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Astronomy
This picture was
taken by the
Hubble Space
Telescope and is
provided
courtesy of the
Space Telescope
Science Institute.
What is out there?
How big is the universe?
Does the universe have an “edge”?
When did the universe begin, and will it end?
Is anybody else out there?
Astronomy
Astronomy is one of the sciences. This class
is designed to fulfill the science requirement
and so needs to emphasize what science is
(and isn’t) and how science works, using
astronomy as the example.
Thus the first question to ask is: What is a
Science? (and what is NOT science?)
Another question we need to consider while
we consider the first question is: Can
Science discover “the truth” ?
Science
To really answer this question, we need to
ask: What is the Scientific Method?
(What are the STEPS in the Scientific
Method?)
Scientific Method
1. Define the problem: what are we trying to
figure out?
2. Gather data: experiment to see what
happens.
3. Hypothesize: try to “explain” what we see;
a good theory should a) organize what we
know, and b) predict new things to look for.
4. Test the hypothesis: Go back to step 2
(gather data) to see if the predictions work.
Scientific Theories
For a theory to be classified as scientific, it
must be testable (and hence falsifiable).
For data to be considered scientific, it must be
repeatable.
As we go through this course, we will see
theories that were quite useful in their time
but were later proved inadequate and were
replaced by better theories. This process is
continuing today.
Scientific Truth
The scientific method as listed on the previous
slide looks like an infinite loop (it keeps
going back to step 2!). Do we ever get out
of this loop - do we ever discover the real
“truth” ?
Scientific Truth
A related question is: can we perform
“perfect” experiments - can we make
“perfect” measurements? No!
If we cannot measure perfectly, there is
always room for a little doubt!
Does this mean that we really don’t know
anything?
Scientific Truth
In some cases, we have tested theories, and
they agree with experiment to a high degree
of accuracy: these theories agree with
measurements to the best that we can
perform the measurements. For example,
we know that gravity attracts, and that the
formula for gravity works extremely well!
Scientific Uncertainty
However, other theories (like weather prediction
and global warming) are still quite uncertain
and do not agree all that well with all the
data. However, we are making good
progress in these difficult areas.
Since science is based on experiment, even
“untrue” theories can still be useful in
making predictions and hence in developing
technologies. However, we must always be
careful in “believing” theories until they
have been thoroughly tested.
Science and Philosophy
In some cases we have theories which cannot
yet be tested. In these cases, choosing
between competing theories is more a
matter of philosophy than of science.
Later, when the technology allows for tests to
be made, the theories then can become
scientific.
Astronomy
The first step in the scientific method is:
Define the situation: what are we trying to
explain?
The “situation” is different for the different
scientific disciplines. So what are we
looking at in astronomy?
Astronomy
Astronomy is the science which describes
the celestial bodies according to their
locations, sizes, motions, constitutions
and evolutions.
The first part of the course considers what
we can find out by using only our unaided
eyes. We will consider how different
people and civilizations “made sense” of
these observations.
Astronomy course overview
In part two we will look at our modern tools
(mainly the telescope and spectrograph) for
extending our data gathering.
In the remaining parts of the course we
consider our current best theories about
astronomy: in part 3 we consider the
objects in our astronomical neighborhood:
the solar system; in part 4 we consider
stars; and in part 5 we consider how stars
are grouped and how big and old the
universe is.
Gathering Data
What are the objects that we see when we
look up? For each, we need to consider
the following questions:
a) how big does it appear in the sky?
(how big an angle does it make with the eye?)
b) how bright does it appear to be?
(how do we measure brightness?)
c) where is it located?
(how do we indicate location?)
d) how does it appear to move?
(is it moving, are we moving, or both?)
Gathering Data
1.
2.
3.
4.
Sun
Moon
Stars
Planets (how are planets and stars different?)
Sun
a) How big?
(what shape is it?)
(how do we measure size
for this shape?)
(how do we measure size
for objects that are far
away?)
This image was taken by SOHO's EIT (Extreme-Ultraviolet
Imaging Telescope) and is courtesy of the EIT Consortium.
Sun
a) How big? A circle (or ball?) in the sky
with a diameter that forms an angle with
the eye of about ½ degree. (There are 360
degrees in a full circle. Who decided that we
break a circle into 360 equal parts instead of, say,
100?)
From horizon to horizon
covers 180 degrees
Angle the sun makes is
about ½ degree
Sun
The 360 degrees in a circle comes from the
observed fact that there are 365 days in a
year (solar cycle). But 365 is an
inconvenient number since it is odd. The
360 is much better since it is divisible by 2,
3, 4, 5 and 6!
b) How bright is the sun?
