ASTR 1210 (O'Connell) Study Guide


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 some of 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.

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*

Experiencing the motions Celestial motions may be complicated, but they are repeatable over periods of months to years, and their cyclic nature was quickly recognized by many cultures. It was this feature that encouraged people to search for a deeper understanding of them, especially in view of what they might portend for themselves.

The cycles also naturally became the basis of practical calendar systems. For instance, Western calendars are based on 7-day weeks (one day for each of the bright moving lights in the sky: the Sun, Moon, and five planets), 30-day months (one lunar cycle), and 365-day years (one cycle of the Sun in motion against the stars).

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 much 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 "apparent motions" 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

G. Effects of Intrinsic Lunar Motions

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Last modified December 2020 by rwo

Text copyright © 1998-2020 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.