Astronomy at Grade 6

The “Big Ideas”

Grade 6 Astronomy – Big Ideas

• Develop an understanding of the position of Earth in Space with respect to its astronomical “neighbours”

• Investigate space research programs involving the human presence in space

• Investigate the astronomical causes (motions) related to day/night, the seasons, moon phases, and eclipse events

Grade 6 Astronomy – Big Ideas

• Investigate and describe points of reference in the night sky (e.g., constellation patterns), and account for the seasonal changes observed

• Examine changing conceptions, historically, about Sun, Earth, Moon and Stars (NoS)

• Provide descriptions of certain historical and cultural views on the cosmos (e.g. cultural astronomy, astrology) (NoS)

A Brief History of Ideas About the Universe

The Greek Mind….

• Much of the Greek method of thinking revolved around philosophy instead of scientific reasoning

• Greeks valued perfection and therefore any models of the universe should involve the perfect shape, the circle

• Greeks also had no reason to believe that the Earth was not the center of the universe. Completely reasonable at the time – and still reasonable today!

• The only 'scientific' data they had available to them was the motion of the Sun, Moon, and planets, which were monitored heavily at the time

Ptolemy ~140 AD

An Earth Centred Cosmos

The Motion of the Planets…A Problem Emerges

• A model of the universe would be very simple except for the fact that the planets undergo a “looping” motion in their movements as seen from Earth

• Remember, in one night, all planets still rise in the east and set in the west

• However, if you keep track of the planet's position versus the background stars night to night, you will see the planet 'move'

• The word 'planetos' means wanderer in Greek

Retrograde Motion

Jupiter and Saturn (June 2000 – May 2001)

Ptolemaic Model

• In order to produce the retrograde motion of the planets, Ptolemy created a model with little circles called epicycles

• All the planets orbited the Earth in a perfect circle

• The planet itself made a smaller orbit centered upon the larger orbit around the Earth

• With the right timing, this model can reproduce the retrograde motion seen from Earth

The “Geocentric” Model

Geocentric model and the “backwards” motion of Mars

Ptolemaic Model• In Ptolemy's complete model,

each planet had its own orbit around the Earth with its own epicycle– By changing the period of the

orbit and the epicycle, the model could match observations relatively well

• The Sun and the Moon traveled around the Earth in perfect circles

• The entire model was composed of more than 80 circles and was very complicated

‘Simplified’ Ptolemaic Model

Ptolemy’s Model Survives

• Since Ptolemy’s model matched observations sufficiently and no contrary evidence was produced, it was supported for nearly 1,500 years!

• After all, if the Earth was moving, shouldn't we feel it?

• On top of all this, the Dark Ages provided relatively little advance in any natural philosophies (sciences) for Europe

Astronomy in the ‘Dark Ages’

• While Europe was enduring the “Dark Ages”, Islamic nations were experiencing a kind of ‘golden age’ of astronomy

• Much of the knowledge of the Greeks was preserved and expanded upon during this time

• Many of the proper names for stars are relics of this era– Rigel, Betelgeuse, Vega ...

and Zuben Al-genubi

“Flat Earth” Models ?

There was never a time in history when the concept of a flat (disc-shaped) Earth was widely held, either by priests, prophets, poets, philosophers, scientists,

or the common people…..it would be a myth to assume this…..and we cannot perpetuate this myth

with our students. The idea has come almost exclusively from literature.

So, what was known back then by the Greek and Arab Cultures….and then

lost?

• The earth is round• Circumference/diameter of the earth; distance to

the moon• The solar system is heliocentric (“sun-centred”)• An estimate of the distance to the sun (while

wrong, the Sun was much further away than commonly thought)

• Length of the year to a high degree of accuracy

Nikolai Kopernik (1473-1543)

• “Who in this most beautiful of temples would put His lamp at a better place than from where it can illuminate them all?. Thus the Sun sitting as on a Royal throne, leads the surrounding family of stars!”

The Copernican Revolution

• At the end of the Dark Ages, a Polish Catholic priest named Nikolai Kopernik (Copernicus) came up with a new model of the universe where the Earth was no longer at the center

• The heliocentric (Sun centered) model placed the Earth out of its central position, yet still maintained many of the observations we see

• The perceived elegance in his model was that it was simpler than Ptolemy’s model, and explained all the stuff going on very well Nikolai Kopernik (Copernicus) (1473-1543)

Position of Mars over Months….

Motion of Mars as explained by the HELIOCENTRIC model….

The Copernican Model – Retrograde Motion

In the Copernican model, retrograde motion is an apparent effect caused by the Earth 'overtaking' an outer planet in its orbit

The Copernican Revolution

• Even though the Copernican model was a ‘simpler’ representation of the solar system, it was not widely accepted due to the incredible hold Ptolemy had on the minds of 16th century astronomers

• While it did provide a much simpler description compared to Ptolemy, it did not necessarily improve the predictive power of the model

• The religious ideas of the time insisted upon Earth being the center of the universe (why would God demote us away from the centre of creation?)

• Copernicus’ works were published in Latin, which was unreadable by the common public who had no theological training

Galileo - The Observer

• A century after Copernicus’ work, other “natural philosophers” began to make strides toward making the heliocentric model popular among ordinary people

• Galileo was the first to use a telescope to make detailed observations of the sky – he was as famous in Italy then as David Suzuki is now in Canada

• Though he did not invent the telescope, he made many working prototypes and trained them on a variety of celestial bodies Galileo Galilei (1564-1642)

Galileo's Observations - I

• Galileo used his telescopes to make observations of many heavenly objects

• The sketch to the right shows Galileo's observations of the four largest moons of Jupiter

• He noticed that the position of these four moons changed night to night, as if they were revolving around Jupiter

• These moons now bear his name– The “Galilean” moons of Jupiter are:

• Io• Europa• Ganymede• Callisto

Galileo's Observations - II

• Galileo also noticed that Venus was not simply a point of light, but actually a disk – this was NEW!!

• He watched Venus go through complete phases, just like the Moon

• This cycle of phases can only be satisfied by the heliocentric model, not the geocentric one – a critical TEST! The phases of Venus

Galileo's Observations - III

• Galileo also pointed his telescope toward the Sun– NEVER DO THIS – DUMB!!

• He discovered that the disk of the Sun was not perfect and was occasionally dotted with small black spots

• By making daily sketches of these spots, he was able to determine that the Sun itself was rotating – WOW!

Galileo's Influences….

• All of Galileo's observations were pointing towards a HELIOCENTRIC view of the universe

• Galileo published his observations and conclusions in multiple works, including some published in Italian to appeal to a wider audience

• Galileo’s popularization of new ideas about the cosmos threatened the status quo, and he was forced to stop publicly teaching in favour of the heliocentric model, and sentenced to house arrest for the rest of his life; he communicated with his daughter regularly through letters (she had been given over to cloistered monastic life at age 13)

• The seeds of the Copernican Revolution, however, had been planted and by the mid-17th century, virtually all philosophers and astronomers were committed to the Copernican model