Sun
b) How bright is the sun? Very! It
overwhelms almost everything else in the
sky when it is visible. (We’ll set up a scale
later.)
c) Where is the sun located?
Sun
c) Where is the sun located?
At different places in the sky at different
times of the day; and at different places in
the sky at the same time of day when
viewed from different locations on the
earth. (For instance, the sun may still be up on the
West Coast when it has already set on the East
Coast.) This will be further analyzed as we
answer the next question:
d) How does the sun move?
Sun
d) How does the sun move?
d1: a DAY is the time from noon on one day
to noon on the next day. Noon is the time of
day when the sun is highest in the sky (and
shadows are the smallest).
d2: The sun rises in the East (more or less)
and sets in the West (more or less). Does
the sun always rise exactly due East and
set exactly due West?
Sun
d2: Does the sun always rise exactly due East
and set exactly due West?
No! The sun rises North of due East and
sets North of due West in the spring and
summer, and it rises South of due East
and Sets South of due West in the fall and
winter. It only rises due East on the first
day of spring and first day of fall. This
change in the rising and/or setting positions
of the sun can be used to mark a yearly
calendar.
Sun
In the first lab meeting, we will use a star
chart to investigate the motions of the sun:
a) how it rises and sets throughout the year;
b) how high in the sky it is at noon at
different days throughout the year;
c) how the sun moves relative to the stars.
There is a separate powerpoint set on the Star
Charts which you can use to familiarize
yourself with this useful tool.
Moon
a: How big is the
moon?
(What shape is it?)
(How does the moon’s
size compare to the
sun’s size?)
The Galileo spacecraft sent back this image of the Moon as it
headed into the outer solar system. The distinct bright ray
crater at the bottom of the image is the Tycho impact basin.
Moon
a) How big is the moon? A circle (or ball?)
that makes about ½ degree with the eye.
This is about the same size (in angle) as the
sun. This will be important when we talk
about eclipses in Part III. The moon
appears to change shape: from a new moon
(which is dark) to a crescent shape to a half
circle to a gibbous shape to a full circle and
then back through these shapes to a new
moon. This cycle takes about a month.
b) How bright is the moon?
Moon
b) How bright is the moon? A full moon is
quite bright - enough to make it hard to
see the dimmer stars, but not nearly as
bright as the sun. The new moon is so
dim it is hard to see (also because it
appears to be so close to the sun).
c) Where is the moon located?
Moon
c) Where is the moon located? At different
places in the sky at different times of the
day; the phases (shape) of the moon are
related to its position during the day.
New moons are always near the sun, and
full moons are always opposite the sun.
d) How does the moon move?
Moon
d) How does the moon move? It follows a
path similar to that of the sun: rising
somewhere in the East and setting
somewhere in the West. It’s time of
rising and setting changes during the
month. When the moon is new, it rises
and sets with the sun; when the moon is
full, it rises and sets opposite the sun.
Stars
a) How big do the stars appear to be?
Betelgeuse, the brightest star in the constellation Orion. (Produced
with ESA's Faint Object Camera (FOC), Hubble Space Telescope.)
Stars
a) How big do the stars appear to be? Each
star appears to be a point, that is, it
makes too small an angle to be measured.
b) How bright are the stars?
Stars
b) How bright are the stars? The stars vary
in brightness - some are so dim that they
can only be viewed by the most powerful
telescopes while others are bright enough
to be seen in faint early dawn or late
evening twilight. (We’ll set up a scale later.)
c) Where are the stars located?
Stars
c) Where are the stars located? Like the sun
and the moon, stars move across the sky;
however, the stars do not move relative to
one another - that is, we can group them
into constellations (like the big dipper).
d) How do the stars move?
Stars
d) How do the stars move? Except for the
North Star, all stars move across the sky.
They move around the North Star and so
most seem to rise in the East and set in
the West just like the sun and moon.
In this connection, the sun and the moon do
change relative position with the stars. The
sun seems to move along a path through the
stars called the ecliptic. The moon follows
a path very close to the ecliptic.
Sun and Stars
The constellations that are found on the path of the
sun (the ecliptic) are called the constellations of
the zodiac. During the year the sun moves along
the ecliptic spending approximately one month in
each of the 12 constellations of the zodiac. The
sun moves from West to East through these
constellations (although it moves from East to
West across the sky), and this causes the sun to
rise a little later than a particular constellation or
star each day, and conversely the constellation and
its stars rise a little earlier each day.
