ASTR 1210 (O'Connell) Study Guide

3. INTRODUCTION TO THE SKY

A. Motivations for Simple Astronomical Observations

• Curiosity: the most enduring (and productive) motivation for trying to understand nature

• Important practical applications:
  • Navigation

  • Time Keeping: during day or night

  • Calendar Keeping: tracking date & seasons

  For early human societies, these could be critical survival technologies.

• Fear/Religious Belief:
  Astrology as an example. This is the idea that the motions of the Sun, Moon, and planets against the stellar background can be used to predict the future and can influence human personalities. It derived from the ancient belief that these objects are living gods, who betray their intentions by their movements. This was obviously a powerful motivation for observing the sky. In the carving shown at the right the Egyptian Pharaoh Akhenaton (ca. 1350 BC) and his family are communing with the Sun god Aten, the source of Akhenaton's power.
  As our scientific understanding grew, astrology lost its interest for most people. We realized that the Sun, Moon, and planets are inanimate objects, moving in highly regular and predictable patterns in response to the well understood force of gravity. Constellations and the Zodiac were recognized to lack physical significance (see below). New planetary bodies, e.g. Neptune, were discovered that astrologers had somehow failed to detect. Statistical tests showed no correlation between "sun signs" and personality or personal history. There is no evidence for astrology, either theoretical or empirical. Astrology lingers only as a form of pseudo-science and casual entertainment. But it, and related ideas, did play an important historical role in encouraging the systematic observations of the sky that ultimately led to the scientific interpretation of the solar system.

B. Naked Eye Measurements of the Sky

1. Angular Separations Measured angles can be all-celestial ("sky", e.g. star-to-star) or celestial-terrestrial (sky to reference point on Earth). Can be between different celestial objects, between a celestial object and a reference point on Earth, or across a celestial object (as in the illustration) Modern Units: Degrees, minutes, seconds of arc Full circle = 360 degrees of arc; 1 degree = 60 minutes of arc; 1 arcmin = 60 seconds of arc Don't confuse these angular units with units of time! Always use the "arc" terminology for clarity.

Measuring an angular diameter

Examples: Angles subtended by a quarter at distance D: [Note: the symbol ~ means "approximately"] 1 degree @ D = 56 in 1 arcmin @ D = 270 feet 1 arcsec @ D = 3 miles The bowl of the "Big Dipper" is ~ 10 degrees long.

Angular scales of "pan" of Big Dipper

The human eye has 1-2 arcmin optical resolution---i.e. it cannot distinguish two stars separated by less than 1-2 arcmin in angle.
The Hubble Space Telescope has 0.1 arcsec resolution; i.e. it can resolve a quarter at a distance of 30 miles




"Hand-y" measuring scale (see illustration):
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




2. Brightnesses
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:


3. Time
Even crude measures of angles and brightnesses, coupled with measurements of time, and 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 one degree per day motion relative to the stars (we must infer its position since stars are 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 in the northern hemisphere..

Interference: sky brightness

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, even when the sky would naturally be dark. Most people today have never experienced a view of the dark sky as it would have appeared to ancient astronomers.

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 zenith is the point on the sphere perpendicular to your horizon plane (i.e. "directly overhead").

• 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.)

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 constellation patterns are transitory. The changing 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 the target's 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 elaborate 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.
• How wide is the Sun in degrees?

Work alone or in a small group, as you wish, and be ready with your answers for the next class..

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)

Previous Guide

Guide Index

Next Guide

---

Last modified January 2013 by rwo

Text copyright © 1998-2013 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 1210 at the University of Virginia.