Lecture Notes

Arny Chapter 1, Section 1

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What is astronomy?
The modern answer to that question is that astronomy is the study of the universe and everyting in it.
Yes, astronomy is a pretty big subject.

We start with the historical answer to this question.
Astronomy is a very ancient subject, and we'll start at the beginning.

A good overview of all we are covering this quarter can be found in the textbook Preview section.
You need to read this!
It not only gives an overview of things, it also introduces some important concepts and terminology.

We start with a discussion of naked-eye astronomy.
Astronomy as it has been done for thousands of years.
This is covered in Section 1.1 of our text.

Chapter One: History of Astronomy

Sec. 1.1: Prehistoric Astronomy


The Motion of Stars
The stars move!
Watch them during the night, some rise in the east, pass overhead (actually to the south of straight overhead), and set in the west.
Stars rising in the northeast set in the northwest.
Stars rising in the southeast set in the southwest.

Some stars in the north don't rise or set, they move in a counter-clockwise circle.
All the stars are moving in circles!
Those in the north move in smaller circles, the circles get bigger as you look further south.
See figure 1.4 in the text (page 24).

There are two ways of explaining this motion:
The stars could be attached to a giant crystal sphere that encloses the Earth and is rotating about it.
[Demonstrate with the celestial globe.]
Note that the stars all move together, always staying in the same constellations.

Or, the Earth could be surrounded by fixed stars and it could be the Earth that rotates.
[Demonstrate.]

Both of these models work equally well. Which is correct?
The stars are fixed and it is the Earth that rotates.
But the view that the stars revolve around us on a celestial sphere is so natural and useful, that we will often describe the sky that way (for example, we'll say "the star rose in the east" rather than "the Earth rotated east allowing us to see the star").


Celestial Sphere:
The surrounding celestial sphere idea has a long history and much of our modern terminology is derived from this.
Explain the Celestial Poles.
The star Polaris happens to be near the North Celestial Pole.
Polaris (the "Pole Star" or "North Star") moves in a tiny circle so that it is always in basically the same place in our sky.
Always! Day and night!
Polaris is one of many northern stars that always stay above the horizon, these are called circumpolar stars.

The stars move in circles.
A star will complete one circle, that is return to the exact same position in our sky, every 23h 56m.
Just under one day, the exact time is 23h 56m 04s.
That's a strange time period, we'll come back to this shortly.


The Motions of the Sun
Nothing is more important to Man than the Sun.
Everyday it rises in the east and sets in the west.
Yet, the Sun doesn't rise at the same place each day!
And the length of the daylight, the time between sunrise and sunset, also varies.

Consider sunset.
If you go out and watch where the Sun sets tonight, you'll find that it looks like this:

Tomorrow, the sunset position will have shifted slightly north (south).

Daylight today was about _____ hours, tomorrow it will be slightly longer (shorter).
The shift can be quite large, you may do an experiment yourself observing this shift later in the quarter.

Over a year, this is how the Sun's position changes for Bakersfield California:

Vernal (Spring) Equinox.
Occurs on or about March 21.
"Equinox" = "equal light", 12 hours daylight, 12 hours nighttime.
The official start of Spring.
Sun rises due east, sets due west.

Sun continues to set (and rise) further north.
Days get longer and longer.

Summer Solstice.
June 21 or so.
Longest daylight of year (14h 30m in Bakersfield).
Start of summer.
Sun rises NE, goes high into southern part of sky, sets in NW.

The Sun has stopped moving north and starts moving back south.
The word "solstice" literally means "Sun stands still".

Autumnal (Fall) Equinox.
September 22, start of fall.
Sun again rises due east, sets due west, 12 hours daylight.

Winter Solstice.
December 21.
Shortest day of the year (9h 45m daylight in Bakersfield).
Sun rises in the SE, stays low in the S, sets in the SW.
Start of winter.
(Longest daylight of year, start of summer in southern hemisphere.)


This cycle repeats in one year.
These variations are what cause the seasons on Earth.

How do we explain this behavior?
The ancient explanation is that the Sun moves around on the celestial sphere.
Path of Sun on celestial sphere called the ecliptic.
It moves further north then south on that sphere, moving across the sky like northern stars in summer and like southern stars in winter.
[Look at figure 1.4 again, in the left figure, the two stars shown rising along the eastern horizon follow paths like the Sun at winter and summer solstice.]

The modern explanation of the seasons (the Sun's north/south motion) is very different.
See figure 1.7 on page 28 and figures 1.9, 1.10, 1.11, 1.12 also.

