that the sun, as well as the moon and the
known planets, all traveled along this same narrow band in the sky. They
divided this band into twelve equal sections, each named after a prominent
constellation that lay within it. These are the constellations of the zodiac.
The concept of the zodiac is a direct consequence of the
Earth's revolution. When we say the sun is "in Aries," it does not
mean the sun is physically located within that constellation of stars. It means
that from our viewpoint on Earth, the constellation Aries is in the same
direction as the sun. We cannot see the constellation because the sun's
brilliance blots it out, but it is there, hidden behind the daytime sky. As the
Earth journeys around the sun, our line of sight to the sun changes, and the
sun appears to move from one zodiacal constellation to the next, taking about a
month to traverse each one.
For example, in late March and April, as the Earth moves into
a position where the constellation Aries lies in the direction of the sun, we
say the sun is in Aries. A month later, the Earth has moved along its orbit,
and our line of sight now points towards the constellation Taurus. The sun has
"entered" Taurus. This cycle continues through the twelve signs of
the zodiac over the course of the year. While modern astronomy has moved beyond
the astrological interpretations of these patterns, the zodiac remains a
fascinating historical artifact, a celestial map created by our ancestors to
track the progress of our planet's grand journey through the cosmos.
While our yearly revolution gives us the seasons, the orbit
itself is not perfectly stable over vast geological timescales. It undergos
slow, cyclical changes that can alter the amount and distribution of solar
energy reaching the Earth. These variations, known as Milankovitch cycles after
the Serbian scientist who theorized them in the early twentieth century,
operate on timescales of tens of thousands of years and are a primary driver of
long-term climate change, including the coming and going of ice ages.
There are three main Milankovitch cycles. The first is the
change in the shape of the Earth's orbit, or its eccentricity. The orbit slowly
oscillates between being more circular and more elliptical on a cycle of about
100,000 years. When the orbit is more elliptical, the difference in solar
energy between perihelion and aphelion is greater, potentially amplifying
seasonal contrasts.
The second cycle is the change in the angle of the Earth's
axial tilt. The tilt does not stay fixed at exactly 23.5 degrees. It wobbles
back and forth between about 22.1 degrees and 24.5 degrees on a cycle of
roughly 41,000 years. A greater tilt means more extreme seasons—hotter summers
and colder winters—while a smaller tilt means milder seasons.
The third cycle is precession, or axial wobble. Like a
spinning top that is slowing down, the Earth's axis does not stay pointed at
the exact same spot in space. It slowly traces out a circle in the sky over a
period of about 26,000 years. This means that while the Northern Hemisphere is
currently tilted towards the sun at perihelion (its closest point), in about
13,000 years, it will be tilted towards the sun at aphelion (its farthest
point). This would cause the Northern Hemisphere's winters to be warmer and its
summers cooler than they are now.
These three cycles do not operate in isolation. They interact
with each other in complex ways, sometimes reinforcing each other's effects and
sometimes canceling them out. The theory is that major ice ages occur when
these cycles combine in a way that reduces the total amount of summer solar
energy received by the high northern latitudes. If the summers are cool enough,
winter snow and ice do not completely melt, and over thousands of years, they
build up into massive ice sheets. The Milankovitch cycles remind us that our
planet's journey is not a simple, unchanging loop. It is a subtle, evolving
dance with deep, slow rhythms that have shaped the climate and the evolution of
life on Earth for millions of years.
To fully appreciate the nature of our own revolution, it is
instructive to compare it to the orbits of our planetary neighbors. Each planet
in our solar system has its own unique orbital characteristics, a product of
its formation and its gravitational interactions with other bodies.
Mercury, the closest planet to the sun, has the most eccentric
orbit of all the planets, with an eccentricity of 0.206. Its speed variation is
dramatic, and it completes a revolution in just 88 Earth days. Venus, our
nearest planetary neighbor, has an orbit that is almost a perfect circle, with
an eccentricity of only 0.007. Its year is about 225 Earth days long. Mars, the
next planet out, has a year of 687 Earth days and an orbital eccentricity of
0.093, more than five times that of Earth. This means its distance from the sun
varies significantly, leading to more pronounced seasonal differences in solar
energy than we experience.
The gas giants live on timescales and at distances that are
difficult to comprehend. Jupiter, the king of planets, takes nearly twelve
Earth years to complete one revolution. Saturn's year is over twenty-nine Earth
years long. Uranus takes eighty-four Earth years, and Neptune, the most distant
planet, a staggering 165 Earth years to make one trip around the sun. Someone
born on Neptune would have only celebrated their first birthday after the
entire American Civil War, both World Wars, the invention of the internet, and
the entirety of human spaceflight had occurred on Earth. Looking at our
planetary siblings highlights the "just right" nature of our own
orbit. Its near-circular shape, its moderate distance from a stable star, and
its one-year period combine to create a remarkably stable and predictable
environment, a crucial ingredient for the long-term evolution of complex life.
