Venus goes through phases. Just like our Moon, Venus can appear as full as a disk or as a thin as a crescent. Venus, frequently the brightest object in the post-sunset or pre-sunrise sky, appears so small, however, that it usually requires binoculars or a small telescope to clearly see its current phase. The time-lapse sequence was taken over the course of many months and shows not only how Venus changes phase but how it's apparent angular size also changes. Astronomy Picture of the Day

Homework #1 will be posted soon. An announcement and will be sent to class members providing information and instructions. You will typically have 5 or more days to complete a homework assignment.

DISTANCE SCALE Seeing the past

Units of Distance in Astronomy Meter: ~10% larger than 1 yard Astronomical Unit (AU): Earth – Sun distance, ~ 150,000,000 kilometers Light Year (LY): distance light travels in one year, ~ 9.46 x km Parsec (pc): ~ 3.26 light years

Big Bang (~ 13.7 x 10 9 yrs ago) Expansion & Cooling Initially only Hydrogen & Helium Growth of structures (gravity) Stars and Galaxies form Nucleosynthesis - “star stuff” Planets possible - “heavier” elements Solar System forms (4.6 x 10 9 yrs ago) TimeTime

Which of the following best describes what we mean by the “universe”? (red) A vast collection of stars that number as many as the grains of sand on all the beaches on Earth. (blue) All the galaxies in all the superclusters. (yellow) The universe is another name for our Milky Way Galaxy (green) The sum total of all matter and energy.

Which of the following best describes what we mean by the “universe”? (red) A vast collection of stars that number as many as the grains of sand on all the beaches on Earth. (blue) All the galaxies in all the superclusters. (orange) The universe is another name for our Milky Way Galaxy (green) The sum total of all matter and energy.

Which of the following has your "cosmic address" in the correct order? You, Earth, solar system, Milky Way Galaxy, Local Group, Local Supercluster, universe You, Earth, Local Group, Local Supercluster, solar system, Milky Way Galaxy, universe You, Earth, solar system, Local Group, Local Supercluster, Milky Way Galaxy, universe You, Earth, solar system, Local Group, Milky Way Galaxy, Local Supercluster, universe

Which of the following has your "cosmic address" in the correct order? You, Earth, solar system, Milky Way Galaxy, Local Group, Local Supercluster, universe You, Earth, Local Group, Local Supercluster, solar system, Milky Way Galaxy, universe You, Earth, solar system, Local Group, Local Supercluster, Milky Way Galaxy, universe You, Earth, solar system, Local Group, Milky Way Galaxy, Local Supercluster, universe

“Spaceship Earth” ● Earth rotates about an axis (day) Earth rotates about an axis (day)

“Spaceship Earth” ● Earth rotates about an axis (day) Earth rotates about an axis (day) ● Earth revolves about ( orbits ) the sun (year)Earth revolves about ( orbits ) the sun (year)

“Spaceship Earth” ● Earth rotates about an axis (day) Earth rotates about an axis (day) ● Earth revolves about ( orbiting ) the sun (year)Earth revolves about ( orbiting ) the sun (year) ● Earth’s rotation axis is tilted by 23.5 degrees to the orbital plane (seasons) Earth’s rotation axis is tilted by 23.5 degrees to the orbital plane (seasons)

“Spaceship Earth” ● Earth rotates about an axis (day) Earth rotates about an axis (day) ● Earth revolves about ( orbiting ) the sun (year)Earth revolves about ( orbiting ) the sun (year) ● Earth’s rotation axis is tilted by 23.5 degrees to the orbital plane (seasons) Earth’s rotation axis is tilted by 23.5 degrees to the orbital plane (seasons) ● The Sun and solar system orbit around the center of the Milky Way Galaxy The Sun and solar system orbit around the center of the Milky Way Galaxy

How do we know these things?

It all begins with observations. What are the “observed facts”? How do we explain these observed facts? Ultimately, what are the underlying laws of nature that underpin these explanations?