The Earth goes around (orbits) the Sun while spinning.
The Earth's spin axis is tilted 23.5 degrees.

The spin axis always points the same direction, towards the North Star.
Sometimes the northern hemisphere is tilted towards the Sun.
During these periods:

the days are longer (24h in some places).
the Sun rises in the NE, sets in NW, goes higher in our skies.
we have summer.

Six months later, Earth is on the opposite side of the Sun,
Sun is low in our skies

rises SE, sets SW
days are shorter
winter (summer in the southern hemisphere)

Fall and Spring occur in-between.
Then the Earth is tilted "sideways", neither towards nor away from the Sun.
The seasons are due to the tilt of the Earth, not its varying distance from the Sun.
Proven by the opposite seasons in northern and southern hemishperes.

[Typical end of first lecture.]

Sun and Stars
The Sun moves around our sky in 24 hours.
Stars move around our sky in 23h 56m.
Both move in circles, the stars move a little faster!

Suppose the Sun and some star are in the same direction right now (point).
In 23h 56m, the star will have gone around the sky and be back to the original position.
The Sun takes a little longer to go around, the Sun will be a little behind the star, another 4 minutes until the Sun returns to the same spot (the Sun also moves slowly north and south).

The Sun doesn't stay aligned with stars.
The Sun falls behind (or the stars pull ahead).
The Sun moves eastward relative to the stars.
The stars move westward relative to the Sun.

So if tonight at say 9 pm, you see some star (not planet or comet!) up in the sky.
The next night again at 9 pm, it will be further west in the sky.
As the days and months go by, the star will move further and further west until it overtakes the Sun and will no longer be visible at that time.
After a year, the Sun and star will have moved so much that they are back to their original positions.
Each year, the same stars (constellations) will return to the night sky.
The Sun, stars, and seasons all vary in the same time period of one year.

The Sun appears to move among the stars.
The path the Sun follows among the stars is called the ecliptic.
It makes one trip around the ecliptic each year.
The Sun follows exactly the same path every year (show on globe?).

As the year goes by, the Sun moves through a sequence of star constellations.
The Sun passes through 12 constellations, these are called the zodiac.
[Zodiac means "circle of animals".]

Keep the motions straight.
Everything whirls around us, east to west, daily.
The Sun and stars move at slightly different speeds, so the Sun shifts position relative to the stars during the year.

Now the Sun doesn't actually whirl around us daily, it just looks that way because the Earth is spinning.
It is actually Earth which orbits around the Sun, see figures 1.5 and 1.6 on page 27.

When we look at the Sun from Earth, there are stars we see behind the Sun, we say the Sun is "in" that constellation.
When the Earth moves, we are looking at the Sun from a different direction and it appears to have moved to a different constellation.
It is actually the Earth's motion that causes the ecliptic path and zodiac constellations.
The Earth orbits the Sun once each year.

This is discussed in greater detail in the textbook in Essay Two (starting page 179), that section includes a lot of important ideas and you need to read it.

The Motions of the Moon
[I'm covering this in a slightly different order than the textbook.]
Every day the Moon rises in the east and sets in the west.
Similar to the Sun and stars.
But the Moon doesn't move at the same rate as either of those.
The Moon moves more slowly across our sky.
The Moon makes one loop around our sky every 24h 48m (varies a lot).

Right now, the Moon is about there (point).
[The Moon can be seen during daylight.]
The Sun is about there (point) in the sky.

As the day goes by, both move around our sky.
The Moon moves slower.
By tomorrow, the Sun will be back to there, but the Moon will be tardy, it will be there.
And by the next day when the Sun is back there, the Moon will be further east, short of its position the previous day.

This shifting will continue day after day.
The Sun will overtake the Moon.
Eventually the two will be back to their original positions.
This cycle takes 29.5 days, this is the origin of our month (named after the Moon!).

The Moon is also seen to go through phases.
Phases: New, Crescent, Quarter (appears half-lit), Gibbous, Full

The cycle of phases of the Moon repeats in 29.5 days, the same as its cycle relative to the Sun.
This is no surprise, the Sun illuminates half the Moon, we on Earth just see the Moon from different angles.
See (study!) figure 1.17 on page 36 and the "Visibility of the Moon" section.
We will go into Moon phases in more detail in labs.

