ASTR 1230 (Whittle) Lecture Notes

A. NAKED-EYE ASTRONOMY

• Determination of 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

• Galileo was first astronomer to use telescopes. These were only 1-2" in size, but they revolutionized astronomy. He 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: the most enduring motivation for trying to understand nature

• Aesthetics

• Fear/Religious Belief:
  • E.g.: Astrology: This is the idea that the motions of the Sun, Moon, and planets can be used to predict the future. It is based on the ancient belief that these objects are 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: on land & sea

• Time Keeping: during day or night

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

C. 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

"Hand-y" 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: the diameter of the Sun or Moon is about 0.5 degree. You can block out either with your index finger held @ arm's length. Try it!

2. Brightnesses
   Star brightnesses are quoted on the magnitude scale: this is logarithmic, open-ended, and runs "backwards." Brighter objects have smaller values.
   The 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
   Even crude measures of angles and brightnesses, if made systematically over days, months, or years, immediately reveal the presence of astronomical time cycles, which became the central concern of early astronomers.

D. EASILY VISIBLE PHENOMENA

• STARS: form the 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. The brighter stars form conspicuous patterns which seem unchanging (to the eye).
  • Motion: stars move in lockstep across sky from East to West. Patterns come back to same location in the sky after slightly less than 24 hours. Star locations in the sky at a given time of night change systematically throughout the year.

  • 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. But our star system (the Milky Way Galaxy) contains about 100 billion stars, and there are billions of galaxies!

• SUN: The most obvious astronomical object (and most important for us!). Steady brightness. Has a slow eastward motion relative to stars (must infer position since stars invisible in daylight). Returns to same place against the star background in one year (365.25 days).

• MOON: Very bright, but much fainter than Sun. More rapid eastward motion relative to stars. 12 cycles per year.
  Shows a drastic change in brightness and phase (bright part as a fraction of a full circle) during each cycle, from totally dark to fully illuminated. Takes 29.5 days to return to the same phase (e.g. "full"). This is the cycle we have formalized in our calendar as the month ("moonth").
  Note: Moonlight was of enormous practical value before electrical lighting was invented, so the phases of the Moon were closely followed in earlier times.

• PLANETS: Less obvious. 5 bright, starlike objects; slow, complex motion relative to the stars. Mercury, Venus, Mars, Jupiter, Saturn. Two of these (Mercury, Venus) are always found relatively near the Sun; others can be up to 180 degrees away.

• 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." Concentrations of debris (from comets) produce "meteor showers," e.g. the Leonids.
  
• 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. E.g. Hale-Bopp (1997).

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

• DIFFUSE NEBULAE: interstellar gas clouds lit up by hot stars. A handful are visible without telescopes. E.g. the Great Nebula in Orion. Mark birthplace of young stars, death of old stars.

• MILKY WAY: appears as a faint, diffuse band of light. It is the combined light of millions of distant stars in the plane of our own Galaxy (a massive, flattened star system), seen edge-on.

• EXTERNAL GALAXIES: other star systems like our Galaxy. 4 visible to eye, 2 of these in northern hemisphere. E.g. the Andromeda Galaxy---the most distant object you can see without a telescope. It is 2 million light years from us, meaning that the light you see from it tonight started its journey 2 million years ago. For more discussion of this lookback effect, see this ASTR 121 page.

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 used to vizualize the 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 the surface of sphere.

The zenith is the point directly overhead on the CS. It is equidistant from all points on the horizon.
The north and south 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. It is a "great circle" (i.e. a circle drawn on the celestial sphere with its center coincident with the center of the sphere).
Your meridian is the great circle on the CS that passes through both celestial poles AND your zenith. It runs from due North to due South. The meridian is not marked on the drawing above; instead, see this figure.

An astronomical object is said to transit when it crosses the meridian. That is the time when it is farthest from the horizon and highest in the sky.

F. NIGHT SKY SIMULATIONS

• Constellation identification is strongly culture-dependent, although some star patterns have similar interpretations across diverse cultures. 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.)

• The 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 a memory aid to navigators. Ptolemy's Almagest (135 AD) listed positions of 1022 stars in 48 constellations. His catalogue was used for the next 1400 years(!)

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

• Arab astronomers were active 700-1600 AD; many modern star names have Arabic roots: Algol ("demon star"), Aldebaran ("the follower"), 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. Click here for an illustration of the boundaries for Orion.