The origins of astronomy and the physical sciences stem from:  observing the sky and nature  finding repeatable patterns  seeking natural explanations for these patterns

Celestial Sphere Large imaginary spherical surface centered on the Earth. Stars and other celestial objects “fixed” on its surface. Conceptual Model, not a physical model

The North Celestial Pole (NCP) and the South Celestial Pole (SCP) are located at the intersection of the earth’s rotation axis with the celestial sphere. The Celestial Equator is the extension of the Earth’s equator onto the celestial sphere.

We view the celestial sphere in the context of the “local sky” Observer’s in different locations see a different sky

Motions of the Earth (rotation, revolution about the sun) cause the portion of the celestial sphere visible in the local sky to change with time.

“Local Sky” positions (defined by observer's location) Cardinal directions – NSEW

Zenith – point directly overhead Nadir - point directly opposite zenith Horizon – half way between zenith and nadir

Meridian – line from north point on horizon, through zenith, to south point on horizon (divides sky into eastern and western halves)

Local sky positions remain fixed: horizon, zenith, cardinal locations, meridian, are always in same location.

In general, most stars are seen to  rise in the eastern half of the sky (i.e., east of the meridian),  increase their altitude (angular distance from the horizon) until they cross the meridian,  set in the western half of the sky (i.e., west of the meridian).

However, some stars (called “circumpolar stars”) are always above the horizon.

What about the Sun’s motion?  Moves east to west, full circuit around the sky, each “day”  Moves slowly eastward each day (~ 1 degree/day), relative to the stars: full circuit in one “year”  Path through sky is a great circle called the “ecliptic”  Constellations ecliptic passes through are referred to as the “zodiac”

The sun follows the same path around the sky (celestial sphere), repeating this journey once every year. This path is called the “Ecliptic”

Why does the sun appear to move like this?

A Geometrical aside… Great Circles and Spheres

Great Circle ● Intersection of a sphere with a plane passing through the center of the sphere. A great circle divides a sphere into two equal hemispheres

Small Circle ● Intersection of a sphere with a plane not passing through the center of the sphere. A small circle divides a sphere into two unequal portions

Great Circles Small Circles

The Ecliptic is a great circle

Locations of planets in the sky  Mercury: always close to Sun in sky  Venus: always close to Sun in sky  Mars: no restrictions on distance from Sun in sky  Jupiter: no restrictions on distance from Sun in sky  Saturn: no restrictions on distance from Sun in sky What causes these differences?

 On short term (diurnal motion), planets appear to move with the stars, east to west, making a full circuit around the sky (meridian to meridian) in approximately one day  Most of the time, planets move slowly eastward each day relative to the stars: different planets at different rates Motions of the planets What causes these motions?

 Planets are always close to the “ecliptic”, the apparent annual path of the sun through the sky.

Close grouping of five planets in April This is a pattern that was well known to the “ancients” Why are the planets restricted to these locations?

Some planets occasionally reverse their motion relative to the stars, moving slowly westward relative to the stars, for a few days apparent retrograde motion What causes this?

The development of Astronomy as a Science  Prehistory - development of myths to explain natural phenomena - no physical model  Later, many threads of physical models were developed, most were lost  We owe the origins of the physical sciences to the Greeks, later threads came from many other cultures.  Physical models are developed to explain observations (everything starts with observations)  These models then make further predictions that can be tested.

 Understanding and explaining motions observed in sky led to geometric models for the geometry of the universe.  These models, in turn, motivated the development of the physical sciences: required to explain this geometry.  Ultimately, this led to an understanding of how stars and planets form.

Understanding the visible sky and motions within the sky is easily accomplished  using the concept of Celestial Sphere  understanding the rotational & orbital motions of the Earth

As you move around on the Earth, the portion of the celestial sphere visible in your local sky changes.

1. At what location on the Earth will the North Celestial Pole appear directly overhead?

Where on Earth is the Celestial Equator directly overhead?