This was comparing the Moon to the Sun.
The Moon moves at a different rate than the stars.
The stars were even quicker than the Sun.
The Moon returns to the same position relative to the stars in 27 days.
The Moon also moves north/south in our sky with a 27 day period.

Why 27 days for some things and 29.5 days for others?
The Moon is between the Earth and Sun, from the Earth we
see the unlit side, a New Moon.

The Moon orbits the Earth in 27 days.
It will be seen back in the same constellation after 27 days.
But because the Earth and Moon are also moving around
the Sun, it takes 29.5 days before the next New Moon.

Summary:
Everything out in space appears to move around us, but at various speeds.
It's like sitting in the infield of a track during a marathon race.
Everyone is going around you but at different rates.

The stars are going fastest, one lap every 23h 56m.
The Sun is second, 24h, 4 minutes behind the stars with each lap.
The Moon is much slower, 24h 48m.

The reason for these times is that the Earth spins around once every 23h 56m.
Because the Moon moves around Earth, and Earth around the Sun, the time for the Sun and Moon to get back to the same spot in the sky is a little bit different than 23h 56m.

Got it?
Good, because now we get to the motions of the planets and things get more complicated.


The Motions of the Planets (we are still in Section 1.1)
To the naked eye, planets just look like (bright) stars.
Every day, the planets, like stars, rise in the east and set in the west.
But the planets move at slightly different speeds through our skies than the stars.
The planets also move north and south like the Sun and Moon.
The planets change their positions relative to the stars, moving from constellation to constellation.
Planets is the Greek word for "wanderers".

Let me illustrate just how strange planetary motion can be:
See figure 1.16 on page 35.

When "moving eastward", the planet is moving around our skies slower than the stars, moving eastward relative to them is falling behind.
Planets generally move more slowly across the sky than the stars.

But occasionally, the planet will start moving faster than the stars and drift westward relative to them.
(Everything moves east to west, just at different rates.)
This unusual faster-than-stars motion (westward) is called retrograde.
["Retrograde" means "backwards".]


Say you go out tonight and see a planet next to a star.
By the next night, the star, at the same time in the evening, will be a little further west - it gains on the Sun.
The planet will also be a little further west, but usually not quite as far as the star.
During a retrograde, the planet will gain on the Sun even faster than the star.


Some other details on the motions of planets:
Mercury and Venus are always near the Sun.
They move ahead and behind, never too far, forever.
Mercury generally stays closer to the Sun than Venus.

Mars, Jupiter, and Saturn are different.
They can be anywhere in the sky relative to the Sun.

Why do planets move differently than the Sun, Moon, and stars?
Why do planets have retrogrades?
Are planets located among the stars or are they closer to us?
Does the Earth move?

How do we explain the motion of the planets?
Two competing explanations are discussed in the next section.

Why do we have seven days in a week? (this is covered in Essay Two)
Because there was seven special objects in the sky.
The Sun, Moon, and five planets.

Saturday = Saturn Day
Sunday = Sun Day
Monday = Moon
Others in English are named after Norse gods.
[Tiw, Woden, Thor, Frig]
In Spanish, Martes = Mars
Miercoles = Mercury
Jueves = Jove/Jupiter
Viernis = Venus

Where did Astrology come from?
The Sun, Moon, and planets moved different from the stars.
They moved in the heavens as if they had a mind of their own.
The planets were named after gods,
Mercury, Venus, Mars, Jupiter, Saturn.
Many believed that these points of light in the sky were the gods!

Those that believed the Sun and planets were gods thought their positions in the sky controlled events on Earth.
Hence the ability to foretell the future depended upon predicting the motion of these points of light in the sky.
So that you'd know when the gods favored marriage or war or . . .
The origin of mathematics and astronomy dates back to these earliest studies of points of light in the sky.

Today we know that the planets are not gods, they are huge rocks in space.
Yet there are still some people today who believe the planets control our future, this belief is called Astrology.
The planets do not control our lives.
They do not tell us when we will win the lottery or where to find true love.
Astrology is nonsense.

Sun O . . . . . . . . . . . . . . o . .o Moon
where that is the Earth between the Sun and Moon.

The Moon is passed by the Sun once every month (29.5 days).
Do we get an eclipse once a month? NO
Why not?
The Moon's orbit is inclined 5 degrees relative to the ecliptic.
When the Sun and Moon pass each other in the sky, they usually "miss" so that no eclipse occurs.
The path of the Sun in the sky (actually due to the Earth's orbit) is called the "ecliptic" because the Moon must be near that path for an eclipse to occur.