History of Astronomy

How our understanding of the Universe evolved…

History of AstronomyBig Questions:

• Why is astronomy often considered the “oldest science”

• What are some of the major discoveries that have shaped our understanding of the universe?

• What is the value of learning about early astronomers like Ptolemy and Aristotle, since their models to describe the universe were WRONG?

Astronomy: The oldest science?

• As far back as the stone and bronze ages, human cultures realized the cyclical nature of the motions in the sky.

• Monuments dating back to ~ 3000 BCE show alignments with astronomical significance.

• Those monuments were probably used as calendars or even as ancient observatories.

Stonehenge, England (3000-2500 BCE)

•Alignments with locations of sunset, sunrise, moonset and moonrise at summer and winter solstices

• Possibly used as a calendar.

Pyramid of Khufu, Giza (Egyptian Culture, 2600-2400 BCE)

• Shafts from the King's chamber indicate location of Polaris 5000 years ago

• Former position of Orion's belt • The pyramid is also aligned perfectly N-S and E-W

Caracol Temple, Mexico (Mayan culture ~ 1000 CE)

• Solstice and equinox alignments• Star alignments• Alignments with Venus

Big Horn Medicine Wheel, Wyoming (Plains Indians, approx. 1200 CE)

• alignments to the rising and setting of the Sun at summer solstice• alignments to rising places of Aldebaran, Rigel, Orion, and Sirius

(all bright stars associated with the Solstice)

Mesopotamian Astronomy ~ 1200 BCE

• Earliest Babylonian star catalogues recorded observations on clay tablets

• Babylonian astronomy was the basis for much of what was done in Greek astronomy later

Greco-Roman Astronomy

Astronomy in ancient GreeceGreek models were based on wrong “first principles”, believed to be obvious and not questioned:

1. Geocentric Universe: Earth at the center of the Universe.

2. “Perfect Heavens”: Motions of all celestial bodies described by motions involving objects of “perfect” shape, i.e., spheres or circles.

3. “Unchanging Heavens”: always was and always will be the same

Major Greek Astronomers• Aristotle (384-322 B.C.E.) – major authority in

ancient Greece, promoted the idea of a geocentric universe and first principles

• Eratosthenes (276 - 194, B.C.E.) – calculated the circumference of Earth using shadows and trigonometry

• Ptolemy (85--165 C.E.) -- Wanted to create a mathematical model of the universe and explain retrograde (backward) motion of the planets

Motions of the “Wanderers”• The ancient Greeks tracked seven objects that

seemed to move against the background of fixed stars in the sky: the Sun, the Moon, Venus, Jupiter, Saturn, Mercury, and Mars

• They named them “planets” (the Greek word for wanderers), and much of ancient astronomy was centered around explaining the motion of these seven bodies

• http://earthsky.org/space/animation-shows-venus-in-evening-sky-late-2014-and-2015?utm_source=EarthSky+News&utm_campaign=5ccd3689e7-EarthSky_News&utm_medium=email&utm_term=0_c643945d79-5ccd3689e7-394144225

Ptolemy’s Epicycles

• http://astro.unl.edu/classaction/animations/renaissance/marsorbit.html

The Ptolemaic system (still geocentric) was considered the “standard model” of the Universe for over 1000 years, through the middle ages

Astronomy in the Renaissance

Major Renaissance Astronomers• Nicolaus Copernicus (1473 – 1543) – caused controversy

by “changing our place in the universe”, leading to what is known as the Copernican Revolution

• Tycho Brahe (1546 –1601) – Invented instruments for measuring angles in the sky, made very precise observations

• Johannes Kepler (1571 – 1630) – used Brahe’s observations to study planetary motion mathematically, discovered that planets’ orbits are elliptical, not perfect circles

• Galileo Galilei (1594 – 1642) – greatly improved the telescope and made many observations in support of the heliocentric model of the solar system

The Copernican Revolution

• Heliocentric Universe– The Sun at the center– Earth was a planet, and the moon revolved

around Earth while the earth revolved around the sun

– His model still accepted Aristotle’s idea of uniform circular orbits, so it did not perfectly predict the motion of the planets

Copernican explanation of retrograde motion in a heliocentric universe

• Retrograde (backward) motion of a planet occurs when the Earth “passes” the planet.

• http://astro.unl.edu/classaction/animations/renaissance/retrograde.html

•

Arrangement of the Planets• Copernicus made observations that allowed him

to determine the order of the 5 planets known at that time– Mercury and Venus can only be observed fairly near to the sun

at dawn or dusk (their orbits must lie inside of Earth’s orbit, since they are always in the same direction as the sun)

– Mars, Jupiter and Saturn can be observed at night when the sun is far below the horizon (only possible if Earth is between the sun and these planets)

– By measuring the time it took planets to return to the same position in the sky, he determined their distances from the sun

Kepler and Galileo

Refining Heliocentric Cosmology

Johannes Kepler (1571-1630)

• Kepler used Tycho Brahe’s meticulous observations to deduce a mathematical explanation for the motion of the planets

• Determined that Copernicus made a mistake-- planets do not orbit the sun in perfect circles!

Kepler’s Laws of Planetary Motion

1. The orbits of the planets are ellipses with the sun at one focus

2. A line from a planet to the sun sweeps over equal areas in equal intervals of time.

3. A planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed:

Kepler’s First Law (orbital shapes)

• The orbits of the planets are ellipses with the sun at one focus.