Significance of constellations:

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

2. Although the eye cannot detect their motions except over 100's - 1000's of years, the stars are all moving with respect to one another. Therefore, constellations are transitory. The changing appearance of the "Big Dipper" (part of Ursa Major) now and 100,000 years from now is shown below. Here is an animation of the motion of the Big Dipper stars over 200,000 years.

   
   

   

   
   

3. Modern astronomers use constellations only as a convenient "address" to roughly locate objects in the sky.

Terminology:

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 ecliptic, and hence zodiac, is determined by the accidental orientation of the plane of Earth's orbit. Most zodiacal constellations are faint and uninteresting (e.g. Libra, Capricorn, Aquarius).

2. Constellation names: Latin, often translated from Greek

3. Star names: the brighter stars have "common" names derived from a mix of Greek, Latin, Arabic. Most stars brighter than 14th magnitude have catalog numbers. Fainter stars---i.e. most stars---are largely uncataloged.
   Bright stars have many synonyms since they 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's Uranometria (1603) assigned Greek letters: alpha, beta, gamma, etc. to stars 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), the order is not necessarily correct today. 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 for brighter stars without Bayer designations.
   Modern catalogs contain up to 10⁸ stars (but this is still only a small fraction of all stars in our galaxy).

4. There are also many catalogues of non-stellar objects, such as nebulae, star clusters, and galaxies. The three you will most frequently encounter are the New General Catalogue ("NGC"), the Messier Catalogue ("M"), and the The Caldwell Catalog of Deep-Sky Objects (an updated version of the Messier Catalogue).

• Read the Lab 1 chapter in the Manual.

• The Lab will be given on the first two usable nights on or after Monday, Sept. 9. You can come to either night. Check the Observatory status via the recorded message (924-7238) after 6:30 PM.

• You will work in groups. Each group will be assigned a set of stars and constellations to learn from the required list in the Lab 1 description. After a group has mastered these, they will teach the other groups.

• Advice on learning constellations:
  • Review and disregard the misconceptions about the sky that are listed in Ten Things to Forget About Astronomy.

  • Because not all objects on the Constellation Quiz are identified by name on your Sky Wheel, consult the following sky charts, which contain labeled objects for the Quiz:
    • Spring semester skychart
      
    • Fall semester skychart

• Find the "North Star," Polaris (in Ursa Minor = "Little Dipper"). Polaris is near to, but not exactly 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. Alternatively, if Ursa Major is low in the sky, you can use the stars in Cassiopeia as pointers. See this illustration.

• 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 and your meridian.

• 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:
  • The scale of the real sky is very different from a chart
    
  • One can't represent brightnesses well on paper. Relative brightnesses of real stars look very different.
    
  • Poor sky transparency, city lights, or 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. (This puts the image on a more sensitive part of your retina.)

After you have had time to learn the constellations, you will be examined individually by a TA on your knowledge of the sky. You will be expected to be able to identify 20 constellations, bright stars, or other features of the sky.

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Assignment

1. The Constellation Lab (Lab I) will take place on the next two usable lab nights, starting Monday, 9/7. You must attend one of the two sessions. Whether the Observatory will be open will be announced on the recorded message (924-7238) by 6:30 PM. We will also send an email alert to the class.
   It would be a good idea to become familiar with using the various forecasts on the ASTR 1230 Weather Page in planning for this and later labs.

2. To prepare for the Constellation Lab: read Lab 1 description (Secs. 1.1 to 1.8); consult constellation descriptions (Sec. 1.9) as needed.

3. Download, print, & read lecture notes for Topic 1

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Web links:

ASTR 1230 Link Page. A good starting point.

Ten Things to Forget About Astronomy

The Sky Tonight (Sky & Telescope)

Ian Ridpath's Star Tales, constellation history & mythology
General information on constellations (guides, charts, illustrations)
Constellations: photos, star ID's (Kaler)
Constellations, Stars, & Deep Sky Objects (Peoria Astronomical Society)
Constellation Myths
Heavens Above Charts for Charlottesville

Heavens Above Constellation Charts/Data

Your Sky, an interactive sky-chart maker
A "live" night sky of your own ("Starry Night" planetarium software)

The Golden Age of the Celestial Atlas (exhibition)
Historical Celestial Atlases on the Web

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Last modified September 2008 by rwo

Text copyright © 1998-2008 Robert W. O'Connell. All rights reserved. Opening fisheye lens picture of comet Hale-Bopp and night sky from Ujue, Spain, April 1997, copyright © J. C. Casado. Illustrations of the celestial sphere copyright © by Nick Strobel. Image of M13 copyright © by J. Ware. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1230 at the University of Virginia.