A. NAKED-EYE ASTRONOMY

• Shape and size of Earth, Moon (Greeks, 500 BC - 200 AD)

• Discovery of Earth's motion around Sun (Copernicus, 1513)

• Kepler's Laws of Planetary Motion (1609, based on Tycho's observations), which led to Newton's Laws of Motion and the modern scientific revolution

• Discovered, for instance, that there were thousands of stars too faint to be seen with the naked eye

B. MOTIVATIONS TO OBSERVE THE SKY

• Curiosity

• Fear/Religious Belief:
  • E.g.: Astrology: Idea that sky can be used to predict future. Based on interpretation of Sun, Moon, planets, stars, as living gods, who betray their intentions by their movements. A powerful motivation for observing the sky, even if (as it turns out to be) sheer superstition.

• Navigation

• Time Keeping: during day or night

• Calendar Keeping: tracking date & seasons
  For early human societies, these were critical survival technologies.

C. EASILY VISIBLE PHENOMENA

• STARS: form backdrop or "reference frame" against which other objects' motions are measured. About 2000-5000 visible to eye (depending on eyesight) over whole sky. About 1000 visible on dark, clear night from given location. Brighter stars form conspicuous patterns which seem unchanging (to eye).
  • Motion: stars move in lockstep across sky from East to West. Patterns come back to same location in sky after slightly less than 24 hours

  • Can you see all the stars that exist? NO! Vast numbers of stars exist which are invisible to the eye. With our 8" telescopes, we can see stars about 800 times fainter than eye limit---up to 5 million over whole sky.

• SUN: Most obvious astronomical object (and most important for us!). Steady brightness. Slow motion relative to stars (must infer position since stars invisible in daylight). One year cycle.

• MOON: Bright, but much fainter than Sun. More rapid motion relative to stars. One month ("moonth") cycle. Drastic change in brightness and phase (bright part as fraction of full circle). Moonlight of great practical value before electrical lighting invented.

• PLANETS: Less obvious. 5 bright, starlike objects; slow, complex motion relative to stars. Mercury, Venus, Mars, Jupiter, Saturn.

• METEORS: streaks of light in sky; typical rate 5-10 per hour. Small pieces of ice, rock burning up in Earth's atmosphere. Definitely not "falling stars."

• COMETS: occasional (once every few years) flamelike objects moving slowly through sky over several weeks. Large (~ 1 km) chunks of ice moving through Solar System, heated by Sun.

• STAR CLUSTERS: compact groups of stars, all formed together. E.g. Pleiades, Hyades, M13. Key to interpretation of stellar evolution.

• NEBULAE: interstellar gas clouds lit up by hot stars. E.g. Great Nebula in Orion. Mark birthplace of young stars.

• MILKY WAY: plane of our own Galaxy (flattened system containing about 100 billion stars), seen edge-on.

• EXTERNAL GALAXIES: other star systems like our Galaxy. 4 visible to eye, 2 of these in northern hemisphere. E.g. Andromeda Galaxy---most distant (2 million light years) object you can see without a telescope.

D. NAKED EYE MEASUREMENTS

1. Angular Separations

Angles can be all-celestial ("sky," e.g. star-to-star) or celestial-terrestrial (sky to reference point on Earth)
Units: Degrees, minutes, seconds of arc
Full circle = 360 degrees of arc; 1 degree = 60 min of arc; 1 arcmin = 60 sec of arc
Don't confuse with units of time! Always use the "arc" terminology for clarity.
Examples:
Angles subtended by a quarter at distance D
• 1 degree @ D ~ 56 in
  
• 1 arcmin @ D ~ 270 feet
  
• 1 arcsec @ D ~ 3 miles

Bowl of "Big Dipper" ~ 10 degrees long.
[~ = "approximately"]
Human eye has 1-2 arcmin resolution.
The Hubble Space Telescope has 0.1" resolution;
i.e. can resolve a quarter @ distance of 30 miles

"Handy" measuring scale:
1 degree = width index finger @ arm's length
10 degrees = closed fist @ arm's length
20 degrees = distance thumb to little finger on outstretched hand @ arm's length
Example: diameter of Sun or Moon is about 0.5 degree. Can cover with index finger @ arm's length. Try it!

