- 4122 - EARTH'S ORBITS - how many, how fast? You'll travel nearly a trillion miles in your lifetime. Everything in the universe is moving, and fast. So how far, on average, does a person travel in their lifetime? The answer depends on whether you consider Earth as your vehicle.
-------------- 4122 - EARTH'S ORBITS - how many, how fast?
- As for distance on Earth's surface, the
typical human will travel 30,000 to 50,000 miles in their lifetime, though some
globe-trotters will go much farther. Considering most people accumulate the
majority of this mileage from commutes and quick errands, that's an
impressively large distance, enough to circumnavigate the globe at least once.
-
- But as large as it is, that number pales in
comparison to the motion we get simply by hitching a ride on Earth. Our planet
spins on its axis. And because Earth is mostly solid, it rotates as a single
rigid body, meaning that everywhere on the planet experiences the same angular
speed and everyone travels a full circle every 24 hours.
-
- But if you were to stand on the north or
south geographic poles, you wouldn't actually travel anywhere; you would just
spin around and around. Those on the equator get a tremendous amount of linear
speed thanks to this rotation, roughly 1,000 mph.
-
- Most people don't live on the equator,
however, so we can say that the average human is constantly traveling at
roughly 930 mph. When you add that up
over a roughly 80-year life span, each person travels around 600,000,000 miles
in their lifetime.
-
- In addition to rotating, Earth orbits the
sun. That orbit is an ellipse, which causes our planet to occasionally move
more quickly or slowly depending on its distance from the sun. But on average,
Earth's orbital speed is about 19 miles per second .
-
- That's about 600,000,000 miles every year.
So over a lifetime, each of us travels roughly 50,000,000,000 miles, which,
again, dwarfs the distance we travel due solely to the rotation of our planet.
-
- But Earth is not the only object in motion
in the universe. The sun travels in a long, lazy orbit around the center of the
Milky Way galaxy. One of these "galactic years" takes roughly
230,000,000 Earth years to complete. To put that into perspective, life first
arose on Earth around 17 galactic years ago, and in only 25 more galactic
years, the sun will die.
-
- Compared with these enormous galactic
scales, a human lifetime is barely perceptible, with the sun barely inching
along its orbit. But on a human scale, it's almost incomprehensible; due to the
motion of the sun orbiting the center of the Milky Way, each of us will travel
around 370,000,000 miles in our lifetime.
-
- Our entire galaxy is in motion, too. All
galaxies are, on average, flying away from each other, but that's due to the
expansion of the universe. On top of that expansion, each galaxy has some
motion of its own.
-
- The Milky Way is on a collision course with
our nearest neighbor, the Andromeda galaxy. The mutual gravitational attraction
is enough to overwhelm the general expansion of the universe, and in about five
billion years, these galaxies will begin to merge.
-
- Both the Milky Way and Andromeda are headed
toward the Virgo cluster, a massive cluster of galaxies about 65 million
light-years away. Beyond that, the Virgo cluster and its surrounding galaxies
are all headed toward the Great Attractor, which is the center of our
supercluster, called Laniakea.
-
- Astronomers can calculate the combined
motion of these gravitational influences by observing the cosmic microwave
background (CMB), which is composed of radiation released when our universe
cooled from a plasma state when it was only 380,000 years old. It completely
soaks the universe and is the same to one part in a million across the entire
sky.
-
- Any motion in the universe will be visible
in the CMB. Light in the direction we're headed in will get Doppler-shifted to
higher frequencies (blueshift), and light in the direction we're moving away
from will be shifted to lower frequencies (redshift). By measuring this shift,
astronomers can calculate our total velocity through the universe, and those
measurements give a number of around 390 miles per second, (1,404,000 miles per
hour)
-
- When you add that up over an 80-year life
span, it gives you a total movement of 930 billion miles. Even if you never
leave home, you will still travel that enormous distance! I thought you should
this!
