ASTR 1230 (O'Connell) 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 -- sometimes in awe and wonder, sometimes in fear of the unknown powers at work in the heavens.

In the past, most people were well acquainted with the basic features of the night sky. We are unfamiliar with the sky in modern times 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 or mirrors) were the only kind possible for most of human history! Telescopes were not invented until 1608 AD.

Nonetheless, great accomplishments were possible without telescopes, e.g.:

The human eye is a remarkably capable astronomical instrument. Follow this link for background information on its function and the important observing considerations of dark adaptation and averted vision.


The concerted study of the sky started long before modern science arose ca. 1500 AD. How long? We don't really know---probably at least 8000 years before. Almost every human society, pre-literate or literate, whose culture we have been able to sample in detail shows some awareness of celestial phenomena --- if not in the form of written records then in other ways, such as the alignment of buildings or other structures to cardinal directions.

Initial motivations:

Study of the sky quickly reveals the existence of regular cycles in time of the Sun, Moon, & planets. These became the central concern of early astronomers because of their immense practical value for:


Only a few kinds of measurements are possible with the naked eye:

1. Angular Separations
    Studying the geometry of the sky by measuring angles is the most basic form of astronomy. Apart from time tracking, this was the only accurate quantitative measurement possible before the advent of modern instrumentation.

    Measured angles can be all-celestial ("sky", e.g. star-to-star) or celestial-terrestrial. They can be between different celestial objects, between a celestial object and a reference point on Earth, or across a celestial object.

    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.


      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

2. Brightnesses

3. Colors, Shapes (in some cases)

4. Time


Other, less conspicuous, features visible to the eye (with the 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 other cosmic objects from our eyesight (though the Moon is often easy to see in daylight, and you can detect Venus if you know where to look). Likewise, 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 (note--not quite 24 hours). 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.


In class, we will use the "Starry Night" planetarium software to simulate the appearance and motions of the night sky. This is an excellent app that is very useful to purchase if you're more serious about observing.


The stars are not uniformly distributed on the sky. Many of the brighter stars form conspicuous patterns. To the eye, the patterns seem unchanging: the stars appear "fixed" relative to one another. Historically, the patterns were very useful for orientation, navigation, and determining the time of night or the date and so were given names.

Each named pattern is called a constellation. Constellations 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 "Orion the hunter" is shown at right.



Significance of the constellations:

  1. Constellations 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 participate in the pattern (as illustrated here in the case of the Orion region.) 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 the Earth's location in 3-D space (a fact not recognized when ancient astrological systems, which attached significance to the shapes, were developed).

      Click here to see a modern, deep telescopic image of Orion that reveals the many beautiful fainter features lying in this direction but mostly invisible to the naked eye.

  2. The stars are all moving with respect to one another, even though the changes would not be apparent to the eye except over thousands of years. Therefore, constellation patterns 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 20th magnitude have catalog numbers (most recently recorded by the Gaia space observatory). 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" in Greek) --- common name
        • = Alpha Canis Majoris --- Bayer listing
        • = 9 Canis Majoris --- Flamsteed listing
        • = HD 48915 --- Henry Draper Catalog listing
        • = BD -16 1591 --- Bonner Durchmusterung listing
        • = SAO 151881 --- Smithsonian Astrophysical Observatory (SAO) listing
        • = 0645-16 --- Right Ascension/Declination coordinate listing

      Bayer's Uranometria (1603) assigned Greek letters: alpha, beta, gamma, etc. to stars, usually 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 "Right Ascension" (west to east) order: e.g. 61 Cygni, 40 Eri, etc.; used today for brighter stars without Bayer designations.

      SAO catalog numbers are needed to locate stars in the database stored in your Celestron telescope computers.

      Modern digital catalogs contain up to one billion stars (but this is still only a small fraction of all stars in our galaxy). Most list objects in Right Ascension order.

  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, 9/5. 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 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 Lec 1

    4. Take Review Quiz--Week 2 on the Collab site.

    Web links:

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    Last modified April 2021 by rwo

    Text copyright © 1998-2021 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. Orion at horizon picture by B. Tafreshi. 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.