Astronomy

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?

This picture was taken by the Hubble Space Telescope and is provided courtesy of the Space Telescope Science Institute.

AstronomyAstronomy 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 UncertaintyHowever, 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 overviewIn 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 DataWhat 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. Sun

2. Moon

3. Stars

4. 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.

Suna) 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

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?

Sunc) 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?

Sund) 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?

Sund2: 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.

SunIn 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.

Moona) 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?

Starsd) 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 ZodiacThe months below refer to when the sun appears to be in the constellation

Constellation Month Constellation Month

Aquarius March Leo September Pisces April Virgo OctoberAries May Libra NovemberTaurus June Scorpius December Gemini July Sagittarius JanuaryCancer August Capricornus February

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.

PlanetsAlthough the stars do not

appear 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.

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 locationsOn 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 ZodiacThe 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 0Pisces April 1 0 Virgo Oct. 13 0Aries 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