Astronomy Unit

• Describe a constellation is its use.• Describe basic structure of the universe.• Describe the historical understanding of the

universe.

KEY WORDSCosmology Astronomy AxisConstellation ZodiacPolarisCoordinate GeocentricHeliocentric Retrograde

Our solar system:Sun (a star) and all the objects in orbit around it.

nine planets and multiple moons, comets etc.

My Very Educated Master Just Showed Us Nine Planets

• Planet: a large collection of matter that is generally spherical and revolves around a star.

Planets / moons are nonluminous (don’t emit light).

Viewed because light from sun reflects off them towards Earth.

Things in our solar system are closer to us than the stars and other objects in the universe.

The universe contains huge groups of stars, called galaxies, separated by great distances.

Our sun is part of a group of stars called the Milky Way galaxy.

Cosmology – study of the origin of the universe.

Astronomy – study of all objects in the universe.

Comparing Stars and Planets

Star: a very large collection of matter that emits an intense amount of energy (light and heat).

Most planets are brighter than stars. Stars twinkle. Planets look round in a telescope; stars look like points of light.

• Only five planets can be seen without a telescope.

Feature Planet StarLocation in the solar system far beyond the solar

systemDistance from Earth fairly near very far

Real size smaller than most stars usually larger than planets

Reason we see it reflects light emits lightSurface temperature usually cool or very

coolvery hot

Composition usually rocks or gases gases under high pressure and temperature

Look does not twinkle twinklesMotion over time very slowly wanders

through constellationsappear to move as part of a constellation

Agricultural revolution: important for people to accurately mark the growing season.

Position of sun allowed creation of a calendar that had 365 days in a year.

Both a religious and practical significance to the movement of the sun and stars in the sky

The Earth rotates on its axis once a day.The axis is tilted at an angle of 23.5 degrees.

The Earth revolves around the Sun once a year.

At any time - half of Earth is in sunlight and half in darkness.

Earth rotates in an easterly direction

summer

winter

Exploration and Trade: navigate water for trade and to conquer other countries.

Star groupings called constellations

Stars and the Sun became the "landmarks" used for navigation.

Named for animals and gods.Zodiac – Greek for animal sign.

• North Star is called Polaris. • Polaris is always due north. • Altitude (height) of Polaris equals the latitude on

the earth's surface from where you are observing the star.

Coordinates of both longitude and latitude are used to mark position on Earth.

Longitude is measured east - west from 0o line through Greenwich, England (prime meridian).

Latitude is measured north - south from the equator.

Astrolabe• Instrument to calculate the position of objects in

sky with high accuracy.

• Perfected in Islamic Golden Age (800-1200s).

Major breakthroughs in math and science.

• Arab ruler - Caliph Harun-al-Rashid preserved all ancient and new scientific writings.

• Collected in a research center calledHouse of Wisdom in Baghdad.

Vertical (upward) angle is called altitude.

Horizon – ground - 0°Zenith – directly above - 90°

Horizontal (sideways) angle measured with a compass is called azimuth.

North – azimuth of 0°South – azimuth of 180°

Position of object in space given with its altitude and azimuth - called its coordinates.

altitude azimuth

Ptolemy (100 AD)

• Summarized simple observations and knowledge of that time.• Earth was centre of the universe - Geocentric.• In good with the Church.

Planet is Greek for “wander.”

Planet seems to move backward in orbits:

Retrograde motion

Copernicus (1473)

• Produced mathematical evidence for a Sun-centered system – heliocentric.

• Easily explained retrograde motion. • Proved Earth rotated on its axis once daily and

traveled around the sun once yearly.• Not in good with Church.

“Founder of modern astronomy”

Ptolemy

Galileo (1564)

• First to state and use the scientific method. • Perfected the telescope (built a few years earlier)

• Discovered moons around Jupiter. (Proof that not all objects traveled around Earth)• Venus had phases (like our moon) indicated it

traveled around the Sun.

• Not in good with the Church.

Would not recant his work as wrong and spent life under house arrest.

1633 - Galileo published his book confirming Copernicus, and is accused

of heresy by the Church.

By the way, Pope John Paul II made a public apology and exonerated Galileo in 1992.

Kepler (1571)

• Brahe built one of the best observatories in the world.

• Worked as assistant for astronomer, Tycho Brahe.Brahe’s goal was to prove Copernicus incorrect.

• Measured positions of 777 stars and five planets that were known at that time.

Kepler used Brahe’s data after his death to described three laws of planetary motion:

1. Planets move around the Sun in ellipses.2. Planets move faster closer to the sun, and slower further away.3. The further away the planets, the longer it will take to complete its orbit.

Measuring Angles in the Sky