Astronomy Picture of the Day Venus goes through phases. Just like our Moon, Venus can appear as full as a disk or as a thin as a crescent. 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. Venus is frequently the brightest object in the post-sunset or pre-sunrise sky. It appears so small, however, that it usually requires binoculars or a small telescope to clearly see its current phase.

Homework #1 will be posted soon. An announcement and email 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 (m): ~10% larger than 1 yard Kilometer (km): 1,000 m (~ 0.62 miles) Astronomical Unit (AU): Earth – Sun distance, ~ 150,000,000 kilometers Light Year (LY): distance light travels in one year, ~ 9.46 x 1012 km Parsec (pc): ~ 3.26 light years (31 x 1012 km or 49 x 1012 miles)

If we shrunk the Universe so that Solar System was the radius of a Quarter:a Our Galaxy (the Milky Way) would extend across the United States. Our whole Solar System Imagine, again, shrinking our solar system to a little larger than a quarter: Our Milky Way Galaxy

Big Bang (~ 13.7 x 109 yrs ago) Expansion & Cooling of the Universe T i m e 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 109 yrs ago)

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

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

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

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

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

Half of the Celestial Sphere is visible in the local sky, the other half is below the horizon. Rotation of the Earth causes the portion of the celestial sphere visible in the local sky to change with time.

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.

(defined by observer's location) “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, 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”

This path is called the “Ecliptic” 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?

Great Circles and Spheres A Geometrical aside… Great Circles and Spheres

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

A small circle divides a sphere into two unequal portions 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 Planets are always seen close to the Ecliptic! How do we understand these “observed facts”?

Motions of the planets 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 What causes these motions?

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

Why are the planets restricted to these locations? Close grouping of five planets in April 2002. This is a pattern that was well known to the “ancients” Why are the planets restricted to these locations?

apparent retrograde motion 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?

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.

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

Circumpolar Stars: never set