2. Brightnesses
   Star brightnesses are quoted on the magnitude scale: logarithmic, open-ended, "backwards." Brighter objects have smaller values.
   Brightest stars are about 0 magnitude; the faintest visible with naked eye are about 5-6 magnitude.
   There are only 11 stars brighter than magnitude 1 visible from Charlottesville but 1630 stars brighter than magnitude 5.
   Faintest objects yet detected (Hubble Space Telescope): 29 magnitude = over 1 billion times fainter than visible to eye.

3. Colors, shapes (in some cases)

4. Time
   Tracking changes in sky with time, e.g. sky motions, with time of day, month, year shows regularities or time cycles.

E. ORIENTATION IN THE NIGHT SKY

The local "horizon plane" is the (imaginary) plane "tangent" to (just touching) the Earth at your location; you can see objects above the plane but not below. The horizon plane sweeps across sky as Earth spins; it determines rising, setting of objects. The horizon plane is different at each location on Earth.
The Celestial Sphere (CS) is a geometric construct to display angular positions of astronomical objects.
The CS is an imaginary hollow sphere centered on Earth. It is shown shaded in the illustration above. Directions to key orienting positions and to each astronomical object are imagined to be marked on surface of sphere.

The zenith is point on CS directly overhead.
The celestial poles are the points on the CS where the projection of Earth's rotation axis pierces the CS. These are fixed points.
The celestial equator is the outward projection of Earth's equator to the CS. A great circle.
Your meridian is the great circle on the CS which passes through the celestial poles AND your zenith.

The rotation of the Earth produces an apparent counter-rotation of the CS and its "attached" stars across your local sky. The Earth rotates eastward, so the sky appears to rotate westward.

Stars rise in the east, move across your meridian, and set in the west. See the figure above. (More on motions in Lecture 3).

F. NIGHT SKY SIMULATIONS

• Constellation identification is strongly culture-dependent. The oldest of the associations used today originated in Mesopotamia ca. 4000 BC. E.g.: Scorpio, Leo. See carved stone at right. (Click for a larger view.)

• Greeks (500 BC - 200 AD) added rich mythological associations and fanciful stories (e.g. Hercules, Perseus, Andromeda). Aratus' (ca. 270 BC) epic poem The Phaenomena describes the constellations as memory aid to navigators. Ptolemy's Almagest (135 AD) listed positions of 1022 stars in 48 constellations. His catalogue used for next 1400 years.

• Romans inherited Greek myths. Names of most constellations are Latin.

• Arab astronomers active 700-1600 AD; many modern star names have Arabic roots: Algol ("demon star"), Antares ("rival of Mars"), Betelgeuse ("armpit of the Central One").

• 1600+: new constellations added to fill in blanks, mostly Southern Hemisphere. E.g. Telescopium, Pyxis (compass). Elaborate & beautiful printed atlases of classical associations (illustration of the north polar constellations from an atlas by Cellerius shown below).

• International Astronomical Union (1930): established formal boundaries for 88 constellations; 74 visible from Charlottesville.

1. No physical significance. Associations are arbitrary & man-made. Constellations are not natural groups and have no intrinsic physical significance. Fainter stars in a constellation don't participate in the pattern. Stars in given constellation lie near the same line of sight from Earth but are not necessarily close to one another in space. Shapes are specific to Earth's location in 3-D space (a fact not recognized when ancient astrological systems were developed).

2. Although eye cannot detect motions except over 100's - 1000's of years, stars are moving with respect to one another. Therefore, constellations are transitory. The appearance of the "Big Dipper" (part of Ursa Major) now and 100,000 years from now is shown below:

   
   

   

   
   

3. Modern astronomers use constellations only a convenience to roughly locate objects in the sky.

1. The zodiac ("circle of animals") is the set of constellations through which the Sun passes in the course of a year. The Sun's path is called the ecliptic, and the Moon and bright planets also stay near this path. Given the modern boundaries of the constellations, there are 13 ecliptic constellations. But in classical astronomy (and current-day astrology) there are only 12---one for each month. The zodiac is determined by the accidental orientation of the plane of Earth's orbit. The zodiacal constellations are mostly faint and boring.

