Unit 8 : Calendar System : Ancient Astronomy : Babylonian Astronomy and the Origin of Astrology : Unit Questionnaire

Unit 1: How to Point to a Star
Unit 2: Where on Earth Are You?
Unit 3: Earth's Rotation and the Sun's Apparent Motion
Unit 4: Yearly Changes in the Sky
Unit 5: Seasons and Climate
Unit 6: Sundials
Unit 7: Navigation
Unit 8: Ancient Astronomy
Unit 9: Constellations

Ancient Astronomy

Prehistoric Astronomy

Much of what we study in the first half of this course could have been observed by humans living at any time in the past.  In fact, they would likely have been more familiar with many of the phenomena we study, because they did not have light pollution making it hard to see the sky, and because they did not have as many other recreational activities available after dark as we do.

Keeping track of the apparent motion of the sun in the sky was a matter of survival for people living in many areas of the world, because it allowed them to anticipate seasonal changes.  There is quite a bit of evidence to show that they did in fact keep track of these motions in one of several different ways. 

The easiest way to keep track of the changing declination of the sun is to track the position of sunset or sunrise.  At the equator, the sunrise azimuth is given by the formula azimuth = (90 deg minus decSun) where decSun is the declination of the Sun on the day you are observing.  For latitudes away from the equator, the total shift of the sunrise position is more than +/- 23.5 deg,  until an observer reaches the arctic circle where the sunrise position shifts from due south to due north between the winter solstice and the summer solstice.

The 3-D view above represents an observer's horizon (who would be standing in the middle of the "field" where the lines intersect).  Our view is a "God's Eye" view taken from the observer's zenith (directly overhead).  This view is specifically for an observer located at the Earth's equator.

At the latitude of Ames, Iowa, 42 deg N, the sunrise and sunset positions are about  32.5 deg from East or West at the solstices.  At Minneapolis/Saint Paul, latitude 45 deg N, they are about 34 deg from East or West at those times.  In Anchorage, Alaska, latitude 61 deg N, they are 55 deg from E and W.  The following images illustrate these ideas.  Please consider them carefully.


The graphic above is for an observer located in Ames, Iowa.  The graphic below is for an observer located in Anchorage, Alaska.  Note the differences in the angle of the solstices between the two locations.

Many ancient monuments have been found to have interesting architectural features that define a line of sight to the position of sunrise or sunset at the winter or summer solstices.  These include ancient Egyptian temples, Stonehenge in England, Mayan temples in Central America, and American Indian stone circles.

It is true that you can keep track of seasons by marking down the dates of the spring and fall equinoxes (i.e. the dates when the sun rises due East and sets due West).  But we can not simply say that ancient peoples marked the equinoxes because various buildings were oriented due North-South or East-West.  There are many other reasons why people orient buildings and structures to correspond with the Earth's poles, so we cannot conclude with confidence that buildings aligned that way were used to keep track of the sun.  For example, in modern times, many streets in the Midwest United States are laid out North-South or East-West, but this is certainly not done in order to keep track of the dates of the equinoxes.


However, it is also possible to track the seasons by keeping track of the sun's right ascension.  One easy way to approximate this is when we see that Orion is up in the early evening, then it must be winter time.




An Astro 120 student at Iowa State shared this bit of old lore:  When in Iowa in the late summer and fall, the weather in Iowa is traditionally dry.  The big dipper appears in the evening sky to be holding water.  In the spring or the winter, the dipper appears in the evening sky as if it were pouring water, or even upside-down.  These are typically Iowa's wetter months, so you might expect more rain or snow.




A more precise method of keeping track of the seasons from the changing right ascension of the sun is the trick called helical rising.  As the sun appears to move around the sky over the course of a year (as the Earth orbits around the sun), a given star will rise earlier by about 4 minutes each day.  A bright star like Sirius will rise with the sun one day and a week later it will rise about half an hour before the sun.  The first time that this star can be seen in the early morning sky before sunrise is called the date of its helical rising.  This event is a pretty good calendar indicator.  There is some evidence that this trick was used both in ancient Babylon and Egypt and by some of the American Indian tribes that lived near the Rocky Mountains in what is now Wyoming.

Go on to the next page