Origins of Modern Astronomy PSCI 131 Astronomical Terms

Origins of Modern Astronomy PSCI 131

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) • European Renaissance (1400 s-1700 s)

PSCI 131: Origins of Astronomy –Ancient Greece (600 BCE – 150 CE)

PSCI 131: Origins of Astronomy – Ancient Greece Assumptions of the Ancient Greeks • Universe – Earth – Seven “planetai” (wanderers in Greek) – Stars • Geocentrism From redorbit. com

PSCI 131: Origins of Astronomy – Ancient Greece Question • Which planets are visible in the night sky without a telescope?

PSCI 131: Origins of Astronomy – Ancient Greece Important Discoveries • Earth is spherical – Aristotle (384 -322 BCE) – Observation of lunar eclipses • Since the Earth always casts a curved shadow on the moon it must be spherical From 8 planets. co. uk

PSCI 131: Origins of Astronomy – Ancient Greece What would Earth’s shadow look like if Earth were a flat disk? A B C

PSCI 131: Origins of Astronomy – Ancient Greece Important Discoveries • Earth’s circumference – Eratosthenes – Measuring angle of shadows – Basic geometry Eratosthenes, 276 -194 BCE From 3 villagecsd. k 12. ny. us

PSCI 131: Origins of Astronomy – Ancient Greece Important Discoveries • Earth’s circumference = 360 degrees = ____ stadia • Need distance between two points in degrees and stadia – Distance from Syene to Alexandria was known to be 5000 stadia (about 480 miles) • 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

6 km = 1 stadia

PSCI 131: Origins of Astronomy – Ancient Greece Important Discoveries • Heliocentric (sun-centered) solar system – Aristarchus (312 -230 BCE) – Sun much larger than Earth, further away than Moon – Basic geometry – First to attempt to measure the relative distance between the Earth-Moon and the Earth-Sun without the aid of trigonometry.

PSCI 131: Origins of Astronomy – Ancient Greece Important Discoveries • Rejection of heliocentric model – Predicted stellar parallax, but none was observed • Need a telescope to see it – Aristotle championed geocentric model • Stronger influence so geocentric model persisted for nearly 2000 years

PSCI 131: Origins of Astronomy – Ancient Greece Important Discoveries • Star catalog – Hipparchus (190 -120 BCE) – First to realize that ideas must be proven with empirical evidence (observation rather than logic) • Also realized that more data meant more certainty in the idea or model – Created highly accurate star atlases in an attempt to measure the length of the year more accurately • He measured the length of year to within 6. 5 minutes of actual time – He made more detailed corrections to locations and distances measured by Eratosthenes

PSCI 131: Origins of Astronomy – Ancient Greece Important Discoveries • Ptolemaic System – Claudius Ptolemy (100 -160 AD) – 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 The Ptolemaic System used epicycles to explain retrograde motion

PSCI 131: Origins of Astronomy – Ancient Greece Important Discoveries: Ptolemaic System • Retrograde motion is really an illusion Earth passes Mars in its orbit; Mars appears to move backward as seen from Earth

PSCI 131: Origins of Astronomy –European Renaissance (1400 s – 1700 s)

PSCI 131: Origins of Astronomy – European Renaissance Timeline: Nicolaus Copernicus 1400 Copernicus 1473 1543 1450 First printing press 1492 1776

PSCI 131: Origins of Astronomy – European Renaissance Nicolaus Copernicus • “Copernican Revolution” • After discovering Aristarchus’ writings he became convinced of the Heliocentric model – His model still contained the epicycles to explain retrograde motion • Published his book for the Suncentered solar system model as he lay on his deathbed. – Set stage for later discoveries

PSCI 131: Origins of Astronomy – European Renaissance Timeline: Tycho Brahe 1400 Copernicus 1473 1543 1450 First printing press 1492 1546 1776 1601 Brahe

PSCI 131: Origins of Astronomy – European Renaissance Tycho Brahe • Very precise astronomical observations – Without a telescope – His observations of Mars were far more precise than any made previously. • Geocentrist – He couldn’t observe the apparent shift (parallax) in the position of the stars that should result if the Earth traveled around the Sun.

PSCI 131: Origins of Astronomy – European Renaissance Timeline: Johannes Kepler 1400 Copernicus 1473 1543 1450 First printing press 1492 1546 Kepler 1571 1630 1601 Brahe 1776

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 • Nothing is at the other focus

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 • Planet speeds up as it approaches the sun and slows down far away

PSCI 131: Origins of Astronomy – European Renaissance - Kepler Second Law of Planetary Motion • Orbital speed varies inversely with orbital distance Sl ow er or b ita ls pe ed From: http: //www. wallpaperswala. com/solar-system/

PSCI 131: Origins of Astronomy – European Renaissance - Kepler Third Law of Planetary Motion • Orbital period proportional to orbital distance • p 2 = a 3 • p: orbital period in Earth years • a: orbital distance in astronomical units (AUs) • Example: if p = 8 yrs, a = 4 AUs (8)2 = 64 then 64 = a 3 3√(64) = 4

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 Kepler 1571 1630 1400 Copernicus 1473 1543 1564 1450 First printing press 1492 1546 Galileo 1642 1601 Brahe 1776

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 The Galilean Moons • 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 Sizes of the Galilean moons shown relative to Jupiter From: en. wikipedia. org

PSCI 131: Origins of Astronomy – European Renaissance - Galileo The Galilean Moons 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 From: splung. com

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

PSCI 131: Origins of Astronomy – European Renaissance - Galileo The Moon’s Surface • Observed the Moon’s surface wasn’t smooth as the ancients proclaimed – Galileo saw mountains, craters and plains – Moon similar to Earth and not a perfect celestial sphere – Thought plains were bodies of water • Ex. Sea of Tranquility

PSCI 131: Origins of Astronomy – European Renaissance - Galileo Sunspots • Dark regions caused by slightly lower temperatures • Refuted celestial immutability Series of Galileo’s 1612 sketches Sunspots through a modern telescope

PSCI 131: Origins of Astronomy – European Renaissance Timeline: Isaac Newton Kepler 1571 1630 1400 Copernicus 1473 1543 1564 1450 First printing press 1492 1546 Galileo 1642 1601 Brahe 1642 1776 1727 Newton

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 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 Universal Gravitation • All objects in the universe exert an attractional force on each other • Force increases with mass and decreases with distance – More massive objects exert a greater gravitational attraction than do less massive objects.

PSCI 131: Origins of Astronomy – European Renaissance - Newton Gravity + Inertia = Orbit An orbit is a balance between gravity and inertia

PSCI 131: Origins of Astronomy – European Renaissance - Newton Summary of Newton’s Laws • 1 st Law – A body at rest, or in uniform motion, will remain so unless acted upon by an unbalanced force • 2 nd Law – The change in motion (acceleration) acceleration is proportional to the unbalanced force • 3 rd Law – For every action there is an equal and opposite reaction

PSCI 131: Origins of Astronomy – European Renaissance - Newton Gravity • Gravity is the force that – – holds us to the Earth causes a rock to fall towards the ground causes the Earth to go around the Sun causes the Sun to be pulled towards the center of the Milky Way galaxy • Gravity acts between any two objects even if they are far apart. – “action at a distance” distance