Archive for the ‘History’ Category

Earth: part II

From my notebook.


The Earth is divided into a core, mantle and crust. The core, hot and iron-rich, consists of a solid inner and liquid outer core. Convection in the outer core creates the magnetic field. The upper mantle and crust constitute the lithosphere, broken up into tectonic plates that ride the asthenosphere, a viscous region of the upper mantle. The science of this movement, plate tectonics, developed in the mid-twentieth century, superseding the idea of continental drift.

71% of the Earth’s surface is covered by water. 97.5% of this water is saline. Of the 2.5% fresh water, 68.7% is ice. The Sun drives the water cycle, a continuous exchange of water within the hydrosphere through evaporation, condensation, precipitation, infiltration and runoff. The gravitational force of the Moon and Sun cause tides, the cyclic rising and falling of the ocean surface. Most places experience a semidiurnal tide: two high and low tides a day. A diurnal tide is one high and low tide a day. The Earth has five oceans: the Pacific, Atlantic, Indian, Southern and the Arctic. The planet experiences periodic ice ages which include colder glacial and warmer interglacial periods. It is currently in an interglacial period of an ice age known as the Quaternary glaciation. The most recent glacial period of this ice age was the Last Glacial Maximum 20 ka.

Earth has had three atmospheres. The first was helium and hydrogen; the second carbon dioxide and water vapour from volcanic outgassing and bolide collisions. The water condensed into oceans. Photosynthesis created the third atmosphere, consisting mostly of nitrogen, oxygen and argon. The atmosphere is divided into the troposphere, stratosphere, mesosphere, thermosphere and exosphere. Most weather occurs in the troposphere. The stratosphere contains the ozone layer, which absorbs ultraviolet light. The mesosphere is where most meteors burn up. The thermosphere includes the Kármán line, the working definition for the boundary between the atmosphere and space, 100km above the Earth’s surface. The sky appears blue due to the atmosphere scattering sunlight, a phenomenon called Rayleigh scattering.

The increasing luminosity of the Sun will end most plant life within the next 600 million years. In 1.1 billion years, the oceans will evaporate. In five billion years the Earth will be swallowed by the Sun.

Scholars in the Western world have known that the Earth is round since the ancient Greeks.

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Earth: part I

FROM MY NOTEBOOK, also titled Things to Keep in Mind.


The Earth is the third planet from the Sun. It formed from the solar nebular (the disc-shaped cloud of gas and dust left over from the formation of the Sun) 4.54 Ga. The Earth is the densest and fifth-largest planet. It orbits the Sun at an average distance of 150 million kms and an average speed of 108,000 km/h. The planet is rotating eastward at an equatorial speed of 1,670 km/h (1970 m/h). This rotation causes the Coriolis effect, impacting oceanic and atmospheric circulation.

The Moon was created when the protoplanet Theia collided with the earth around 4.5 Ga. The blow also produced the 23.5 degree axial tilt responsible for the four seasons. Due to changes in the orbits of the gas giants, the Late Heavy Bombardment followed from 4.1 – 3.8 Ga. During this period, the inner planets were bombarded with meteorites. These might have brought much of Earth’s water.

The Earth: bitch-slapped by a girl

I can haz oxygen?

Life appeared at least 3.5 Ga. By this time Earth’s magnetic field was in place. Photosynthesis caused the Great Oxygenation Event of 2.4 Ga, which in turn triggered the Huronian Glaciation from 2.4 – 2.1 Ga. This glaciation might have been a Snowball Earth, as in those that occurred around 650 Ma. In 530 Ma, the Cambrian explosion saw the rapid appearance of most animal phyla. The movement of tectonic plates created the supercontinent Pangea around 250 Ma. Dinosaurs appeared about 230 Ma. In 65 Ma, a meteorite crashed into the Yucatan Peninsula, causing the K-T (Cretaceous-Tertiary) extinction event. The genus homo appeared about 2.3 – 2.4 Ma.

Things smashing:

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Earth: timeline

From my notebook, sometimes called Everything We Should Already Know.



