Lecture 2: Some Historical

Perspective

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Announcements

The first homework is now accessible via the class website: http://localgroup.ps.uci.edu/cooper/Cosmology18S/assignments.html

Due Wednesday, April 11th by 5pm in the P20B (wooden) dropbox in the hallway outside of FRH 4129 — see details included in Lecture 1.

Announcements

Homework #1

Thales (~624 BCE - 546 BCE)

•Lived in Miletus and was one of the Seven Sages of Greece •Engineer, Businessman, and Politician: credited with inventing the potter’s wheel and the anchor, selling olive oil, etc.

•Favored a radical idea: humans can understand the world according to logic rather than myth. e.g. postulated that earthquakes are caused by waves hitting the Earth (and not by gods fighting).

Democritus (~400BCE)

• Considered by some to be the “father of modern science” –proposed that many worlds existed throughout the cosmos (other stars were other Suns with planets).

–some of those worlds populated with life. –believed everything is made of indivisible units he called “atoms”. Everything, even the human soul, consisted of atoms.

• People 2500 years ago were not lacking ideas. What they lacked was the technology to test those ideas.

Aristotle (384 BCE - 322 BCE)

•Argued that the Earth was round, not flat.

As we discussed on Monday, the cosmological models adopted by many ancient cultures included a spherical Earth.

Aristotle (384 BCE - 322 BCE)

•Argued that the Earth was round, not flat.

wait, so why don’t we fall off ???

Aristotle (384 BCE - 322 BCE)

•Argued that the Earth was round, not flat.

wait, so why don’t we fall off ???

On the Heavens: 1a. Earthly material (air, earth, fire, water) tends to fall towards the center of the Universe. 1b. Thus, Earth is at the center of everything. 2. The heavens are made of perfect material called aether & move in perfect shapes (circles). 3. Earth is fixed (no motion or spinning); star, planets revolve around us on (55) spheres.

Aristarchus (310 BCE - 230 BCE)

• Put forth first heliocentric theory. • Measured size of & distance to Moon/Sun

(w/measurements of an eclipse plus geometry) • Since Sun is much larger, it must be center of Universe. • This idea was not generally accepted.

Aristarchus (310 BCE - 230 BCE)

• Put forth first heliocentric theory. • Measured size of & distance to Moon/Sun

(w/measurements of an eclipse plus geometry) • Since Sun is much larger, it must be center of Universe. • This idea was not generally accepted.

Why wasn’t this idea accepted?

Aristarchus (310 BCE - 230 BCE)

• Put forth first heliocentric theory. • Measured size of & distance to Moon/Sun

(w/measurements of an eclipse plus geometry) • Since Sun is much larger, it must be center of Universe. • This idea was not generally accepted.

Why wasn’t this idea accepted?

Because this would imply the Earth is moving and we don’t

observe stellar parallax.

Parallax

As you move, stationary objects appear to shift in the opposite direction.

However, some objects do not appear to shift…why?

When things are very far away, they appear to be motionless.

There’s an anti-correlation between distance from you and apparent motion.

This is perhaps a better example, where instead of stars shifting we see

mountains shifting.

If Earth is moving, then we should see nearby stars shift relative to the background

(more distant) stars…just like for these mountains.

Parallax

As you move, stationary objects appear to shift in the opposite direction.

Parallax

Stellar ParallaxUse the earth’s motion as a baseline to measure the distance to a nearby star

As the earth moves around the sun, a nearby star will appear to move slightly relative to distant background stars

What you see in the sky

Stellar ParallaxUse the earth’s motion as a baseline to measure the distance to a nearby star

As the earth moves around the sun, a nearby star will appear to move slightly relative to distant background stars

What you see in the sky

June

June

Stellar ParallaxUse the earth’s motion as a baseline to measure the distance to a nearby star

As the earth moves around the sun, a nearby star will appear to move slightly relative to distant background stars

What you see in the sky

December

December

Stellar Parallax

What you see in the sky

Use the earth’s motion as a baseline to measure the distance to a nearby star

As the earth moves around the sun, a nearby star will appear to move slightly relative to distant background stars

Stellar Parallax

What you see in the sky

June

June

Use the earth’s motion as a baseline to measure the distance to a nearby star

As the earth moves around the sun, a nearby star will appear to move slightly relative to distant background stars

Stellar Parallax

What you see in the sky

December

December

Use the earth’s motion as a baseline to measure the distance to a nearby star

As the earth moves around the sun, a nearby star will appear to move slightly relative to distant background stars

Stellar Parallax

A star’s parallax is defined as one-half of the angular shift of the star over the course of a year.

One parsec is the distance to a star whose parallax angle is one arcsecond

parsec = parallax second of arc

1 parsec = 3 x 1013 km

Distance (in parsecs) = 1 / parallax angle (in arcseconds)

(or, d = 1/p)

George Lucas’ Poor Screenwriting…[as if creating Jar Jar Binks wasn’t bad enough]

It’s the ship that made the Kessel Run in less than twelve parsecs!

(Writing credit: George Lucas, 1977)

Stellar parallaxesFrom ground-based telescopes, parallaxes can be measured for stars out to distances of around 200 parsecs.

This corresponds to just a tiny portion of the Milky Way galaxy!

Stellar Parallax

All stars have parallaxes smaller than 1 arcsecond.

The nearest star to the Sun, Proxima Centauri, has parallax 0.77 arcseconds

A parallax angle of ~0.3 arcseconds, is equivalent to the apparent size of a dime as viewed from ~10 miles away.

Parallax: Evidence for/against the heliocentric model

The Greeks never saw such a shift in the stars’ positions. As such, Aristotle’s Earth-centered model prevailed.

The only way that this could be so is if the stars are very very far away.But this seemed unlikely, as it would imply that the Universe was very large… - We now know that the Universe is very large.

A Problem with Aristotle’s Model: The Retrograde Motion of Mars

for more see https://mars.nasa.gov/allaboutmars/nightsky/retrograde/

A Problem with Aristotle’s Model: The Retrograde Motion of Mars

Mars is following a longer orbital path and moving at a slower speed along that orbit.

The slower Vorbit for Mars is not fully understood until we understand gravity.

Claudius Ptolemy (90-168 CE)

Added circular orbits on top of circular orbits.

Claudius Ptolemy (90-168 CE)

Claudius Ptolemy (90-168 CE)

-The planets also seemed to change speeds at times. In reality, this occurs because the orbits are not circular.

-To explain this, Ptolemy assumed the planets rotated about a different point from that of Earth, called an “equant”.

Ptolemy’s model of the Universe was the accepted picture in the Western world for the next ~1400 years.

Claudius Ptolemy (90-168 CE)

Nicolaus Copernicus (1473-1543)

- A Polish priest who was among the first scholars to have his own private library. - Found Ptolemy’s model to be too ugly. - Wanted Sun at the center, “from which it can light up everything...” and planets on concentric spheres, orbiting in circles around the “lantern of the universe”. - Easily explained retrograde motion, but failed to predict position of planets in the sky. - This idea would eventually prevail....with time.