-
- The length of Earth's day is only 24 hours,
rather than over 60 hours, thanks to a temporary balance between the
gravitational tidal forces from the moon and the sun.
-
- When the moon formed about 4.5 billion years
ago, probably from a giant impact, it was much closer to Earth than it is now
and our planet was spinning much faster, with a day length of less than 10
hours. Since then, the moon has been gradually moving outward, stealing some of
Earth's angular momentum, with the result that Earth's rotation has slowed.
Today a day on Earth lasts 24 hours.
-
- However, at the rate at which the moon is
moving away from us, measured to be 1.49 inches per year by experiments using
laser reflectors left on the moon by Apollo astronauts, our planet should have
slowed to the point of having 60-hour-long days. So what slowed the slowing
down?
-
- It's all related to a balance in the torques
created by thermal tides in Earth's atmosphere and the gravitational tides
emanating from the moon. The moon's
gravity pulls on Earth's oceans, resulting in high tides on opposite sides of
the planet as the ocean bulge follows the moon around our planet. More mass in
the ocean tidal bulge means the moon's gravity pulls on it more, and, coupled
with the effects of friction between the ocean tides and the sea floor, the end
result is a slowing of Earth's spin by about 1.7 milliseconds each century.
-
- However, thermal tides in Earth's atmosphere
are able to counteract this breaking effect if the period with which they
reverberate around the planet enters into a resonance with Earth's rotation.
The temperature of the atmosphere controls the velocity of the thermal waves,
and as the atmosphere warms, it swells, creating another kind of bulge.
-
- Sunlight also produces an atmospheric tide
with the same types of bulges. The sun's
gravity pulls on these atmospheric bulges, producing a torque on the Earth, but
instead of slowing down Earth's rotation like the moon, it speeds it up.
-
- For most of Earth's history, the “lunar
tides” have been 10 times stronger than the “thermal tides”, resulting in
Earth's rotation slowing down. However, based on atmospheric global circulation
models and geological evidence of bands in sedimentary rocks corresponding to
spring and neap tides in the past, this all changed between 2.2 billion and 600
million years ago.
-
- As the atmosphere warmed as evidenced by
the lack of glaciation during this period, the thermal tides grew larger and
faster until they entered a resonant frequency with Earth's rotation. A
resonance is a kind of amplification. The common analogy is a child on a swing,
give them a push at just the right time, synchronous with the arc of their
swing, and they swing faster and higher. Something similar happens with
resonances in nature.
-
- About 2.2 billion years ago, the thermal
tides began traveling around Earth with a period of nearly 10 hours, while
Earth's day length was 19.5 hours. The thermal tides traveled around Earth
twice for every single rotation of Earth on its axis, a resonance of 2:1. This
resonance just amplified the thermal tides, so the atmospheric bulge grew
larger and the sun's pull became significant enough to match that of the moon.
-
- Consequently, the slowing down of Earth's
rotation by the moon's tides began to be counterbalanced by the speeding up
incurred by the thermal tides. For that long period of time between 2.2 billion
years and 600 million years ago, Earth's length of day did not continue to
slow, but remained at 19.5 hours.
-
- Eventually, the two tidal forces moved out
of sync, and over the past 600 million years Earth's rotation has subsequently
begun slowing down again. Today, the length of day is 24 hours, while the
thermal tides take 22.8 hours to travel around Earth.
-
- However, this situation is not fixed.
Although recent measurements have found Earth's rotation to be speeding up
marginally, over the long term Earth probably won't return to the days of the
tidal forces balancing each other out. Instead, climate change could move the
thermal tides farther out of sync with planetary rotation, increasing the effect
that lunar tides have on slowing the planet.
-
- As we increase Earth's temperature with
global warming, we're also making this resonant frequency move higher. We're moving our atmosphere farther away
from resonance. As a result, there's less torque from the sun and therefore the
length of day is going to get longer sooner than it would otherwise.
-
- If you can't keep up take notes!
-
-
August 16, 2023 EARTH'S ORBITS
- how many, how fast? 4122
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