Astronomical Terms Rotation –Motion around an axis –Produces day/night From: physicalgeography.net.

Astronomical Terms

• Rotation– Motion around an axis– Produces day/night

From: physicalgeography.net

Astronomical Terms

• Revolution– Motion around another object; orbiting– Produces seasons

From: science-class.net

Astronomical Terms

• Orbital period– Time required for one revolution– Length of one year for a given

planet

school-for-champions.com

Astronomical Terms

• Orbital distance– Average distance from the Sun

Solar system shown to size and distance scale

From: commons.wikimedia.org

Astronomical Terms

• Astronomical unit (AU)– Distance from Earth to Sun

– Used as a “measuring stick” for solar system distances

From: wchs-astronomy.wikispaces.com

Astronomical Terms

•Parallax– Apparent shift of

objects due to motion of observer

– Stellar parallax:

apparent shift of stars due to Earth’s movement around Sun

Astronomical Terms

• Geocentric: Earth-centered

• Heliocentric: Sun-centered

ORIGINS OF ASTRONOMY

• Ancient Greece (600 BCE – 150 CE)

• Islamic Golden Age (700s-1400s)

• European Renaissance (1400s-1700s)

• Earth Motions

• Earth-Moon System

Ancient Greece

(600 BCE – 150 CE)

PSCI 131: Origins of Astronomy –Ancient Greece

Assumptions of the Ancient Greeks

• Universe– Earth– Seven “planetai”– Stars

• Geocentrism

• Celestial immutability

PSCI 131: Origins of Astronomy – Ancient Greece

From redorbit.com

Important Discoveries• Earth is spherical

– Aristotle

– Observation of lunar eclipses

– What evidence did he observe?

PSCI 131: Origins of Astronomy – Ancient Greece

From 8planets.co.uk

Important Discoveries• Earth is spherical

– What would Earth’s shadow look like if Earth were a flat disk?

PSCI 131: Origins of Astronomy – Ancient Greece

? ? ?

Important Discoveries• Earth’s circumference

– Eratosthenes

– Measuring angle of shadows

– Basic geometry

PSCI 131: Origins of Astronomy – Ancient Greece

From 3villagecsd.k12.ny.us

Eratosthenes, 276-194 BCE

Important Discoveries: Earth’s circumference

• Two ways to measure circumference– Degrees and length (stadion: ancient unit of length)

• Earth’s circumf. = 360 degrees = ____ stadia

• Need distance between two points in degrees and stadia– Distance from Syene to Alexandria in stadia was known to be

5000 stadia (about 480 miles)

PSCI 131: Origins of Astronomy – Ancient Greece

?

Important Discoveries: Earth’s circumference

PSCI 131: Origins of Astronomy – Ancient Greece

• How did Eratosthenes find distance in degrees?– Angle of shadows in both cities on same day– At Syene: no shadow– At Alexandria: shadow made 7 degree angle

Important Discoveries: Earth’s circumference

PSCI 131: Origins of Astronomy – Ancient Greece

• Since 360°÷ 7°= about 50, Earth’s circumference must be about 50 times the Syene-Alexandria distance

• 5000 stadia x 50 = 250,000 stadia or 24,500 miles (actual: 24,901 mi)

