Ch by absences


									                                              Ch.    1
                  An Earth-Centered Universe
The following is a commentary on the various sections and subsections of the chapter. In it I
point out what is important and less important, raise questions related to the text's material, and
point to various aids, such as the animations on the CD that comes with your text.

The Animations
I will often rate the animations. Zero means "Forget it" or "It's wrong so you're better off by not
viewing it." "5" means worth the time to view it, but just barely. "7" or higher means "Don't miss

1.1 The View from Earth
A basic introduction to the road ahead. A good read but not a lot of stuff you need to know.

1.2 The Celestial Sphere
Watch the sky and you will notice some things (assuming you pay attention and DO NOT look at
the Sun). Over the course of a night stars seem to rise in the east and go down or set in the west.
Most stars move this way as does most everything else. It looks as if celestial objects are circling
on great spheres, celestial spheres (CS).
The Celestial Sphere's. axis of rotation coincides with that of earth's (we will learn later that it is
the Earth that rotates not the CS). The North Celestial Pole (NCP) is directly over the Earth's
north pole (replace north with south and you have what happens in the southern hemisphere of
Earth but we will concentrate on the northern hemisphere). The NCP is very near the star Polaris.
The Celestial Equator (plane midway between the celestial poles) coincides with earth’s equator.

The Fig. 1-7 animation can help you see the relation of the Earth and its spin to the celestial
sphere, celestial equator, and the N and S celestial poles. [Value rating = 6 out of 10]

Constellations are primarily used to make partitions to make it easier to identify regions of the
sky. They are also good to impress your friends. Other than that they are of no use in this class.
Notice that stars DO move, but very, very slowly.

Measuring the Positions of Celestial Objects
In this section the important idea is that we measure distances between stars in the sky by angles,
not by actual distance. Angles are easy to measure; distances are hard.

Celestial Coordinates
This subsection is relevant in the laboratory (Astr 1101), but we will not concern ourselves with
celestial coordinates in this course.

1.3 The Sun's Motion Across the Sky
The Fig. 1-17 animation takes some careful viewing, but has real value. The upper part shows
the Sun's motion among the stars during one full year. The lower part shows the Sun orbiting the
Earth (!) which is the geocentric view of the solar system, not the way it really happens (Ch. 2).
But that's OK because we really do sort of see things from a geocentric viewpoint since we are
on the Earth and all. Note that the Sun's "orbit" is tilted relative to the Earth's equator, which is
why the Sun appears to go up and down in the upper picture. [Value Rating = 6-7]

The Ecliptic
The reason for a whole section on the ecliptic is that it plays an important role in understanding a
wide range of phenomena. Pay close attention to the figures and explanations. Try to envision
the motions in three dimensions.

The Sun and the Seasons
There are really two reasons for the seasons, both arising from the tilt of the Earth's axis relative
to the ecliptic plane (Figs. 1-19 and 1-20).

The Fig. 1-19 animation shows that the Sun is highest at noon in June and lowest in December. It
also shows that the Sun is "up" a lot longer in June than December, which contributes to its being
hotter in June. The Fig. 1-20 animation shows how this looks to someone standing on Earth. The
two together can be quite helpful. [VR = 5]

The question arises: Why is it hottest in August, not June, and coldest in February, not

A Scientific Model
Based on our observations we have developed an idea of how things occur. We have placed the
stars and the Sun on great spheres circling us. We have developed a model. So, what else is
happening out there?

1.4 The Moon's Phases
The dark side of the Moon
One thing should be clear: there is no such thing as "the dark side of the Moon," not really - Pink
Floyd notwithstanding - only in the sense that Africa used to be known as "the dark continent,"
which meant "dark" in the sense of unseen, unknown.
  Similarly, the far side of the Moon is not visible from Earth, but it is no darker than the side
we see. Since half of the Moon is always lit (except during lunar eclipses), the fraction of the
Moon that we see as unlit equals the fraction of the far side that is lit.
When viewing Fig. 1-23 be sure to notice that the Moon is rotating in one case (righthand side)
and not rotating in the other case. Here, "rotation" refers to rotation with respect to the stars.
  You might find it a help to see that one is rotating and the other is not if you only view the
Moon. This takes the Earth out of the picture and the rotation or non-rotation is easy to see with
no distraction.