2. Constellation names: Latin, often translated from Greek

3. Star names: brighter stars have "common" names based on mixture of Greek, Latin, Arabic. Most stars brighter than 14th magnitude have catalog numbers. Fainter stars---i.e. most stars---are largely uncataloged.
   Bright stars: many synonyms since appear in many catalogs
   E.g: the star at the mouth of Canis Major (the large dog); brightest star in the sky
   • Sirius ("the scorched one") --- common name
     
   • = Alpha Canis Majoris --- Bayer listing
     
   • = 9 Canis Majoris --- Flamsteed listing
     
   • = HD 48915 --- Henry Draper Catalog listing
     
   • = BD -16 1591 --- Bonner Durchmusterung listing
     
   • = 0640-16 --- RA/DEC coordinate listing

   Bayer, Uranometria (1603): assigned Greek letters: alpha, beta, etc. in order of brightness in each constellation; about 1300 stars have Bayer designations. But: because of errors (or, in some cases, actual changes in brightness), order is not necessarily correct. E.g. Alpha Ori (Betelgeuse) is fainter than Beta Ori (Rigel).
   Flamsteed (1712): numbered stars in each constellation in RA order: e.g. 61 Cygni, 40 Eri, etc.; used in case of brighter stars without Bayer designations.
   Modern catalogs contain 10⁵ to 10⁸ stars.

• Do it soon!
  
• Read the Lab 1 writeup in the Manual.
  
• Best is to choose a clear night near new Moon (see sky events).
  
• Find a dark site. Take your Manual and sky wheel.
  
• Find the "North Star," Polaris (in Ursa Minor = "Little Dipper"). Polaris is near to, but not coincident with, the North Celestial Pole. It is not a very bright star. Easiest method is to use the two "pointers" at the end of the bowl of Ursa Major (the Big Dipper). See the sketch below.
  
• Then orient yourself N/S/E/W. When you face Polaris, your right arm is toward the East and your left is toward the West.
  
• Find your zenith. Your meridian is the imaginary great circle passing through both the zenith and North Celestial Pole. It runs from due North to due South.
  
• Orient your sky wheel to match the time of night by aligning the date and time tickmarks. (Note: time marked is always Standard---not Daylight--- time.) Using the wheel and pattern recognition, "hop" to other bright stars and groups. Use brighter, more conspicuous constellations to locate others. Practice locating all the constellations, stars, and other features on the Manual list for this semester.
  
• Difficulties with using charts:
  
  • Scale of sky very different from chart
    
  • Can't represent brightnesses well on paper. Relative brightness of real stars looks very different.
    
  • Poor sky transparency, city lights, moonlight reduce visibility.
    
• Practice using averted vision to find fainter objects: rather than looking directly at the target, stare about 15 degrees away, but concentrate on target location.
  
• When you feel ready, take the constellation identification quiz from the TA at the Student Observatory 8-11 PM Monday-Thursday nights. You will be asked to identify 20 constellations, bright stars, or other features of the sky. This semester, use the "Spring" list.

• You can do Lab 1 (Constellations) and Lab 2 (Intro to Binoculars) in either order. You may find it helpful to use binoculars as you familiarize yourself with the constellations.

1. Read Syllabus

2. Read Introduction to Manual

3. Read Lab 1 description (secs. 1.1 to 1.8). Consult constellation descriptions as needed.

4. Mark up your Sky Wheel with names of objects on the Spring Constellation Quiz. Consult Norton's table of Proper Star Names. Identify corresponding stars on Norton's Star Maps. Transfer to Sky Wheel.

5. Download, print, & read lecture notes for Lec 1

ASTR 130 Link Page. A good starting point.
The Sky Tonight (Sky & Telescope)
General information on constellations (guides, charts, illustrations)
Constellations, Stars, & Deep Sky Objects (Peoria Astronomical Society)
The Golden Age of the Celestial Atlas (exhibition)
Your Sky, an interactive sky-chart maker
A "live" night sky of your own ("Starry Night" planetarium software)