Important Discoveries• Heliocentric (sun-centered) solar

system

– Aristarchus

– Sun much larger than Earth, further away than Moon

– Basic geometry

PSCI 131: Origins of Astronomy – Ancient Greece

From amazing-space.stsci.edu

Important Discoveries• Rejection of heliocentric model

– Predicted stellar parallax, but none was observed

• Need a telescope to see it

– Aristotle championed geocentric model

PSCI 131: Origins of Astronomy – Ancient Greece

Important Discoveries• Ptolemaic System

– Ptolemy

– Geocentric model

– First organized explanation of celestial motion

PSCI 131: Origins of Astronomy – Ancient Greece

Important Discoveries: Ptolemaic System

• Deferent: orbital path• Epicycles: smaller circles

– Used to explain retrograde motion

PSCI 131: Origins of Astronomy – Ancient Greece

Important Discoveries: Ptolemaic System

• Retrograde motion: “backward” motion of a planet along its orbital path

PSCI 131: Origins of Astronomy – Ancient Greece

The Ptolemaic System used epicycles to explain retrograde motion

Important Discoveries: Ptolemaic System

• Retrograde motion is really an illusion

PSCI 131: Origins of Astronomy – Ancient Greece

Earth passes Mars in its orbit; Mars appears to move backward as seen from Earth

Islamic Golden Age

(700s – 1400s)

PSCI 131: Origins of Astronomy

• Preserved and expanded work of ancient Greeks and Indians

• Detailed star catalogs, many observatories

• Most star names are Arabic– Altair, Deneb, Aldebaran, etc.

European Renaissance

(1400s – 1700s)

PSCI 131: Origins of Astronomy –European Renaissance

PSCI 131: Origins of Astronomy – European Renaissance

Timeline: Nicolaus Copernicus

1400 1776

1450First

printing press

1473 1543

Copernicus

1492

PSCI 131: Origins of Astronomy – European Renaissance

Nicolaus Copernicus

• “Copernican Revolution”

• Heliocentrism

• Set stage for later discoveries

PSCI 131: Origins of Astronomy – European Renaissance

Timeline: Tycho Brahe

1400 1776

1450First

printing press

1473 1543

Copernicus

1492 1546 1601

Brahe

PSCI 131: Origins of Astronomy – European Renaissance

Tycho Brahe• Very precise astronomical

observations– Without a telescope

• Geocentrist– Apparent absence of stellar

parallax

PSCI 131: Origins of Astronomy – European Renaissance

Timeline: Johannes Kepler

1400 1776

1450First

printing press

1473 1543

Copernicus

1492 1546 1601

Brahe

1571 1630Kepler

PSCI 131: Origins of Astronomy – European Renaissance

Johannes Kepler• Brahe’s assistant

• Used Brahe’s observations

– To prove heliocentrism

– To develop laws of planetary motion still in use today

PSCI 131: Origins of Astronomy – European Renaissance - Kepler

First Law of Planetary Motion• Orbits are ellipses, not circles

• Sun located at one focus of the ellipse

The pushpins are at the foci of the ellipse

PSCI 131: Origins of Astronomy – European Renaissance - Kepler

First Law of Planetary Motion• Orbits are ellipses, not

circles

• Sun located at one focus of the ellipse

From: csep10.phys.utk.edu

PSCI 131: Origins of Astronomy – European Renaissance - Kepler

Second Law of Planetary Motion

• Law of equal areas

• During a given amount of time, a planet will always “sweep out” the same area in its orbit over that time

• http://www.youtube.com/watch?v=_3OOK8a4l8Y From: dallaskasaboski.blogspot.com

PSCI 131: Origins of Astronomy – European Renaissance - Kepler

Second Law of Planetary Motion• Orbital speed varies inversely with orbital distance

From: http://www.wallpaperswala.com/solar-system/

Slower orbital speed

PSCI 131: Origins of Astronomy – European Renaissance - Kepler

Third Law of Planetary Motion• Orbital period proportional to orbital distance

• p2 = d3

• p: orbital period in Earth years• d: orbital distance in astronomical units (AUs)

• Example: if p = 8 yrs, d = 4 AUs

PSCI 131: Origins of Astronomy – European Renaissance - Kepler

Laws of Planetary Motion

• Significance

– First mathematical model of celestial motion

– Not based on philosophical assumptions

PSCI 131: Origins of Astronomy – European Renaissance

Timeline: Galileo Galilei

1400 1776

1450First

printing press

1473 1543

Copernicus

1492 1546 1601

Brahe

1571 1630Kepler

1564 1642Galileo

PSCI 131: Origins of Astronomy – European Renaissance

Galileo Galilei• “Father of modern

observational science”