• http://science.sbcc.edu/physics/flash/Keplers%20Laws.html

Eccentricities of Ellipses

• Eccentricity = how far from circular an ellipse is

Eccentricity e = c/a

c

center

Eccentricity of Ellipses

• Planet’s orbits are very close to circular with small eccentricities

e = 0.4 e = 0.6

Kepler’s Second Law (changing speeds)• A line from a planet to the sun sweeps over

equal areas in equal intervals of time.

• Planet’s move faster when closer to the sun

Eccentricity of ellipse is exaggerated for effect

Kepler’s 2nd law

• http://science.sbcc.edu/physics/flash/Keplers%20Laws.html

• There is a mathematical relationship between the distance from the sun and the time it takes the planet to complete its orbit.

• A planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed:

P2 ∝ a3P = period in years

a = distance in AU

∝ = is proportional to…

Kepler’s Third Law (length of years)

Kepler’s 3rd Law

• http://science.sbcc.edu/physics/flash/Keplers%20Laws.html

Galileo Galilei (1564- 1642)

• Greatly improved upon the newly invented telescope and made discoveries in support of the Heliocentric model of the solar system

Major Discoveries of Galileo• Moons of Jupiter (4 Galilean moons)

• Surface structures on the moon; • first estimates of the height of mountains on the

moon • moon isn’t a perfect

sphere

Major Discoveries of Galileo

• Sun spots -proving that the sun is not perfect

Major Discoveries of Galileo

• Phases of Venus- proving that Venus orbits the sun and not the Earth!

Major Discoveries of Galileo

Conflict of Religion and Science

• Galileo’s observations and evidence in support of the Copernican Theory created a conflict between a faith-based system and the scientific ways of thinking based on observation and evidence

• His methods and conclusions are considered the birth of the modern view of science

A New Era of Science

Newton

A New Era of Science

• Astronomy continued to evolve after the Renaissance

• Mathematics as a tool for understanding the physics of the universe became the central focus

• Evolving telescopes made new observations possible

Isaac Newton (1643 - 1727)• Added physics interpretations to the mathematical descriptions of astronomy by

Copernicus, Galileo and Kepler• Major Achievements:

• Invented Calculus as a necessary tool to solve mathematical problems related to motion• Discovered the three laws of motion• Discovered the universal law of mutual gravitation

Newton’s Laws of Motion

• http://science360.gov/obj/video/642db496-d506-432e-85b4-4e38f75d9142/newtons-three-laws-motion

Newton’s 1st Law of MotionA body at rest continues to rest, and a body in motion will continue in uniform motion in a straight line unless it is acted upon by some net force.

(AKA- Inertia)

Newton’s 2nd Law of Motion

Acceleration (a) of a body is inversely proportional to its mass (m), directly proportional to the net force (F), and in the same direction as the net force.

a = F/m F = m a

Newton’s 3rd Law of Motion

• To every action, there is an equal and opposite reaction.

Newton’s Universal Law of Gravity• Any two bodies are attracting each other

through gravitation, with a force proportional to the product of their masses and inversely proportional to the square of their distance:

F = force

∝ = is proportional to

m1 = mass of first body

m2 = mass of second body

d = distance

Fm1 * m2

d2

• More Mass = more gravitational force• More Distance = less gravitational force

∝

Mass vs. Weight

• Because of Newton’s Law of Gravity, we know that:– Your weight is a measure of the force of gravity

acting on your body• Your weight will be different on the moon because it

has a different mass, and therefore a different gravitational “pull”

– Your mass is a measure of how much matter you contain.

• Your mass will be the same or the moon

Charles Messier

• Published a catalog in 1774 that contains over one hundred deep sky objects, including nebulae and galaxies.

• These are now known as Messier Objects• Also discovered 13 comets (his true interest)

Cosmic Times???????

• Replace modern astronomy section with Cosmic Times lessons in jigsaw groups?– http://cosmictimes.gsfc.nasa.gov/teachers/guide/

index.html

Modern Astronomy in the 20th and 21st centuries

Albert Einstein• Einstein observed that Newton’s laws of motion

are only correct in the limits of low velocities and masses

• Space and time are not absolute and are affected by large masses and an observer’s relative motion– Theory of Special Relativity (E = mc2) published in

1905– Theory of General Relativity (modification of

Newtonian Gravity) published in 1916

Henrietta Leavitt

• In 1912, Leavitt worked at Harvard College Observatory as a “computer,” one of several women paid 25 to 30 cents per hour to extract data from thousands of photographic plates

• She identified a pattern in the brightness and periods of Cepheid variable stars

• Her calculations formed the basis of how astronomers now calculate distances in the universe

Edwin Hubble • In 1925 he calculated that M31 was much further

away previously though, and therefore not part of the Milky Way galaxy– We now know that M31 is actually the Andromeda

Galaxy and that there are many other galaxies beyond the Milky Way

• In 1929 published he findings that the universe is expanding– The farther away a galaxy is, the faster it is moving

away-- a relationship now known as Hubble's Law.

Arno Penzias and Robert Wilson

• Discovered cosmic microwave background radiation (CMB) in 1965

• This “leftover” energy is evidence of the Big Bang and the origin of the universe

Stephen Hawking

• In 1970 the English physicist provided theories indicating that black holes are actually a common feature of general relativity.

• He also predicts that black holes should in theory emit radiation (known today as Hawking radiation)

Michel Mayor and Didier Queloz

• Swiss astronomers detected the first exoplanet orbiting a sun-like star in 1995

• The planet is officially called 51 Pegasi b, but also known as Bellerophon

• Since that discovery, nearly 2000 extrasolar planets have been identified

Astronomy in Action

Rosetta Mission lands on a

Comet

Nov. 2014

New Horizons Pluto Fly-byJuly 2015

Dawn Mission to study CeresMay 2015

What will the NEXT great discovery be?