3. INTRODUCTION TO THE SKY

1 degree = width of index finger @ arm's length
10 degrees = width of closed fist @ arm's length
20 degrees = distance thumb to little finger on outstretched hand @ arm's length

Astronomers quote star brightnesses on the magnitude scale. This scale has ancient roots, based originally on ranking the stars by their apparent brightness as seen with the unaided eye. Without instruments, this kind of ranking is about the best observers can do.
Today, the scale has been quantified in terms of the light power deposited by an individual star per unit area and tied to telescopic measurements made with electronic detectors.
For the purposes of this course, all you need to know is that the magnitude scale runs "backwards" (with brighter objects having smaller or negative magnitudes), that with the naked eye you can't see fainter than about 6th magnitude, and that the brighter planets and stars have magnitudes in the range -4 to +2. See chart below:

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 most ancient astronomers (discussed in the next several lectures).

C. EASILY OBSERVABLE SKY PHENOMENA

• STARS: form the backdrop or "reference frame" against which other objects' motions are measured. About 2000-5000 are visible (depending on eyesight) over the whole sky. About 1000 are visible at a time on a dark, clear night from a given location. The brighter stars form conspicuous patterns which seem unchanging (to the eye).
  • Motion: stars move in lockstep across the sky from East to West during the night. Stellar patterns come back to the 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.

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

• MOON: Bright, but much fainter than Sun. More rapid motion relative to stars. 12 cycles per year.
  Shows a drastic change in brightness and phase (bright part as fraction of full circle) during each cycle. 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 motions relative to 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. Large brightness changes for the first three listed.

• METEORS: brief streaks of light in sky; typical rate 5-10 per hour.

• COMETS: occasional (once every few years) flamelike objects moving slowly through sky over several weeks.

• THE MILKY WAY: a diffuse, irregular band of faint light most conspicuous June-August.

Your view of the sky is strongly affected by background sky light, both natural and man-made. During the day, Sunlight scattered by molecules in the Earth's atmosphere produces the bright "blue sky" that completely shields almost all cosmic objects from our eyesight. Near full Moon, only the brightest objects are visible in the night sky because of atmospheric scattering of Moonlight. City lights create enough local "light pollution" to rival or exceed the effects of the full Moon.

D. THE CELESTIAL SPHERE

• The Celestial Sphere (CS) is a geometrical construct on which to display the angular positions of astronomical objects. See the image above.

• It 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 the sphere.

• The horizon plane is the (idealized) plane "tangent" to (just touching) the Earth at your location. You can see objects above the plane but not below. The horizon will be discussed further in the next lecture.

• The north & south celestial poles: the points on the sky directly "over" Earth's poles. They are the points where the projections of Earth's rotation axis pierce the CS. These are fixed points.

• The celestial equator: the outward projection of Earth's equator to the sphere.

• Astronomical coordinate systems are based on measuring angular distances along the celestial sphere from poles, equator, and other reference points to astronomical objects (similar to geographical latitude and longitude).

E. CONSTELLATIONS

(The human brain is wired for this kind of pattern recognition: people who could recognize the tiger lurking in the forest shadows survived better than those who could not.)

Constellation associations are strongly culture-dependent, and the same patterns can have very different interpretations in different cultures. For the brighter constellations, there are often interesting mythological contexts, dating back to Greek and Roman times. Some associations are ancient, going back to around 2000 BC (Leo the lion, Scorpio the scorpion); some are new (Microscopium). Few resemble their namesake closely. Classical atlases of traditional associations can be elaborate & beautiful (an illustration of the north polar constellations from an atlas by Cellerius is shown below).

The associations had at least two functions: (1) as a mnemonic for remembering the patterns that served as important aids for navigation or time-keeping; (2) as religious or mythological symbols of meaning in nature.
The Zodiac is a set of constellations that were usually assigned greater importance. These are the constellations through which the Sun appears to move in the course of a year. In its traditional form there were 12 Zodiacal constellations, one corresponding to each month. (According to the modern boundaries of the constellations, however, the Sun also passes through a 13th: Ophiuchus.)

