Astronomy

1
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?

2
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 isnt)
  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 ?

3
Science

• To really answer this question, we need to ask
  What is the Scientific Method?
• (What are the STEPS in the Scientific Method?)

4
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.

5
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.

6
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 ?

7
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 dont know anything?

8
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!

9
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.

10
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.

11
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?

12
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.

13
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.

14
Gathering Data

• What are the objects that we see when we look up?
  For each, we need to consider the following
  questions
• how big does it appear in the sky?
• (how big an angle does it make with the eye?)
• how bright does it appear to be?
• (how do we measure brightness?)
• where is it located?
• (how do we indicate location?)
• how does it appear to move?
• (is it moving, are we moving, or both?)

15
Gathering Data

• 1. Sun
• 2. Moon
• 3. Stars
• 4. Planets (how are planets and stars
  different?)

16
Sun

• 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.
17
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?)

Angle the sun makes is about ½ degree
From horizon to horizon covers 180 degrees
18
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?

19
Sun

• b) How bright is the sun? Very! It overwhelms
  almost everything else in the sky when it is
  visible. (Well set up a scale later.)
• c) Where is the sun located?

20
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?

21
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?

22
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.

23
Sun

• In the first lab meeting, we will use a star
  chart to investigate the motions of the suna)
  how it rises and sets throughout the yearb)
  how high in the sky it is at noon at different
  days throughout the yearc) 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.

24
Moon

• a How big is the moon?
• (What shape is it?)
• (How does the moons size compare to the suns
  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.
25
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?

26
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?

27
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?

28
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. Its
  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.

29
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.)
30
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?

31
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. (Well set
  up a scale later.)
• c) Where are the stars located?

32
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?

33
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.

34
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.

35
Constellations of the Zodiac

• The months below refer to when the sun appears to
  be in the constellation
• Constellation Month Constellation Month
• Aquarius March Leo September
• Pisces April Virgo October
• Aries May Libra November
• Taurus June Scorpius December
• Gemini July Sagittarius January
• Cancer August Capricornus February
• Note The months are approximate

36
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.

37
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.

38
Planets

• Although the stars do notappear to move
  relativeto one another (and hence we can make
  constellations out of them),there are five
  visible exceptions to this(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.

Voyager 2 captured this image of Neptune in 1989.
39
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)?

40
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.

41
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?

42
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.

43
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).

44
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.

45
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 .

46
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.

47
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).

48
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 March 23 0 Leo Sept. 11 0
• Pisces April 1 0 Virgo Oct. 13
  0
• Aries May 3 Libra Nov. 15 -
• Taurus June 5 Scorpius Dec. 17
  -
• Gemini July 7 Sagittarius Jan.
  19 -
• Cancer August 9 Capricornus Feb. 21
  -
• Note The months and Right Ascension are
  approximate