ASTR 1210 (O'Connell) Study Guide


Night Sky & Comet Hale Bopp
"Night hides the world but reveals a universe." (Persian proverb)

This lecture introduces the basic features of the sky which you can easily see without telescopes. "Naked eye" observations were the starting point for the explorations of the sky that culminated in our understanding of the universe described in the last lecture.

This lecture also discusses constellations and prepares you for the out-of-class Constellation Quiz.

A. Culture and Scientific Discovery

It took about 500 years of scientific effort to put together the picture of the structure and evolution of the universe described in the last lecture. A vast amount of evidence underpins the elements of this understanding (and the details make up the bulk of the textbook). We believe that this picture is right in its essentials --- so, for instance, when science is taught 300 years from now, it will still be a valid first-cut description.

On the other hand, our scientific understanding of the cosmos differs drastically from those of pre-scientific cultures. This raises a fundamental question about human societies: Why didn't we know all this thousands of years ago?

More importantly, why didn't we know those other crucial scientific facts with more immediate practical ramifications---like the role of microorganisms in causing disease or the value of refined petroleum as an energetically dense, portable fuel?

Most early cultures had never moved very far toward these conclusions. With the striking exception of the ancient Greeks, they may have collected a great deal of information about the motions and appearance of astronomical objects, but they failed to interpret it critically. The idea that human beings would one day walk across the face of one of those godlike, glowing lights in the sky would have been inconceivable to most early cultures.

It must also be admitted that our scientific understanding of the universe, however well-founded, is not congenial to everyone. The human race, the Earth, even our galaxy, have no special place in it. From a human point of view, the universe as revealed by science may seem cold, dangerous, and purposeless. It is certainly not the universe most people had hoped to find.

As a contrast, we will explore one of the most fascinating pre-scientific cosmologies---that of the Mesoamerican cultures that flourished between about 500 BC and 1500 AD---in Study Guide 5.

But first, we turn to the basis of all cosmologies: the study of the night sky.

B. Motivations for Simple Astronomical Observations

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 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 to cardinal directions.

In prehistoric times, astronomy consisted of simple observations that any interested person could make. In fact, up to the nineteenth century 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. We no longer need to use the sky as a pathfinder either.

Systematic observations of the sky, ranging from crude to highly sophisticated, were made by many historical cultures, pre-literate and literate. Fascination with the sky drew not simply on its appearance, as impressive as that might be on a dark and clear night, but more importantly on the fact that things in the sky moved continuously, some of them in complex ways. The mysterious cycles exhibited by the sky denizens betrayed the existence of powers that were (at least initially) beyond human understanding. Of course, the dominant object in the sky, the Sun, also moved in a complicated pattern, and this was important to understand for many practical reasons.

So, there were several different motivations for study of the sky:

Thought experiment

C. Naked Eye Measurements of the Sky

Only "naked eye" observations (i.e. without optical aid from lenses or mirrors) were possible for most of human history! Telescopes were not invented until 1609 AD.

The human eye is excellent at pattern recognition and can sense the color and the shape of sufficiently bright and extended cosmic objects. However, only a few kinds of quantitative measurements are possible with the naked eye:

    1. Angular Separations

    Measured angles can be all-celestial ("sky", e.g. star-to-star) or celestial-terrestrial (sky to reference point on Earth). They 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: [Note: the symbol ~ means "approximately"]

      Angles subtended by a quarter at distance D:
      • 1 degree @ D ~ 56 inches
      • 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

D. Easily Observable Sky Phenomena

Other, less conspicuous, phenomena:

We will illustrate the bright objects in the sky and their main motions in class using a computer sky simulation program called Starry Night.

Interference: sky brightness

Celestial Sphere

E. The Celestial Sphere

over Monument Valley
Orion and Mars over Monument Valley
(Wally Pacholka)

F. Constellations

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

Each named pattern is called a constellation. It was natural for people to seek deeper meaning in these remote, silent, but majestic figures at the limit of the visible world. So, the constellations often were given important mythological or religious associations. Some have traditionally been associated with animals, instruments and other features from the natural or human worlds. An example of the figures and star outlines associated with "Orion the hunter," "Taurus the bull," and "Lepus the hare" is shown at right (click for enlargement).

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

Polar Constellations
Functions of the constellations? Significance of the constellations?

  1. They have no physical significance.

  2. Although the eye could not detect the 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.

    The Big Dipper now and in 102,000 AD.
    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.

G. Doing the Constellation Quiz

Finding North

Reading for this lecture:

Reading for the next lecture:

Web Links:

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Last modified July 2017 by rwo

Text copyright © 1998-2017 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.