Is our journey eternal? In the grand cosmic scheme, the answer
is no. The Earth's orbit is evolving, and its future is tied to the life cycle
of our sun. In the short term, over millions of years, the gravitational
interactions between the planets and the slow loss of mass from the sun as it
converts hydrogen to helium will cause the Earth's orbit to slowly migrate
outwards. It is a minuscule change, about 1.5 centimeters per year, but over
billions of years, it adds up.
However, the sun's own evolution will render this slow drift
irrelevant. In about five billion years, our sun will exhaust the hydrogen fuel
in its core. It will swell into a red giant, becoming so large that it will
likely engulf Mercury, Venus, and possibly the Earth itself. Even if our planet
escapes being consumed, its surface will be scorched and sterilized by the
immense heat of the expanding sun. The oceans will boil away, and the
atmosphere will be stripped away. Our gentle, life-giving journey will end in a
fiery apocalypse.
But that is a fate so distant it is beyond comprehension. For
now, and for the foreseeable future of humanity, our planet will continue its
silent, steadfast revolution. It will continue to trace its elegant elliptical
path through the void, a tiny speck in the vastness, yet a world teeming with
life, consciousness, and wonder. Every sunrise, every change of season, every
page we turn on our calendars is a direct consequence of this grand cosmic
motion.
To truly understand the revolution of the Earth is to gain a
new perspective on our place in the universe. It is to feel the connection
between the mundane rhythms of our daily lives and the majestic, clockwork
machinery of the cosmos. We are not merely inhabitants of a planet; we are
voyagers on a celestial ship, sailing through the ocean of space on a journey
that defines our existence. The next time you feel the warmth of the summer sun
or the crisp chill of a winter morning, take a moment to remember the
incredible, unseen journey that made it possible. Remember the tilt, the orbit,
the speed, and the delicate balance that allows us to be here, at this very
moment, on this beautiful, revolving Earth.
What is the difference between Earth's rotation and
revolution?
Rotation is the spinning of the Earth on its axis, which takes
approximately 24 hours and causes the cycle of day and night. Revolution is the
Earth's orbit around the sun, which takes approximately 365.25 days and defines
the length of a year and causes the seasons.
Does the Earth's distance from the sun cause the seasons?
No, this is a common misconception. The seasons are caused by
the 23.5-degree tilt of the Earth's axis. This tilt means that different parts
of the Earth receive more direct sunlight at different times of the year. In
fact, the Earth is closest to the sun (at perihelion) in early January, during
the Northern Hemisphere's winter.
Why is our orbit an ellipse and not a perfect circle?
According to Newton's laws of motion and gravity, an orbit is
the path of an object that is falling towards a massive body (like the sun) but
also has sufficient sideways velocity to keep missing it. An ellipse is the
natural path for such an object under the influence of gravity. A perfect
circle is a special type of ellipse that would require a perfectly precise
initial velocity, which is highly unlikely in nature.
How fast is the Earth moving as it revolves around the sun?
The Earth's speed is not constant. It moves fastest at its
closest point to the sun (perihelion) at about 30.3 kilometers per second
(67,000 miles per hour) and slowest at its farthest point (aphelion) at about
29.3 kilometers per second (65,000 miles per hour). The average speed is about
30 kilometers per second.
What are the solstices and equinoxes?
The solstices and equinoxes are points in the Earth's orbit
that mark the changing of the seasons. The summer solstice (around June 21st in
the Northern Hemisphere) is when that hemisphere is tilted most directly
towards the sun, resulting in the longest day of the year. The winter solstice
(around December 21st) is when it is tilted farthest away, resulting in the
shortest day. The equinoxes (around March 20th and September 22nd) occur when
the tilt is sideways relative to the sun, and day and night are of roughly
equal length worldwide.
What is a leap year and why do we have them?
A leap year is a year with an extra day, February 29th. We
have leap years to keep our calendar in sync with the Earth's revolution. A
complete orbit takes about 365.25 days, so we add an extra day every four years
to account for the accumulated quarter days. The Gregorian calendar refines
this with a rule that skips leap years on century years unless they are
divisible by 400.
What are Milankovitch cycles?
Milankovitch cycles are long-term, predictable changes in the
Earth's orbit and its axial tilt. They include changes in the shape of the
orbit (eccentricity), the angle of the tilt (obliquity), and the wobble of the
axis (precession). These cycles, which operate over tens of thousands of years,
affect the amount and distribution of solar energy on Earth and are a major
driver of long-term climate patterns like ice ages.
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