ASTRONOMYNot to be confused with Horoscopes and Zodiac signs.

The subject of Astronomy

Astro- means “pertaining to stars or celestial bodies”. -nomy means “distribution, arrangement, or management” The field of astronomy began when the earliest humans

gazed to the heavens and pondered just what it was that they were seeing.

The earliest astronomers concocted stories that attempted to explain how terrestrial events were tied to celestial events.

In this way, both astronomy and astrology were born. Astronomy differs from Astrology in that one is based on

observable and quantifiable data. The body of knowledge in this field has grown through the centuries due to measurable and reproducible investigations. The other is more subjective.

Early Astronomers

The earliest astronomers were the Greeks and their study of the heavens occurred between 600 BCE– 150 CE.

The early Greeks astronomers held a geocentric view, believing the Earth was stationary and all known bodies (the moon, sun, Mercury, Venus, Mars, Jupiter, and Saturn) were in orbit around the Earth.

They believed there existed beyond the planets a transparent, hollow sphere (celestial sphere) on which the stars traveled daily around the Earth.

Geocentric view

Early Astronomers (continued) The Greeks believed the Earth was too

large to move, and exhibited no evidence of motion, so this was the basis of their view.

The planets observed were known to wander (planetai is Greek for “wanderer”) across the sky, with the rest of the universe seeming frozen in place.

Aristotle (384-322 BCE) deduced the Earth must be spherical, because it always cast a curved shadow on the Moon during an eclipse.

Breaking with tradition

Aristarchus (312-230 BCE) proposed a heliocentric model for what we know as our solar system.

He also used basic geometric principles to measure the size of the Earth and Moon, as well as their relative distance to the Sun.

Unfortunately, because his calculations were in error, all proposals made by Aristarchus were roundly dismissed as incorrect and the geocentric model persisted.

More about the early Astronomers Eratosthenes (276-194 BCE) was the first to

accurately measure the size of the Earth. He measured the angle of the Sun in two Egyptian

cities that lie roughly North and South of one another (Aswan and Alexandria).

The difference in angles was 7 degrees, or 1/50 of a circle, so he deduced that the circumference of the Earth was 50 times the distance between the 2 cities.

His estimate was the Earth’s circumference is 24,428 miles.

The actual circumference is 24,902 miles. This percent error is 1.9%.

More about the early Astronomers

The greatest and most productive Greek astronomer was Hippocarpus.

He charted more than 850 stars, developed a method of measuring brightness, and grouped them according to that brightness.

He also determined, to within minutes, the actual length of a year.

All of these discoveries were done prior to the development of the telescope, the calculator, a computer, and an accurate time-piece.

The Ptolemaic System

Ptolemy published Almagest (“the great work”) in 141 CE, which was able to accurately predict planetary motion.

In this model, the planets were believed to orbit the Earth in perfect circles (which the Greeks considered the pure and perfect shape).

It was though this tome that retrograde was identified and explained.

Planetary retrograde

Modern Astronomy

Nicholas Copernicus (1473-1543) first proposed a model that made the Earth one of the planets orbiting the Sun.

Further, after observing planetary motion that defied a circular orbit, Copernicus predicted elliptical orbits for some of the planets.

Copernicus published his predictions and findings in a manifesto that was published as he lay dying.

These predictions were determined by the Church to be heretical.

Giordano Bruno attempted to defend the findings of Copernicus, but was tried during the Inquisition, found guilty of heresy, and burned at the stake.

Modern Astronomy (continued) Tycho Brahe (1546-1601) established an

observatory in Copenhagen, Denmark. This observatory served as a model for accurate and precise measurements of celestial bodies.

Brahe also noted that a phenomenon of a parallax, which made stars appear to move slightly over time.

When the Ruler of Denmark died, Brahe lost his patron and moved to the Czech Republic to continue his work.

Parallax

Modern Astronomy (continued) Johannes Kepler worked under Brahe

and continued his work after his death. Kepler developed three laws of

planetary motion – The path of each planet is an ellipse with

the Sun at one of the foci. Each planetary orbit covers an equal area

in an equal amount of time. The orbits of planets and their distance to

the Sun are proportional. (p2 = d3)

Kepler’s 1st and 2nd Laws

Modern Astronomy (continued) Galileo Galilei (1564-1642) heard about a

Dutch invention that used lenses to magnify distant objects.

Without ever seeing an example, Galileo constructed his own telescope (30x) and began making observations of the night sky.

With this telescope, Galileo discovered: Four moons orbiting Jupiter, That planets possess a circular shape (Earth-like), Venus has phases just like the Moon, The pock-marked surface of the Moon, Sunspots.

Galileo (continued)

Galileo proposed, but never demonstrated from the Leaning Tower of Pisa, that objects fall at the same rate. (Gravity is a constant)

In 1630, Galileo published his opus (Dialogue of the Great World Systems) and traveled to Rome to ask for permission to publish.

After publication, it was realized just what he was proposing and Galileo was ordered to refute his work.

He refused, was arrested, tried, and convicted of proclaiming doctrines contrary to religious doctrine. He was sentenced to house arrest and died 10 years later.

In 1992, the Church formally overturned Galileo’s conviction.

Sir Isaac Newton

Born the year of Galileo’s death (1643), Newton was the first to accurately identify, describe, and quantify the force of gravity.

He also defined inertia and described its effect on the movement of objects.

Additionally, he explained variances in planetary periods (perturbation) by accounting for the gravitational forces by nearby planets.

Other Astronomers

The Mayan people combined astronomical observations with religious beliefs.

Along with the Aztecs, Mayan astronomers refined other Mesoamerican calendars.

Many of their observations dealt with periods.

Modern (days)    Maya in days Ptolemy Lunar month 29.53059  29.53086  29.53337 Period of Venus 583.93 583.92027  583.94267 Period of Mars 779.94 780 779.94 Solar year 365.24198 365.242 365.24667

Other Astronomers

Aztecs refined a calendar, which allowed for ceremonial observations on specific days.

Druids appeared more than 300 years BCE.

They left many structures with which astronomical measurements could be made.

Much speculation surrounds their culture and practices, but their observatories leave little doubt that they were keen astronomers.

Druid structures

Egyptian Astronomers Egyptian Astronomers are recorded as early as

5000 BCE. Early observations were concerned with plotting

movement of celestial objects during the pre-Dynastic period using stone circles much like Stonehenge.

During the Dynastic period, a 365 day calendar was developed and used to chart, record, and predict annual flooding of the Nile.

The pyramids at Giza are aligned with (what was then) the pole star. Because of variations in the tilt of the Earth, our current pole star (Polaris) is different from what was the pole star during this period.

The Temple of Amun-Re was aligned with the midwinter Sun.

These structural observatories allowed early Egyptians to accurately predict seasons and perform ceremonial rites during specific periods.

Egyptian Astronomers (cont.) During the last century BCE,

Alexandria, Egypt was a primary site for scientific investigation.

The brightest supernova (exploding star) was noted and described in 1006 from a location in Egypt.

Many astronomical instruments used in Egypt and elsewhere were developed by Egyptian mathematicians.