Fig. 1-24 (VR = 9)
This is, of course, an important figure. Imagine yourself standing on the Earth and visualize how
the Moon would appear at its different positions in its orbit. If you imagine well, you should
agree with the figure in the upper right, showing the "View from Earth."
   Don't hesitate to stop the motion at various points so you can correlate the orbital position with
the phase at your leisure.

For a web-based animation of the Moon's phases click here. Select "Both" and then click on
"Animation" for the best veiw of what we see at the same time as what is happening in space.
Graduating Harvard seniors were famously unable to explain the phases of the Moon, but you
should be able to do a lot better than they did after you view and study this animation. This is
similar to the previous animation.

1.5 Lunar Eclipses
A lunar eclipse is caused by the Moon passing through the Earth's shadow
Fig. 1-27 (VR = 8)
This animation can be a great help with one of the more difficult situations of 3D visualization. If
you can really see why eclipses occur during only some new and full moons, you are in good
  Stop the animation at points A and C so you can easily see that the Moon passes through the
Earth's shadow at those points (but does not at other points).

1.6 Solar Eclipses
Total Eclipses
   If you ever get the chance to see a total solar eclipse and don't, I will look up your grade and
lower it as far as I can. Just joking, of course, but I'm not joking when I say that you will
probably never see anything as visually stunning as a total solar eclipse. It is absolutely
indescribable. The view of the corona, the chromosphere and prominences makes it
unforgettable; it is like a new astronomical object. You see the parts of the Sun that you never
get to see otherwise. Don't dare miss it.
   There will be a total eclipse in 2017 that cuts right across the country, passing close to
Arkansas and lasting 2min 40sec. In 2024, just seven years later, the eclipse path passes right
through Arkansas including Little Rock and totality at the center of the path will last for 4min
28sec! The people who live near where Missouri, Illinois and Kentucky come together will be
able to see both eclipses, weather permitting, without leaving home, lucky them.

We do not see Solar eclipses every month for the same reason we do not see lunar eclipses every
month. The Moon's orbit is tilted so it does not pass in front of the Sun each orbit. Fig. 1-31 (VR
=8 )
The animation does an excellent job of showing how only a narrow strip of the Earth lies within
the Moon's shadow, and also why any point within the path of the shadow sees the eclipse for
only a short period of time, never longer than 7 1/2 minutes as the shadow sweeps past.
   The corresponding figure in the text is OK, but the point described by the caption is not very

Partial and Annular Eclipses
  These are nice, but are nothing like total eclipses. These happen when only part of the Sun is
covered from the viewer's location .

1.7 Observations of Planetary Motions
When watching the planets, say you see it next to a star. That night it will stay very close to the
star. Several nights later you see the planet and it will be at a slightly (very slightly) different
location. It will stay at approximately this location on the sky as it travels east to west. The
model we have would suggest the planets are on different spheres rotating at slightly different
speeds compared to the stars and the Sun. Pay attention to the caption of Fig. 1-36, which
explains why East and West are reversed on maps of the sky compared to maps of the Earth. For
help in visualizing "east" and "west" in the sky, see this link: scroll down to the bottom until you
come to the relevant figure. (If you have not discovered this site yet - on the "Useful Links" page
- now is a good time, so click on the link just above and begin to explore the site.)

Sometimes these planets seem to move backward, retrograde motion. This motion is the long
term motion (motion we see over days, months, and years) not the nightly east west motion.
Retrograde motion is the property of planetary motion that was most difficult to explain. You
have to understand both what it is and how the geocentric and heliocentric models (Ch. 2)
explain it.