1. They have no physical significance.
   The associations are arbitrary & man-made. Constellations are not natural groups of stars. The fainter stars in a constellation don't follow the pattern. Stars in a given constellation lie near the same line of sight as viewed from Earth but are not necessarily close to one another in space. Click here for an illustration in the case of Orion.
   The shapes are specific to Earth's location in 3-D space (a fact not recognized when ancient astrological systems were developed and attached significance to the shapes).

2. Although the eye could not detect motions except over 1000's of years, all stars are moving with respect to one another. Therefore, the constellations are transitory. The appearance of the "Big Dipper" now and 100,000 years from now is shown below. Here is a GIF animation of the motion of the Big Dipper stars over 200,000 years.

   
   

   

   Click on the image for a QuickTime animation.

3. There are now 88 "official" constellations. Astronomers use constellations mainly as a convenience to roughly locate objects in the sky, like a ZIP code. They are, however, important for orienting yourself in the night sky when you observe it with the naked eye, binoculars, or small telescopes. They can also help you determine geographic directions and the time of night.

   
   

F. DOING THE CONSTELLATION QUIZ

• The procedure is explained on the Student Lab Information page.
  
• No preparation is required (beyond reading this Study Guide). It is better to write your answers in pencil than ink. Bring a red flashlight and a clipboard if you can.
  
• Remember that there are two sessions each night, Mondays through Thursdays, but that you must reserve a place in a session ahead of time.
  
• Be at the Student Observatory by the starting time.
  
• Don't wait until the end of the semester! Clear skies are always at a premium.
  
• Best is a night without a bright Moon (see sky events).

• Tips for recognizing constellations:
  • Locate the Big and Little Dippers (officially in Ursa Major and Ursa Minor, respectively). Then find Polaris, the North Star, at the end of the handle of the Little Dipper. This allows you to orient yourself N/S/E/W. (See below),
    
  • Use a star map & pattern recognition to "hop" to other bright stars. Use brighter, more conspicuous constellations (e.g. Orion) to locate others.
    
  • Warning! the scales and brightnesses of objects in the sky are NOT well represented on any printed map.
    
  • Use averted vision to find fainter objects: rather than looking directly at target, stare about 15 degrees away, but concentrate on target location. Light pollution is a serious problem in Charlottesville.
    

Bennett textbook: Ch 2.1, 3.5
Study Guide 3
Optional: if your copy of the text has the "Starry Night" DVD, you can use it to explore the appearance of the night sky and elabrate on the simulations we showed in class.

Bennett textbook: Ch 2.1, 2.2
Study Guide 4

Measure the angular diameter of the Sun as follows. You can do this on any day (clear or partly cloudy) when you can see the disk of the Sun. Don't look directly at the sun. Instead put your hand (palm out & fingers together) in front of your eyes at arm's length. Close one eye. Then, carefully fold down fingers, keeping the Sun's light covered until you can't remove any more fingers without letting sunlight pass. Remember that your index finger will subtend about 1 degree in width when held at arm's length.
1. How wide is the Sun in degrees?

Work alone or in a small group, as you wish, and be ready to turn in answers on Thursday, 2/5.

Astronomy Without a Telescope (introduction to naked-eye astronomy by Nick Strobel)
Important sky phenomena (Sky & Telescope)
Observing highlights this week (Sky & Telescope)
General information on constellations (guides, charts, illustrations)
The Golden Age of the Celestial Atlas (exhibition)
Your Sky (an interactive sky-chart maker)
Heavens Above (information on observing artificial satellites and natural sky features, including charts)
A "live" night sky of your own ("Starry Night" planetarium software)
A skeptic's take on astrology (from Bad Astronomy)

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Last modified January 2009 by rwo

Text copyright © 1998-2009 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. Celestial sphere drawing by Nick Strobel. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.