Constellations of the Zodiac
The months below refer to when the sun appears to be in the constellation
Constellation Month
Constellation Month
Aquarius
Pisces
Aries
Taurus
Gemini
Cancer
Leo
September
Virgo
October
Libra
November
Scorpius
December
Sagittarius January
Capricornus February
March
April
May
June
July
August
Note: The months are approximate
Heliacal Rising
Since the stars appear to rise a little earlier
each day than the sun, for each star there is
one day each year that the star can first be
seen before sunrise (assuming clear skies).
After that, the star can be seen earlier and
earlier. This first day is called the “heliacal
rising” of that star. Some cultures based
their calendars on such heliacal risings of
certain bright stars.
Moon, Sun and Stars
The moon also moves along a path very close to the
one the sun moves on (the ecliptic). It makes a
complete cycle around the path through the
constellations of the zodiac once a month (rather
than once a year like the sun). It also moves East
along its path through the stars (but moves West
through the sky as we see it), so it also rises a little
later each day than the stars, and since the moon
moves faster than the sun, the moon rises a little
later (relative to the sun) each day.
Planets
Although the stars do not
appear to move relative
to one another (and
hence we can make
constellations out of them),
there are five visible
exceptions to this
Voyager 2 captured this image of Neptune in 1989.
(besides the sun and moon
which makes the total seven).
These the Greeks named wandering stars which
today we call planets. The five planets visible to the
naked eye are: Mercury, Venus, Mars, Jupiter and
Saturn.
Planets
Although without a telescope the planets do
not appear to have a measurable size, with a
modest telescope they do!
Do the planets wander all over the place, or
do they have definite paths (like the sun
and moon along the ecliptic)?
Planets
Do the planets wander all over the place, or do
they have definite paths (like the sun and
moon along the ecliptic)?
All the planets have definite paths through
the stars, and all these paths are very
close to the ecliptic.
Planets
Like the sun, moon and stars, the planets
move across the sky from East to West; but
do the planets move across the ecliptic from
West to East like the sun and moon?
Planets
Like the sun, moon and stars, the planets
move across the sky from East to West; but
do the planets move across the ecliptic from
West to East like the sun and moon? All
the planets do move from West to East
along the ecliptic MOST OF THE TIME,
but occasionally each goes backward
(from East to West). This is called
RETROGRADE motion.
Planets
Another interesting piece of data: Mercury
and Venus are always close to the sun in the
sky. You never see Mercury or Venus at
midnight - only a little before dawn or a
little after sunset. The other three: Mars,
Jupiter and Saturn can sometimes be seen
close to the sun but can also be seen far
from the sun (you can sometimes see these
at midnight).
Positions in the sky
To locate a position on the earth, we can tell
where it is in relation to other landmarks, or
we can give its latitude and longitude.
In the same way in the sky, we can tell where
something is near to something else, e.g.,
which constellation it is in or near, or we can
give its declination and right ascension.
Declination
Declination is like latitude: it gives the north-south
position. The North Star is directly above the
North Pole. So we give the North Star a
declination of +90o (just like the North pole has a
latitude of 90o North). The positions of the sky
directly over the equator have a declination of 0o
just like the equator has a latitude of 0o. Anything
that is directly above places South of the Equator
(latitude of xo South) is given a declination of -xo .
Right Ascension
Right Ascension is like longitude. It describes
the East-West position. Just like longitude has to
have some starting place (on Earth, we assign the
North-South line through Greenwich, England as having 0o
longitude), so right ascension needs some line to
call 0. This line is the North-South line from the
North Star through the location of where the sun is
on March 21 (equal day and night = equinox). Instead
of degrees we use hours: there are 24 hours in the
complete circle.
Star Charts and locations
On one type of star chart the celestial equator (0o
declination) is marked with a solid white line. [On
the other, the one used in the Star Chart
powerpoint set, the celestial equator is the pink
circle added to the slide.] The North Star is the
brass ring (since it is the one place that does not move).
The ecliptic (path of sun, moon and planets) is marked
with a dotted white line. [On the other type, it is
marked as the “railroad tracks”.] Note that the
dotted white line crosses the solid white line in
two places: the sun is at one place on March 21
and on the other on September 21 (the date of the
two equinoxes).
Constellations of the Zodiac
The months below refer to when the sun appears to be in the constellation
Constellation Month RA Dec
Constellation
Month RA Dec
Aquarius
Pisces
Aries
Taurus
Gemini
Cancer
Leo
Virgo
Libra
Scorpius
Sagittarius
Capricornus
Sept.
Oct.
Nov.
Dec.
Jan.
Feb.
March
April
May
June
July
August
23
1
3
5
7
9
0
0
+
+
+
+
11
13
15
17
19
21
0
0
-
Note: The months and Right Ascension are approximate