ASTR 1210 (O'Connell) Study Guide



4. MOTIONS IN THE SKY


Star trails over the Himalayas in a
2-hour exposure (Anton Jankovoy).


Cyclical motions of bright objects in sky were the main historical stimulus for the study of astronomy. The motions became the subject of intensive study by many ancient cultures, even if they could not interpret the motions properly. Ultimately, analysis of these motions led to some of the most important developments in the history of modern science. In this guide, we describe these motions as they might have been seen by ancient astronomers but explain them from a modern perspective.


A. Motions of Bright Objects

The table below lists celestial motions which are easily detectable by someone on the Earth without telescopes.

OBJECT PERIOD MOTION
ALL Daily ("diurnal") Rotation Westward
SUN Annual (365 days) (a) 1 degree/day Eastward*
(b) North/South*
MOON** Monthly (29 days) (a) Eastward, N/S*
(b) Phase change
PLANETS (5)** Months-Years Generally Eastward, but with
Westward loops*

Most of these motions are so slow that if you aren't a practicing amateur astronomer, you probably aren't aware of them. The best way to visualize them is in a planetarium or with a good computer sky simulation program. We will use the Starry Night simulator in class.

Thought experiment


B. Explanation of Motions

In the rest of this guide, we explain these phenomena from a modern scientific perspective. It took many centuries for astronomers to arrive at the correct interpretation described here. Ancient Greek astronomers understood most of this after several hundred years of work, but the knowledge was lost and only rediscovered during the Renaissance, 1300 years later.

The key to complete understanding of celestial motions was introduced by the Greeks: mathematics.

The listed motions of celestial objects are produced by two entirely different effects: The motions discussed in this guide are all in the second category and are induced by the fact that:


C. Effects of Earth's Shape and Spin

Day/Night; Horizon Plane


D. Effects of Earth's Motion in Orbit


E. Tilt of Earth's Axis


F. Astronomical Effects on the Weather



Reading for this lecture: Reading for next lecture:



Web links:



Previous Guide Guide Index Next Guide

Last modified January 2014 by rwo

Text copyright © 1998-2014 Robert W. O'Connell. All rights reserved. Zodiac and Earth tilt diagrams copyright © by Nick Strobel. Equator/ecliptic diagram copyright © Edmund Scientific Corp. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.