- 4075 - SUMMER SOLTICE - and more mysteries? The mid-year solstice in 2023 falls on 21 June.. Depending on where you are, this will either be your winter solstice (for those in the southern hemisphere) or the summer solstice (for our northern hemisphere).
----------------- 4075 - SUMMER SOLTICE - and more mysteries?
- Solstice all boils down to orbits, the way Earth
whirls and wobbles as it winds its way around the sun. Earth is a moving platform orbiting the sun
in a little more than 365 days. Despite our incredible orbital speed (around 30
kilometers per second), we don't feel this motion. Instead, it appears to us as
though the sun is moving through the year.
-
- The background
stars at the same time as the sun rise and set every 23 hours, 56 minutes and 4
seconds, the true rotation period of Earth.
The sun, though, rises and sets roughly every 24 hours, making the
"solar day" 3 minutes and 56 seconds longer than Earth's true
rotation period.
-
- That difference is
the result of the sun's apparent motion against the background stars. From our
imaginary airless Earth, we would see the sun gradually sliding through the
constellations of the zodiac, making one full lap of the sky in one year.
-
- But things are a
little more complicated. Our moving
Earth platform is tipped over, tilted on its side by about 23.5 degrees.
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- As we move around
the sun, our planet alternately tilts one hemisphere towards our star, then
away again. This is the cause of the seasons.
When your hemisphere is tilted towards the sun, you have summer, long
days, with the noonday sun high in the sky. Six months later, when you are
tilted away, you have winter, the noonday sun is low, days are shorter.
-
- Between those
extremes, the sun gradually drifts north and south. At the extremes of its
motion, it would be overhead from 23.5° north of the Equator (northern
hemisphere midsummer) or 23.5° south (southern midsummer).
-
- In total the sun's
motion moves it between two extremes some 47° apart. Low in the sky in winter,
and high in summer.
-
- The two solstices
are the points at which the sun is either the farthest north in the sky or at
its most southerly location.
-
- When the sun is
farthest north in the sky, it will appear lowest in the sky at noon from
locations in the southern hemisphere. This also means the shortest period of
daylight of the calendar year.
-
- For the northern
hemisphere, the situation is reversed, the summer solstice places the noonday
sun high in the sky, with the longest period of daylight of the year. In six months' time, on December 22, we will
have the other solstice, marking the point at which the sun is at its most
southerly point in the sky. That will bring with it the longest day for those
in the southern hemisphere, and the shortest for those in the north
-
- In a simple
universe, one would expect the longest day to be the hottest (with most time
for the sun to heat the Earth) and the shortest day to be the coldest (the most
hours of darkness for things to cool down).
However, things are somewhat more complex. The atmosphere, the ground,
and particularly the oceans, take a long time to heat up and to cool down. The
result? The warmest time of the year for many places comes a few weeks after
midsummer.
-
- We imagine the
seasons are of equal length—three months of each, in a 12-month year. But we forget. Not all months are alike.
Some are shorter than others (especially February).
-
- The southern
hemisphere summer (northern winter), from December 22 to March 21, lasts just
89 days. The southern winter (northern summer), by contrast, is almost 94 days
long! The southern autumn (March to
June) is almost 93 days long, while the northern autumn (September to December)
is only 90 days.
-
- The reason behind
these variations is, once again, all down to Earth's orbit. As we move around
the sun, the distance to our star varies slightly. Sometimes, we are closer to our star, and
Earth moves faster in its orbit. At other times, we are more distant, and move
slower.
-
- This year, 2023, on
July 7, Earth will reach its farthest point from the sun, which astronomers
call "aphelion." On that date, we will be more than 152 million
kilometers from our star.
-
- Six months later,
on January 3, 2024, we will be at our
closest to the sun, "perihelion", just over 147 million kilometers
distant.
-
- When the first
Earth Day was held in 1970, geologists were still putting the finishing touches
on plate tectonics, the model that explains how the Earth's surface takes
shape. Why are we all wet?
