A laser is used at the Very Large Telescope (VLT) site in Chile to measure the distortions of Earth's ever changing atmosphere. Constant imaging of high-altitude atoms excited by the laser - which appear like an artificial star - allow astronomers to instantly measure atmospheric blurring. This information is fed back to a VLT telescope mirror which is then slightly deformed to minimize this blurring.

The VLT consists of four 8.2 meter (27.3 ft) wide telescope units: Yepun (Sirius), Antu (Sun), Kueyen (Moon), & Melipal (Southern Cross). The light from each of these is combined, achieving an equivalent aperture of 16.4 meters (54.7 ft)

Homework #1 will be posted soon Homework #1 will be posted soon. An announcement and instructions will be sent to the class once it is posted.

Where do we fit in the universe?

Science begins with observation:

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

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.

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.

Motions in the sky:

Stellarium: www. stellarium Stellarium: www.stellarium.org -- A free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope.

Screenshots from Stellarium

Planets Known in Ancient Times Mercury: difficult to see; always close to Sun in sky Venus: very bright when visible; always close to Sun in sky, but can be farther than Mercury Mars: noticeably red Jupiter: very bright Saturn: moderately bright

Q: Why are there seven days in the week Q: Why are there seven days in the week? A: There were seven objects in the sky that moved independently of the stars. Roman gods associated with planets 1. Saturn: Saturday 2. Sun: Sunday 3. Moon: Monday Anglo-Saxon or Nordic analogues to Roman gods 4. Mars: Tuesday (Tiw’s day) 5. Mercury: Wednesday (Woden’s day) 6. Jupiter: Thursday (Thor’s day) 7. Venus: Friday (Freys’ day)

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

Patterns Observed in the sky: (observed facts)

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

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?

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?

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?

It did a reasonably good job explaining these motions. What causes the observed motions of the stars, sun, moon, and planets in the sky? The Greeks developed a model for the Universe that lasted for nearly 15 centuries. It did a reasonably good job explaining these motions.

Claudius Ptolemy (100-170 CE) Developed a model of the universe designed to fit the observational data.

“all natural motion is circular” Ptolemy and later scientists were strongly influenced by the belief of Plato that … “all natural motion is circular”

Ptolemy’s Geocentric Model Earth is at center (Geocentric) Sun orbits Earth Planets orbit on small circles (epicycles) whose centers orbit the Earth on larger circles (this explains retrograde motion)

Apparent retrograde motion in geocentric model

Planet orbits lie in approximately the same plane (this explains why the planets are always near the ecliptic) Inferior planet epicycles were fixed to the Earth-Sun line (this explained why Mercury & Venus never stray far from the Sun). Geocentric Model

Ptolemy’s model fit the data and made accurate predictions, but was horribly contrived!

Although the geocentric model of Ptolemy gained dominance, Aristarchus of Samos actually proposed that the earth rotated daily and revolved around the sun

Ptolemy’s Geocentric Model Relied upon circles upon circles (epicycles & defferents) to explain the motions of planets and the sun. Tied to Plato & Aristotle’s belief that “all natural motion is circular” With modifications (e.g., additions of epicycles upon epicycles), remained the standard through the middle-ages.

Plato proposed that the orbits of the planets have what shape? conical circular elliptical equal-angular epicycles

Plato proposed that the orbits of the planets have what shape? conical circular elliptical equal-angular epicycles

The diurnal (daily) motion of stars is due to the motion of the earth around the sun the rotation of the earth the epicyclic nature of the celestial sphere the rotation of the celestial sphere

The diurnal (daily) motion of stars is due to the motion of the earth around the sun the rotation of the earth the epicyclic nature of the celestial sphere the rotation of the celestial sphere

What is the ecliptic? when the Moon passes in front of the Sun the constellations commonly used in astrology to predict the future the Sun's daily path across the sky the Sun's apparent path across the celestial sphere

What is the ecliptic? when the Moon passes in front of the Sun the constellations commonly used in astrology to predict the future the Sun's daily path across the sky the Sun's apparent path across the celestial sphere

About how long does it take the Sun to complete one “trip” around along the ecliptic around the entire sky? One day One month One year The time varies from one trip to the next This never happens

About how long does it take the Sun to complete one “trip” around along the ecliptic around the entire sky? One day One month One year The time varies from one trip to the next This never happens