Earth 4.54 Ga

Moon 4.5 Ga

Late Heavy Bombardment 4.1 – 3.8 Ga

Life 3.5 Ga

Magnetic Field 3.5 Ga

Great Oxygenation Event 2.4 Ga

Huronian Glaciation 2.4 – 2.1 Ga

Snowball Earths 650 Ma

Cambrian explosion 530 Ma

K-T extinction event 65 Ma



Supereon: Eon: Era: Period: Epoch: Age



– Hadean

– Archean

– Proterzoic

– Phanerzoic


We are in the: Phanerzoic eon; Cenozoic era; Quaternary period; Holocene epoch; Subatlantic age.

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The Solar System

MORE FROM my notebook:


The Solar System consists of the Sun and the bodies that orbit it. It formed from the gravitational collapse of a molecular cloud about 4.57 Ga. Located on the inner rim of the Orion Arm in the Local Fluff inside the Local Bubble, it is 25 000 – 28 000 light years from galactic centre, which it orbits at about 970 000 km/h (600 mph) – completing one revolution every 225 – 250 million years (i.e. a galactic year). The closest stars are those of the triple star system Alpha Centauri.

The Sun is a yellow dwarf, although its colour is actually white. It appears yellow due to Rayleigh scattering in the Earth’s atmosphere. It is a population I star – young and metal-rich. A star is a luminous ball of plasma held together by gravity. Stars form from collapsing molecular clouds and shine due to the conversion of hydrogen into helium through nuclear fusion. They create the heavier elements through stellar nucleosynthesis. The Sun radiates a stream of charged particles, the solar wind, creating the heliosphere. It becomes 10% more luminous every billion years. In five billion years it will become a Red Giant, expanding to 250 times its present size. It will cast off a planetary nebula and the core will become a white dwarf. A star with greater mass would explode into a supernova, becoming a neutron star or black hole.

Eight planets orbit the Sun. A planet (from the Greek “wanderer”) is an astronomical body orbiting a star, with enough mass to be rounded, and which has cleared the neighbourhood of planetesimals. In the inner Solar System are the four terrestrial planets, composed primarily of rock and metal: Mercury, Venus, Earth, and Mars. Between the inner and outer planets is the asteroid belt. In the outer Solar System are the four gas giants: Jupiter and Saturn, composed primarily of hydrogen and helium, and the ice giants Uranus and Neptune, composed primarily of ices. These four have planetary rings of cosmic dust, the most spectacular of which are Saturn’s. Around the outer planets is the Kuiper belt. There are also five confirmed dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. A dwarf planet is an astronomical body that orbits a star and is rounded but has not cleared the neighbourhood around its orbit. There are probably hundreds to thousands in the Solar System. The orbits of all these bodies through the interplanetary medium describe a flat plane, the ecliptic.

Small Solar System bodies are objects that orbit the sun, but are not rounded and have not cleared the neighbourhood around their orbits. This excludes moons, which orbit planets, and includes asteroids, comets, meteoroids, meteors and meteorites. Asteroids are small celestial bodies of rock and metal that orbit the Sun; comets differ in being composed of ice, dust and rocky particles and displaying a coma and/or tail due to vaporization when close to the Sun. A meteoroid is a small asteroid. A meteor is a meteoroid that enters an atmosphere; a meteorite is a meteoroid that impacts a planet’s surface.

Geocentrism dominated astronomy until recent times. Aristarchus of Samos had proposed a heliocentric system in the third century BCE, as did his follower Selecus of Seleucia in the second. It was nonetheless the geocentric model of Claudius Ptolemy, also of the second century BCE, that became standard. Nicolaus Copernicus proposed a heliocentric model in the 16th century. It found confirmation in the observations of Johannes Kepler (German) and Galileo Galilei (Italian) in the 17th century.

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The Milky Way

HERE IS MORE from my notebook, a collection of general historical overviews and brief summaries.


The Milky Way: a barred spiral galaxy in the Local Group (which is part of the Virgo Supercluster).

A galaxy is a massive gravitationally bound system of stars, gas and dust. The first galaxies formed during the Dark Ages from hydrogen and helium atoms due to quantum fluctuations. Galaxies can be elliptical, spiral or irregular.