• Improvements to telescopes– Did not invent them

• First astronomer to use telescope

PSCI 131: Origins of Astronomy – European Renaissance - Galileo

• Discovered four of Jupiter’s moons– Io, Ganymede, Callisto,

Europa

– First observation of moons other than Earth’s

– Showed that Earth isn’t unique in having a natural satellite

From: en.wikipedia.org

Sizes of the Galilean moons shown relative to Jupiter

The Galilean Moons

PSCI 131: Origins of Astronomy – European Renaissance - Galileo

From: splung.com

Galileo’s notebook showing changing positions of Galilean moons over two weeks

Jupiter and the Galilean moons through a small telescope, as Galileo saw them

From: physics.wisc.edu

The Galilean Moons

PSCI 131: Origins of Astronomy – European Renaissance - Galileo

The Phases of Venus• Observed that Venus goes through

phases like Earth’s moon

– Proof of heliocentrism

From: oneminuteastronomer.com

http://astro.unl.edu/classaction/animations/renaissance/venusphases.html

PSCI 131: Origins of Astronomy – European Renaissance - Galileo

Sunspots

From: galileo.rice.edu From: galileo.rice.edu

Series of Galileo’s 1612 sketches in a “flip-book” animation Sunspots through a modern telescope

• Refuted celestial immutability

PSCI 131: Origins of Astronomy – European Renaissance

Timeline: Isaac Newton

1400 1776

1450First

printing press

1473 1543

Copernicus

1492 1546 1601

Brahe

1571 1630Kepler

1564 1642Galileo

1727

Newton

1642

PSCI 131: Origins of Astronomy – European Renaissance

Sir Isaac Newton• Law of Universal Gravitation

• Law of Inertia

• Explained why planets move in orbits

PSCI 131: Origins of Astronomy – European Renaissance - Newton

Universal Gravitation

• All objects in the universe exert an attractional force on each other

• Force increases with mass and decreases with distance

PSCI 131: Origins of Astronomy – European Renaissance - Newton

Universal Gravitation

PSCI 131: Origins of Astronomy – European Renaissance - Newton

Law of Inertia(Newton’s First Law)

• An object in motion continues in motion with the same speed and in the same direction unless acted upon by an external force

From: drcruzan.com

PSCI 131: Origins of Astronomy – European Renaissance - Newton

Gravity + Inertia = Orbit

An orbit is a balance between gravity and inertia

Earth Motions

PSCI 131: Origins of Astronomy –Earth Motions

Revolution

•Period of revolution: 365.25 days– Leap year

•Perihelion: closest to Sun (January)•Aphelion: furthest from Sun (July)•Solstices & Equinoxes

PSCI 131: Origins of Astronomy –Earth Motions

Revolution: Perihelion & aphelionPSCI 131: Origins of Astronomy –Earth Motions

From: universetoday.com

Revolution: Solstices & equinoxesPSCI 131: Origins of Astronomy –Earth Motions

From: oceanservice.noaa.gov

Precession•Wobble of Earth on its axis; 26,000-yr cycle

PSCI 131: Origins of Astronomy –Earth Motions

The Earth-Moon System

PSCI 131: Origins of Astronomy –Earth-Moon System

The Earth-Moon System

• Mean Earth-Moon distance: ~238,000 mi

• Period of rotation: 27.5 days• Period of revolution: 27.5 days• “Synchronous” orbit

– Same side always faces Earth

PSCI 131: Origins of Astronomy –Earth-Moon System

Lunar Phases

•Half of lunar surface always lit– Exception: lunar eclipse

•Current phase depends on how much of lit surface is visible from Earth

PSCI 131: Origins of Astronomy –Earth-Moon System

PSCI 131: Origins of Astronomy – Earth-Moon System-Lunar Phases

End of Origins of Astronomy

PSCI 131: Origins of Astronomy