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. 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
1. Angular Separations
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.
[Note: the symbol ~ means "approximately"]
Angular scales of "pan" of Big Dipper
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.
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Last modified September 2015 by rwoText copyright © 1998-2015 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.