ASTR 1230 (Whittle) Lecture Notes


Night Sky & Comet Hale Bopp

Astronomy is primarily an observational science. It is driven more by new observational discoveries than by interpretive insights. Few important astronomical discoveries were predicted, and many were actually accidental. The human imagination has never been a match for the universe.

Astronomical discovery began with the simplest of observations: people looking at the night sky and trying to understand what they were seeing. In the past, most people were well acquainted with the basic features of the night sky. Today, most are unfamiliar with the sky mainly because of the advent of artificial lighting, which makes it difficult to see the night sky in urban areas (and also unnecessary to know the sky as a pathfinder).

This lecture introduces you to the basic features of the night sky which are visible to the unaided eye and prepares you for the Constellation Laboratory.


"Naked eye" observations (i.e. without optical aid from lenses, mirrors) were the only kind possible for most of human history! Telescopes not invented until 1608 AD.

Great accomplishments were possible without telescopes, e.g.:


Astronomy is the "oldest science." Systematic observations of the sky, ranging from crude to sophisticated, were made by nearly all historical cultures, pre-literate and literate.

Initial motivations:

Study of the sky quickly reveals existence of fixed star patterns and regular cycles in time. These became the central concern of early astronomers because of their immense practical value for:


  1. Angular Separations

  2. Brightnesses

  3. Colors, shapes (in some cases)

  4. Time


Other, less conspicuous, features (with modern interpretation):

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 "blue sky" that completely obscures 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.


It is important, but difficult, to try to visualize your situation when you look into the sky at night: you are standing on a spherical, spinning, moving planet. What you can see in the sky is determined by the Earth's orientation and position in its orbit around the Sun. Your view of the sky is always made in your "local reference frame".

Local Reference System
Sky Motion

The diurnal motion: The daily spin of the Earth on its axis produces an apparent counter-rotation of the CS and its "attached" stars across your local sky. One complete rotation around its axis with respect to the stars takes 23h56m. The Earth rotates eastward, so the sky appears to rotate continuously westward. Objects "rise" in the east or "set" in the west when they cross your local horizon plane. See the figure above.


We will use the "Starry Night" planetarium software to simulate the appearance and motions of the night sky.


Many of the brighter stars form conspicuous patterns on the sky. (The human eye/brain is wired for this kind of pattern recognition.) To the eye, the patterns seem unchanging: the stars appear "fixed" relative to one another. Historically, the patterns were very useful for orientation, navigation, determining time of night, date, etc., and so were given names.

Each named pattern is called a constellation. They are associated with mythological figures, animals, instruments, and other features from the natural, human, or religious worlds. An example of the stick-figure pattern associated with the hunter Orion is shown at right.


Polar Constellations

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.


  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.

  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).


  • 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.

    Finding North


    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

    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
    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.