How much of the Celestial Sphere can you see at any given time?

Half of the sky is always visible above the horizon.

Moving north or south on the Earth (changing your latitude), leads to viewing a different portion of the celestial sphere.

If you are located in the northern hemisphere, what is the point in the sky around which objects on the Celestial Sphere appear to rotate over the course of a day? Do they appear to rotate clockwise or counter- clockwise?

Discuss with your neighbours What is the relationship between the altitude of the North Celestial Pole and the latitude of an observer on Earth. (hint: imagine observer’s at the North Pole, at the Equator, and at a mid- latitude location, such as Bloomington).

Today we divide the celestial sphere into 88 regions, referred to asconstellations (in a manner similar to dividing the Earth’s surface into countries).

Circumpolar Stars: never set

ConceptTest As you move from the Earth's equator toward the North Pole, the number of circumpolar stars: increases decreases stays the same

ConceptTest As you move from the Earth's equator toward the North Pole, the number of circumpolar stars: increases b. decreases c. stays the same

On the Earth any location can be given by two quantities: Longitude (angular distance east or west from the “Prime Meridian”)  Latitude (angular distance north or south of the equator)

Two quantities (coordinates) + Locations that define the origins (“zero points”) = all that is needed to locate anything on a sphere

On the Celestial sphere any location can be given by its:  Declination (Celestial Latitude, measured north or south of the Celestial equator)  Right Ascension (Celestial Longitude, measured (angular distance from the “Vernal Equinox”)

Back to our story…

The north celestial pole (NCP) and the south celestial pole (SCP) are located at the intersection of the earth’s rotation axis with the celestial sphere. The Celestial equator is the extension of the Earth’s equator onto the celestial sphere.

CONSTELLATIONS The celestial sphere is divided into 88 regions, referred to as constellations, in a manner similar to dividing the Earth’s surface into countries.

On the Earth any location can be given by two quantities: Longitude (angular distance east or west from the “Prime Meridian”)  Latitude (angular distance north or south of the equator)

Two quantities (coordinates) + Locations that define the origins (“zero points”) = all that is needed to locate anything on a sphere

On the Celestial sphere any location can be given by its:  Declination (Celestial Latitude, measured north or south of the Celestial equator)  Right Ascension (Celestial Longitude, measured (angular distance from the “Vernal Equinox”)

Local Sky: Defined by the observer We view the celestial sphere in the context of the local sky

So, we have a sphere (the Earth) within a sphere (Celestial sphere). If both were fixed and unchanging, we would always look to the same point in the sky to see the same object. BUT…  We can be located anywhere on the Earth  The Earth is rotating  The Earth is orbiting the Sun  The Moon is orbiting the Earth  The planets are also orbiting the sun We need to take all of this into account to understand changes in the sky, time, seasons, etc.

GROUP ACTIVITY  Assemble into groups of 5 or 6 students  Select a moderator for the day  Introduce yourselves & “schmooze” for a couple minutes  Discuss why you are taking the class  Put together a list of names of the group to turn in  After a few minutes, we will have an exercise Schmooze: (from a Yiddish word meaning to chat) (1)To converse idly or casually and in a friendly manner, especially in order to make a social connection. (2) The act or an instance of schmoozing.

Do the instructed exercise within the group - have everybody do it but discuss within the group) Think about: What changes with rotation? What changes with “latitude”? What doesn't change? What can you see, what can’t you see?

“Local Sky” positions (defined by observer's location) Cardinal directions – NSEW Zenith – point directly overhead Nadir - point directly opposite zenith Horizon – half way between zenith and nadir Meridian – line from north point on horizon, through zenith, to south point on horizon (divides sky into eastern and western halfs)

Local sky positions remain fixed: horizon, zenith, cardinal locations, meridian, are always in same location. Motions of the Earth (rotation, revolution about the sun) cause the portion of the celestial sphere visible in the local sky to change with time.