The Milky Way owes its name to the Latin Via Lactea, from the Greek Galaxius (“milky circle”). The oldest star dates to 13.2 Ga (although the thin disk formed 8.8 plus or minus 1.7 Ga). The galaxy is composed of 200-400 billion stars. On average, it is 10 000 light years thick and 100 000 light years in diameter. At the centre is Sagittarius A*, probably a supermassive black hole. The galaxy rotates at about 970 000 km/h (600 mph), completing one rotation every 225-250 million years. The bar is composed primarily of red stars. There are two major interstellar arms (there were once thought to be four): the Perseus and Scutum-Centaurus Arms.

The Milky Way’s closest neighbour is the Canis Major Dwarf Galaxy, an irregular galaxy of an estimated one billion stars. The Milky Way’s gravitational field is pulling it apart. It has torn away a stellar stream, the Monoceros Ring, and wrapped it around our galaxy thrice. Another galaxy merging with ours is the Sagittarius Dwarf Elliptical Galaxy. Our largest neighbour is the Andromeda Galaxy, a spiral galaxy of one trillion stars. The Andromeda Galaxy is the largest in the Local Group, although the Milky Way might be the most massive due to more dark matter. Orbiting the Milky Way are the Small and Large Magellanic Clouds, irregular dwarf galaxies connected via the Magellanic Stream, a stream of neutral hydrogen gas.

The Milky Way will collide with the Andromeda Galaxy in about four billion years. The combined galaxies could form a giant elliptical galaxy.

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The Big Bang

IF YOU WANT TO READ all these notebook entries, click on the “History” category to the right. This is the bare minimum, in the simplest possible language, that I decided I should know about the Big Bang. Corrections welcome.


Big Bang: the expansion of the universe from a hot and dense initial state 13.73 Ga. The standard cosmological model is the Lambda-Cold Dark Matter Model (ΛCDM). Lambda refers to dark energy, used to explain the accelerating rate of expansion since 5Ga. The existence of dark matter is used to explain the speed of rotation of galaxies. The major elements produced were hydrogen, helium and lithium. Stars produced the heavier elements.

This giant space worm is about to eat a satellite

The observable universe is a sphere with a diameter of 93 billion light years. It is believed to be mostly composed of dark energy and matter. There are probably more than 170 billion galaxies. The universe could end in a Big Crunch, Big Freeze, or Big Rip. It will end in a Big Crunch if the mass density of the universe is greater than the critical density; otherwise, in a Big Freeze. An alternative is that dark energy will tear it apart in a Big Rip.

Here is how we know about the Big Bang. In 1912, American astronomer Vesto Slipher determined that light from other galaxies is redshifted (shifted to longer wavelengths – an example of the Doppler effect). In 1915 Einstein published his field equations for general relativity, which Belgian Catholic priest Georges Lemaître used to propose that the universe is expanding in 1927. This found empirical confirmation in 1929, when American astronomer Edwin Hubble, building on Slipher’s observations, formulated Hubble’s Law: the further away a galaxy, the faster it is receding from us. Therefore either we are at the centre of exploding galaxies or the universe is uniformly expanding everywhere. Fred Hoyle, a proponent of steady state theory who was critical of the idea that the universe had a beginning, coined the term Big Bang in 1949. The theory was confirmed with the discovery of cosmic background radiation by Americans Arno Penzias and Robert Wilson in 1964. It finds further support in the abundance of light elements and in the morphology and distribution of galaxies.

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In the Beginning

I WANT TO COMPLETE SOMETHING before I die. It’s a notebook that contains broad historical overviews. It’s skeletal – sparse on detail – and biased toward my interests. I wrote it for me. Since we’re soon to be space dust, I’m going to write it out here. I found it crazy that I knew so little about this world when the information is out there. How old is the Earth? What is life? How long has our species been around? With these questions in mind, I called my notebook EVERYTHING THAT YOU SHOULD KNOW AT ALL TIMES.

Why at all times? I’ll answer that at the end of these posts.

Here’s the first page:


Big Bang 13.73 Ga

Milky Way 13.2 Ga

Sun 4.57 Ga

Earth 4.54 Ga

Moon 4.5 Ga

Life 3.5 Ga

Man 200 ka

Space 1961

Notes for readers who are not me:

Ga means “billion years ago”.

ka means “thousand years of ago”.

13.2 refers to the oldest known star in the Milky Way.

“Man” refers to the emergence of the species Homo sapiens, not of the genus Homo.

1961 is when the first man – Soviet astronaut Yuri Gagarin – entered space.

Next post: the Big Bang

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