-
- Scientists think
Earth was a dry rock after it coalesced 4.5 billion years ago. So where did
this essential chemical, H2O, come from? Perhaps an interstellar delivery system,
in the form of massive impacts about 4 billion years ago.
-
- Pummeled by icy
asteroids, the Earth could have replenished its water reservoirs during the
period, called the “Late Heavy Bombardment”. But the beginnings of Earth's
water are shrouded in mystery because so little rock evidence remains from this
time period.
-
- What's down there
in the core? The stuff of legend and
lore, Earth's core has long fascinated writers as well as scientists. For a
while, the composition of Earth's unreachable core was a solved mystery. With
meteorites, scientists gauged the planet's original balance of essential
minerals, and noted which were missing. The iron and nickel absent in Earth's
crust must be in the core, they surmised.
-
- But gravity
measurements in the 1950s revealed those estimates were incorrect. The core was
too light. Today, researchers continue to guess at which elements account for
the density deficit beneath our feet. They're also puzzled by the periodic
reversals in Earth's magnetic field, which is generated by the outer core's
flowing liquid iron.
-
- How did the moon
get here? Did a titanic collision
between the Earth and a Mars-size protoplanet form the moon? The chemical
composition of both rocky bodies matches so closely it suggests the moon was
born from Earth, not a separate impactor. But a fast-spinning young Earth could
have flung off enough molten rock during impact to form a chemically similar
moon.
-
- Where did life
come from? An even tougher
question. Was life brewed on Earth or
sparked in interstellar space and delivered here on meteorites? The most basic
life components, such as amino acids and vitamins, have been found on ice
grains inside asteroids and in the most extreme environments on Earth.
-
- Figuring out how
these parts combined to form the first life is one of biology's biggest
hurdles. And no direct fossil traces of Earth's first inhabitants, which were
probably primitive, rock-chewing bacteria, have yet been found.
-
- Where did all the
oxygen come from? We owe our existence
to cyanobacteria, microscopic creatures that helped to radically transform
Earth's atmosphere. They pumped out oxygen as waste, and filled the skies with
oxygen for the first time about 2.4 billion years ago.
-
- But rocks reveal
oxygen levels cruised up and down like a roller coaster for 3 billion years,
until they stabilized around the Cambrian Period about 541 million years ago.
So did bacteria spike the air, or was there another contributing factor?
Understanding the shift to an oxygen-rich Earth is a key factor in decoding the
history of life on our planet.
-
- What caused the
Cambrian explosion? The appearance of
complex life in the Cambrian, after 4 billion years of Earth history, marks a
unique turning point. Suddenly there were animals with brains and blood
vessels, eyes and hearts, all evolving more quickly than during any other
planetary era known today.
-
- When did plate
tectonics start? Thin plates of hardened
crust knocking about Earth's surface make for beautiful mountain sunsets and
violent volcanic eruptions. Yet geologists still don't know when the plate
tectonics engine revved up. Most of the evidence has been destroyed.
-
- Just a handful of
tiny mineral grains called zircons survive from 4.4 billion years ago, and they
tell scientists the first continental-like rocks already existed. But the
evidence for early plate tectonics is controversial. And geologists still
wonder how continental crust forms.
-
- Will we ever
predict earthquakes? Statistical models can tease out a forecast of
future earthquake probability, similar to weather experts who warn of coming
rain. But that hasn't kept people from trying to predict when the next one will
hit, with no success.
- Even the biggest
experiment failed by 12 years, when geologists predicted an earthquake at
Parkfield, California by 1994, and set up instruments to catch the coming
temblor. The actual quake hit in 2004. One of the biggest hurdles is that
geologists still don't understand why earthquakes start and stop.
July 4, 2023 SUMMER SOLTICE
- and more mysteries? 4075
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Tuesday, July 4, 2023
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