Fig. 1-36 animation is excellent in helping understand what retrograde motion is, but says
nothing about why it happens. Between which two days does the retrograde motion occur? [VR
= 8-9]

1.8 Rotations
We will not concern ourselves with rotations at the moment. We will bring it up later so read it.
Do not spend a lot of time trying to master it though.

1.9 Units of Distance in Astronomy
There are lots of units of distance out there: inch, meter, yard, kilometer, mile, an Astronomical
Unit (AU), and a light-year(lt-yr). Hopefully you have an idea of what the inch, mile, and the
kilometer units are (or what they represent), but what is an AU or a lt-yr? What units do we use
for heights of people?
Feet, inches, meters. Why these units? Why not measure in miles or kms or light years (LY) or

Within the solar system: astronomical unit, AU are used (sometimes light minutes or light
hours). In AU, distances from the Sun to Earth, Mars and Jupiter are 1, 2.77, and 5.20,
respectively. In miles these same distances are 93 million, 258 million and 484 million.
Which distances would you rather work with? Distances between stars are measured in lt-yrs or
parsecs (= 3.26 LY), and between galaxies there are millions of lt-yrs or millions of pcs.

1.10 The Scale of the Universe
Be sure to view the animation Fig. 1-38 .It gives some idea of the scale of the universe and of our
place in it. Actually, much better is this web site which does the same thing, but much more
nicely. It is a poor man's version of the famous movie, Powers of Ten, which was gussied up and
made into the IMAX movie, "Cosmic Voyage."

Look over the Tools of Astronomy: Powers of Ten, where the idea of scientific notation is
discussed. You will see this notation often so make sure you understand what it means (but you
will not compute anything using this notation).
Our Universe is a big place and we make up only a tiny fraction of it. Notice the distances
between things.

1.11 Astronomy Today
A good read, but not a lot of real knowledge here.

Study Guide (in addition to the multiple-choice questions at the end of the chapter)
The student who has mastered this material should be able to understand and/or describe:

1. The concept of the celestial sphere and the celestial poles.

2. Why we have seasons. This should be explained in terms of the tilt of the Earth's axis (relative
to what?), and how this produces the seasons (more than one reason).

3. The diurnal (daily) motions of Sun, Moon, stars and planets.

4. Why we see different constellations at different times of the year.

5. How the ancients could tell the difference between stars and planets.

6. The difference between normal and retrograde motion of the planets in terms of what you see
in the sky.

7. How the ecliptic path of the Sun and the ecliptic plane are related.
8. Why the Sun is high in the sky at noon in summer and low in the winter.

9. How the Sun moves during the year as seen by people at various latitudes, e.g., at the equator
or at the poles.

10. The meaning of the equinoxes and the solstices, including why they are so named.

11. The difference between rotation and revolution.

12. Why we always see the same side of the Moon; what this means about its periods of rotation
and revolution.

14. The meaning of the various terms describing the phases of the Moon: first and third quarter,
new and full, crescent and gibbous, waxing and waning.

15. Be able to draw a diagram of the Earth, Sun and Moon illustrating the different phases.

17. Where to look for the Moon at sunset, midnight and sunrise for any of its major phases (new,
full, first and third quarters). Example: At sunset where would you look to find the full moon.
Answer: near the eastern horizon.

18. Similarly, what the phase is if you know the time of day and where the Moon is located in the

19. Why "the dark side of the Moon" is a misnomer.

20. What causes an eclipse of the Moon; be able to draw a diagram illustrating how it happens.

21. The main reason that we don't have a lunar eclipse at every full moon.

22. Why the Moon is not completely dark during a total lunar eclipse.

23. The geometry (the relation between Sun, Moon and observer) during a total, partial and
annular eclipse of the Sun.

24. Why a total solar eclipse is visible from only a small part of the Earth, while a total lunar
eclipse is visible from about half of the Earth.

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