Ch3: The Science of Astronomy

Eclipses (Ch2)

  • The Earth and Moon cast shadows.

  • When either passes through the other’s shadow, we have an eclipse

Why don’t we have an eclipse at every new and full moon?

  • The Moon’s orbit is tilted 5 degrees to ecliptic plane.

  • So we have about two eclipse seasons each year, with a luner eclipse at full moon and solar eclipse at new moon.

Acquire Measurements

  • Full circle = 360 degrees

  • 1 degree = 60’ (arcminutes)

  • 1’ = 60” (arcseconds)

Thought Question

The angular size of your finger at arm’s length is about 1 degree. How many arcseconds is this?

→ 60 × 60 = 3600 arcseconds

Chapter3: The Science of Astronomy

In what ways do all humans use scientific thinking?

  • Scientific thinking is based on everyday ideas of observation and trial-and-error experiments.

How is modern science rooted in ancient astronomy?

  • Many of our current systems had their roots in the achievements of ancient astronomy.

    • Daily timekeeping

    • Tracking the seasons

    • Calendar

    • Monitoring lunar cycles

    • Monitoring planets and stars

    • Predicting eclipses

    • And more…

Why does modern science trace its roots to the Greeks?

Greek geocentric model (c. 400 B.C.)

  • Greeks were the first people known to make models of nature

  • They tried to explain patterns in nature without resorting to myth or the supernatural.

But this made it difficult to explain apparent retrograde motion of planets….

  • But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion.

Why was planetary motion so hard to explain?

  • Review: Over a period of 10 weeks, Mars appears to stop, back up, then go forward again.

Ptolemy (A.D. 100 - 170)’s model ~The Ptolemaic Model~

  1. Geo-centered Model

  2. Combination of Big & Small circles

How did Copernicus, Tycho, and Kepler challenge the Earth-centered model?

  • Copernicus (1473-1543)→ “Copernican REVOLUTION” (1543)

  • Sun-Centered model: Sun at the center + Circular planetary orbits

Tycho Brahe (1546-1601)’s model

  • Made the most accurate naked eye measurements of planetary positions

  • Geo-centered model + Planets orbiting around Sun

Kepler (1571-1630)’s model (1619)

  • “If I had believed that we could ignore these eight minutes [of arc], I would have patched up my hypothesis accordingly. But, since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy.”

What is an eclipse? (Don’t be confused with ECLIPSE!)

  • An eclipse looks like an elongated circle

What are Kepler’s three laws of planetary motion?

  • Kepler’s First Law: The orbit of each planet around the Sun is an eclipse with the Sun at one focus

  • This means that a planet travels faster when it nearer to the Sun and slower when it is farther from the Sun.

Kepler’s Third Law

  • More distant planets orbit the Sun at slower average speeds, obeying the relationship

  • p = orbital period in years

  • a = average distance from Sun in AU

Kepler’s Third Law

  • Graphical version of Kepler’s Third Law

How did Galileo solidify the Copernican revolution?

  • Galileo overcame major objections to the Copernican view. Three key objections rooted in Aristotelian view were:

    1. Earth could not be moving because objects in air would be left behind

    2. Non-circular orbits are not “perfect” as heavens should be,

    3. If Earth were really orbiting Sun, we’d detect stellar parallax.

What have we learned?

  • How did Copernicus, Tycho, and Kepler challenge the Earth-centered idea?

    • Copernicus created a sun-centered model; Tycho provided the data needed to improve this model; Kepler found a model that fit Tycho’s data.

  • What are Kepler’s three laws of planetary motion?

    1. The orbit of each planet is an eclipse with the Sun at one focus.

    2. As a planet moves around its orbit it sweeps out equal areas in equal times.

    3. More distant planets orbit the Sun at slower average speeds: p²=a³

  • What was Galileo’s role in solidifying the Copernican revolution?

    • His experiments and observations overcame the remaining objections to the Sun-centered solar system model.

So, is Geocentric Model really wrong?

  • Still in USE in Magnetospheric Physics