ASTRO 101 Principles of Astronomy

Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: Telephone: WWW: Office: Physics 241, hours T TH 2:00-3:20

Text: “Discovering the Essential Universe, Fifth Edition” by Neil F. Comins

Course WWW Page Note the underline: … ast101_spring2014.html … Also check out Nick Strobel’s Astronomy Notes:

Homework Homework due February 11: Question 11 from Chapter 2 (In what ways did the astronomical observations of Galileo support a heliocentric cosmology?) Write down the answer on a sheet of paper and hand it in before the end of class on February 11.

Homework Go to a planetarium show in PA 209: The days and times of the shows will be (all shows last less than 1 hour): Tuesday February 4 2:00 PM Wednesday February 5 1:00 PM Thursday February 6 5:00 PM Friday February 7 1:00 PM Monday February 10 1:00 PM Tuesday February 11 10:00 AM Wednesday February 12 11:00 AM Thursday February 13 12:00 PM Friday February 14 12:00 PM Get 10 points extra credit for homework part of grade. Sign up for a session outside PA 209. Hand in a sheet of paper with your name and the data and time of the session.

Next: Lunar and Solar Eclipses

Lunar and Solar Eclipses But first, let’s discuss “angular size” and “linear size”…

Angular Size The physical size is measured in meters, light-years, etc. The distance is measured in the same units. The angular size is how large something “looks” on the sky, and is measured in degrees.

Angular Size The angular size is how large something “looks” on the sky, and is measured in degrees. As you move the same object further, its angular size gets smaller.

Angular Size The angular size is how large something “looks” on the sky, and is measured in degrees. If two objects are at the same distance, the larger one has the larger angular size.

Angular Size Trick photography often involves playing with different distances to create the illusion of large or small objects:

Angular Size This figure illustrates how objects of very different sizes can appear to have the same angular sizes. The Sun is 400 times larger than the Moon, and 390 times more distant.

Lunar and Solar Eclipses A solar eclipse is seen when the Moon passes in front of the Sun, as seen from a particular spot on the Earth. A lunar eclipse is seen then the Moon passes into the Earth’s shadow.

Shadows If the light source is extended, then the shadow of an object has two parts: the umbra is the “complete” shadow, and the penumbra is the “partial shadow”.

Shadows If the light source is extended, then the shadow of an object has two parts: the umbra is the “complete” shadow, and the penumbra is the “partial shadow”.

Lunar Eclipses During a total lunar eclipse, the Moon passes through Earth’s shadow.

Solar Eclipses The umbral shadow of the Moon sweeps over a narrow strip on the Earth, and only people in that shadow can see the total solar eclipse.

Solar Eclipses The umbral shadow of the Moon sweeps over a narrow strip on the Earth, and only people in that shadow can see the total solar eclipse.

Solar Eclipses The umbral shadow of the Moon sweeps over a narrow strip on the Earth, and only people in that shadow can see the total solar eclipse. During totality the faint outer atmosphere of the Sun can be seen.

Annular Eclipses The angular sizes of the Sun and Moon vary slightly, so sometimes the Moon isn’t “big enough” to cover the Sun

Lunar and Solar Eclipses Why isn’t there an eclipse every month? Because the orbit of the Moon is inclined with respect to the orbital plane of the Earth around the Sun.

How often do we see an Eclipse? Roughly every 18 months there is a total solar eclipse visible somewhere on the Earth.

Next: The Scientific Method Gravity and the motions of the planets

Outline of the Scientific Method Gather data, make observations, etc. Form a hypothesis on how the object of interest works. Determine the observable consequences of your idea, using reasonable assumptions and well-established “laws.” Formulate experiments to see if the predicted consequences happen.

Outline of Scientific Method If the new observations agree with the predictions: great, keep going. If the new observations don’t agree with the predictions: start over!

Outline of Scientific Method

A Good Recap From Nick Strobel

Next: The motion of the planets

A Brief History of Astronomy

Stonehenge (c B.C.) Stonehenge was probably used to observe the sun and Moon. Image from FreeFoto.com

The great pyramids of Egypt were aligned north-south.

A Brief History of Astronomy An early view of the skies:  The Sun: it rises and sets, rises and sets…  The Moon: it has a monthly cycle of phases.  The “fixed stars”: the patterns stay fixed, and the appearance of different constellations marks the different seasons. Keep in mind there were no telescopes, no cameras, no computers, etc.

A Brief History of Astronomy But then there were the 5 “planets”:  These are star-like objects that move through the constellations.  Mercury: the “fastest” planet, always near the Sun.  Venus: the brightest planet, always near the Sun.  Mars: the red planet, “slower” than Venus.  Jupiter: the second brightest planet, “slower” than Mars.  Saturn: the “slowest” planet.

A Brief History of Astronomy By the time of the ancient Greeks (around 500 B.C.), extensive observations of the planetary positions existed. Note, however, the accuracy of these data were limited. An important philosophical issue of the time was how to explain the motion of the Sun, Moon, and planets.

What is a model? A model is an idea about how something works. It contains assumptions about certain things, and rules on how certain things behave. Ideally, a model will explain existing observations and be able to predict the outcome of future experiments.

Aristotle ( B.C.) Aristotle was perhaps the most influential Greek philosopher. He favored a geocentric model for the Universe:  The Earth is at the center of the Universe.  The heavens are ordered, harmonious, and perfect. The perfect shape is a sphere, and the natural motion was rotation.

Geocentric Model The motion of the Sun around the Earth accounts for the rising and setting of the Sun. The motion of the Moon around the Earth accounts for the rising and setting of the Moon. You have to fiddle a bit to get the Moon phases.

Geocentric Model The fixed stars were on the “Celestial Sphere” whose rotation caused the rising and setting of the stars.

This is the constellation of Orion

The constellations rise and set each night, and individual stars make a curved path across the sky. The curvature of the tracks depend on where you look.

Geocentric Model The fixed stars were on the “Celestial Sphere” whose rotation caused the rising and setting of the stars. However, the detailed motions of the planets were much harder to explain…

Planetary Motion The motion of a planet with respect to the background stars is not a simple curve. This shows the motion of Mars. Sometimes a planet will go “backwards”, which is called “retrograde motion.”

Planetary Motion Here is a plot of the path of Mars. Other planets show similar behavior. Image from Nick Strobel Astronomy Notes (

Aristotle’s Model Aristotle’s model had 55 nested spheres. Although it did not work well in detail, this model was widely adopted for nearly 1800 years.

Better Predictions Although Aristotle’s ideas were commonly accepted, there was a need for a more accurate way to predict planetary motions. Claudius Ptolomy (85-165) presented a detailed model of the Universe that explained retrograde motion by using complicated placement of circles.

Ptolomy’s Epicycles By adding epicycles, very complicated motion could be explained.

Ptolomy’s Epicycles Image from Nick Strobel’s Astronomy Notes (

Ptolomy’s Epicycles

Ptolomy’s model was considered a computational tool only. Aristotle’s ideas were “true”. They eventually became a part of Church dogma in the Middle Ages.

The Middle Ages Not much happened in Astronomy in the Middle Ages ( A.D.).

Next: The Copernican Revolution