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Astronomy - PowerPoint PPT Presentation

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Astronomy. Environmental Science. Astronomy is the science that studies the universe. Astronomy deals with the properties and laws of objects in space. The universe is made up of galaxies. There are billions of galaxies in our universe. . Our galaxy is called the Milky Way.

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Environmental Science

There areeightplanets andfoursmaller bodies (dwarf planets) that orbit the center of our solar system, the sun.

Scientists have been researching the sky for almost 3000 years!

  • Early astronomy was centered in Greece.

The “Golden Age” of Astronomy in Greece: (600 B.C. – 150 A.D.)

  • Used philosophical arguments and observation.

Greeks developed Geometry and Trigonometry to measure planet size and distances.

  • Aristotle (384-322 B.C.) believed that the Earth was round because of the curved shadows it formed on the moon. Idea abandoned in Middle Ages.
early greeks use philosophy personal observations and geometry to make astronomical theories
Early Greeks use philosophy, personal observations and geometry to make astronomical theories.
aristotle concluded the earth is round because the shadow the earth casts on the moon is curved
Aristotle: concluded the earth is round because the shadow the earth casts on the moon is curved.
in reality the solar system is a heliocentric model
In reality, the solar system is a Heliocentric Model

Aristarchus first proposed the heliocentric model, but no one believed him.


Ptolemaic System:Ptolemy proposed a motionless earth at the center of our solar system (Geocentric) and people supported this for 13 centuries!


Retrograde motion is when the planet appears to stop, reverse its direction for a time and then resume the initial, eastward motion.

  • Retrograde motion cannot be possible, can it? Why? Then HOW does it exist?
modern astronomy
Modern Astronomy!

“Modern” Astronomy uses natural laws as opposed to philosophical or religious views to determine astronomical theories.


The Scientists involved in the modern Astronomical theory were from a variety of European countries.

copernicus conclusions
Copernicus’ Conclusions
  • Determined that the earth is a planet, just like other five known planets, and that it rotates.
  • Also proposed a heliocentric solar system with circular orbits for the planets.

Became interested in astronomy while viewing astronomer-predicted solar eclipse.

Persuaded King Frederick II to build observatory.

Measured the locations of heavenly bodies for 20 years.


Brahe made observations of Mars and made instruments with which to view the skies.

This angle-

measuring device

was more precise

than any previous



In the last year of his life, Brahe found an able assistant to continue his work: Johannes Kepler. Kepler would outshine his predecessor.


Brahe’ data did not make sense using a geocentric model. But when Keplerplaced the sun in the middle of the solar system, and suddenly the data made sense!

kepler s laws of planetary motion
Kepler’sLaws of Planetary Motion

Kepler determined the laws of planetary motion.

  • Elliptical Orbits
  • Orbits sweep out equal areas in equal time intervals.
  • Square of period equals cube of distance.
kepler s laws of planetary motion1
Kepler’sLaws of Planetary Motion

Orbits sweep out equal areas in equal time intervals.

kepler s third laws of motion
Kepler’s Third Laws of Motion….
  • States that the square of the length of time that it takes a planet to orbit the sun (orbital period) is proportional to the cube of its mean distance to the sun.
  • Huh?
in other words
In other words….

T2 d3

Comparing two planets:

for the right units
For the right units….

T2 = d3

Distance in astronomical units

(1 AU = 150 million km = length of earth’s semi-major axis of its elliptical orbit around the sun)

Period in Earth years.


For example:

  • Mars has an orbital period of T = 1.88 years. The square of 1.88 is 3.54 (T2). The cube root of 3.54 is 1.52. So the distance from Mars to the sun is 1.52 AU = 228 million km.
  • The AU stands for astronomical unit is the average distance between the earth and sun is about 150 million km.
galileo discovered
Galileo discovered:
  • Jupiter’s four moons;
  • That planets are circular disks;
  • That Venus has phases like our moons;
  • That the surface of our moon is not smooth;
  • The Sun’s sunspots.
universal gravitation
Universal Gravitation
  • Is a Scientific Law that every body in the universe attracts every other body.
  • This force of attraction is proportional to their masses and inversely proportional to the square of the distance between their centers of mass.

This force (called gravity) decreases with the square of the distance (inversely proportional).

  • The larger the mass, the larger the gravitational pull (directly proportional).
  • The moon has a strong enough pull that it causes tides, but a satellite does not have a large enough pull to have a measurable effect on Earth.