Skyward Bodies

Skies often fill our field of vision. By night, starlight punctuates the panorama. By day, clouds drift through a river channel of saturation and hue by minutes and hours, and then we imagine, over centuries.

To jump over centuries
In one step is impossible.
Jump too high or far,
You’ll be way too late.
― Dejan Stojanović

People, poets, and philosophers have long imagined clouds traveling over the landscape, including the noted Sanskrit poet Kālidāsa. Meghadūta, Sanskrit for cloud messenger, is a lyrical poem written by Kālidāsa in 375 CE.


Kālidāsa's poem portrays demigod Yakṣha who has been condemned by his master for neglecting his duties and exiled from his home in the Himalaya mountains. Yaksha wishes to comfort his wife. He entreats a passing cloud to carry a message to her. Yakṣha describes the glorious sights that will be revealed to the cloud as it travels back to the Himalayas where his wife awaits his return. In this stanza, Yaksha recounts a mountain stream personified as a loving woman:

The neighboring mountain stream that gliding grants
A glimpse of charms in whirling eddies pursed,
While noisy swans accompany her dance
Like a tinkling zone, will slake thy loving thirst--
A woman always tells her love in gestures first.

For Ptolemy (AD 100 – c. 170), clouds and stars pass to and fro overhead much to his sensory delight.

I know that I am mortal by nature, and ephemeral; but when I trace at my pleasure the windings to and fro of the heavenly bodies I no longer touch the earth with my feet: I stand in the presence of Zeus himself and take my fill of ambrosia
― Claudius Ptolemy

REFERENCES

• Almagest, by Claudius Ptolemy, AD 150.
• Circling: 1978-1987, by Dejan Stojanović.
• Cloud Messenger, by Kālidāsa, 375 CE.

Extraterrestrial Worldview

Earth orbits the Sun at sixty-six thousand miles per hour needing 365¼ days to notch a round trip.

The conceptual model of Earth's extraterrestrial relationship to the cosmos has changed since antiquity. Natural scientists and philosophers have put forth three paradigms:

1. The Sun, Moon, stars & naked-eye planets circle the Earth;
2. The Earth & planets circle the Sun; and
3. The Earth & planets orbit the Sun in an elliptical path.

Period	Model	Belief	References
350 BCE to 1500	Geocentric	The astronomical system of many civilizations, including ancient Greece, held that the Sun, Moon, stars, and planets circled Earth.	Aristotle's cosmological treatise On the Heavens, recorded in 350 BC, and Claudius Ptolemy's astronomical treatise Almagest, recorded in 150 AD, consider a stationary Earth as center of the universe.
16th C	Heliocentric	The Copernican Revolution held that Earth and planets follow circular orbits around the Sun.	Nicolaus Copernicus published On the Revolutions of the Heavenly Spheres in 1543. Copernicus provided the seminal work of Renaissance astronomy that describes the paradigm-shifting heliocentric model.
17th C	Elliptic orbits	Earth, a small body, orbits the Sun, a large body following an elliptical path, with the Sun located at one of the foci of an ellipse.	Johannes Kepler improved on Copernicus's heliocentric theory. Kepler's first law of planetary motion (published between 1609 and 1619) describes the elliptical path of planets traveling at variable speeds around the Sun.

American philosopher John Dewey wrote of the Copernican Revolution:

Knowledge falters when imagination clips its wings or fears to use them. Every great advance in science has issued from a new audacity of imagination.
— John Dewey

Heliocentric
Mars & Earth circling Sun

Geocentric
Mars & Sun orbiting Earth

Kepler's elliptical orbit is our current conceptual model. Planets orbit the Sun on an elliptical path with the Sun located at one foci of the ellipse.

The Sun shown as the foci of the elliptical orbit of a planet.

Reflecting on Copernicus's paradigm-shattering hypothesis in The Quest for Certainty, John Dewey cautions us against inflexible beliefs and urges us to beware obstructions to advancing our worldview.

It has to search out and disclose the obstructions; to criticize the habits of mind which stand in the way; to focus reflection upon needs congruous to present life; to interpret the conclusions of science with respect to their consequences for our beliefs about purposes and values in all phases of life.
— John Dewey

REFERENCES

• Copernican Revolution, Wikipedia.
• Heliocentrism, Wikipedia.
• The Quest for Certainty: A Study of the Relation of Knowledge and Action, John Dewey, Gifford Lectures 1929. London.
• Ptolemaic system, Britannica.

Earthly Rotation

Like a whirligig Earth rotates on its own axis.

Primitive observers from the inhabited middle latitudes of Earth would've have mused about something we rarely contemplate:

The Sun rises in the East
The Sun sets in the West

Like most celestial phenomena, Sunrises and Sunsets occur over a repeatedly observable and predictable cycle. During one day, the Sun appears to follow a half-circular arc through the sky.

As observers of the night sky our ancestors would've also noticed that stars moves along a similar circular path.

Our ancestors might have reasonably believed that the bespeckled night sky was an extraterrestrial backdrop that rotates.

"Before we invented civilization our ancestors lived mainly in the open out under the sky." ― Carl Sagan

Whirling Southern Star Trails over ALMA by ESO

Ancient ancestral astronomers might have hypothesized a stationary Earth centered about a rotating backdrop of stars.

"Mortal as I am, I know that I am born for a day. But when I follow at my pleasure the serried multitude of the stars in their circular course, my feet no longer touch the earth."
― Ptolemy, Almagest, a 2^nd century astronomical treatise

Today we know it's the Earth that's rotating on its axis that creates this illusion. How might we devise a model that matches our observations and proves the Earth is rotating?

Supporting Model

Foucault pendulum
at the North Pole
by Theresa Knott

A less intuitive but valid hypothesis is that the Earth is rotating on its own axis. Of course we know now that Earth rotates on its own axis, but we were helped by a model to clinch the deal of corroborating what others had observed for generations.

The classic model that helped prove Earth is rotating on its own axis is a simple experiment with a pendulum known as the Foucault pendulum.

Foucault Pendulum

The Foucault pendulum (pronounced foo-koh) was introduced in 1851 to visually demonstrate Earth's rotation. The experiment is named after French physicist Léon Foucault.

An experimental Foucault pendulum, commonly suspended 39–98 feet and freely swinging back and forth, will be visibly affected by Earth’s rotation.

Simulated pendulum rotation
by Dominique Toussaint

The affect is not visible after a few swings, rather becomes gradually visible by a matter of degrees over the course of a day.

To maintain the bob of a pendulum swinging in museum demonstrations, an electromagnetic or other drive is used, or the pendulum is manually restarted.

A pendulum day is the time needed for the plane of a Foucault pendulum to complete 360 degree rotation. The duration of a pendulum day is one day divided by the sine of the latitude.

"I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." ― Umberto Eco

REFERENCES

• But it moves! How we know the Earth rotates, Ethan Siegel,17 September 2010.
• Foucault pendulum, Wikipedia.
• Pale Blue Dot: A Vision of the Human Future in Space, Carl Sagan, Random House 1994.