Archaeoastronomy

Prehistoric stone structures, created by early astronomers, plotted the sun's motion against the horizon.
These structures registered the ominous shortening of daylight as winter approached.

At Stonehenge, pictured above, Druid priests used the carefully placed stones to follow the sun's journey.
As the day's shortened, elaborate ceremonies appealed to the gods to stop the sun and lengthen the days.
Every year the ceremonies worked, the sun paused for a few days and then began moving back north.

Fortunately that is still true today, every year around December 21, the sun stops its southward journey
the amount of daylight increases, temperatures moderate and spring arrives. We know that this phenomenon,
the winter solstice, is the result of a tilted earth, orbiting the sun. A similar event occurs around June 21.
At summer solstice, the sun pauses its northward motion and begins its journey south towards the winter solstice.

To avoid confusion with summer and winter in the two hemispheres it is easier to refer to the two solstices as
June solstice and December solstice.

Studying these ancient observatories and the people who created them is called archaeoastromony.
The image below shows what may be another ancient observatory.

The 13 ancient stone towers pictured above are part of a large complex at Chankillo, Peru.
Estimated to be 2400 years old, some researchers think these towers are an ancient solar observatory.
I decided to test this idea without traveling to Peru - although traveling to Peru would be more exciting!

I started this investigation by opening Google Earth and checking the resolution of the images around Chankillo.
Fortunately the images are excellent.

To view the 13 towers at Chankillo, enter the latitude and longitude in Google Earth's "Fly To" option.
Copy and Paste using the format below:
(These are the coordinates for the 13 Towers)
latitude: 9 33 40.27 S
longitude: 78 13 38.53 W

The Google image shows the ruins of some structures to the west of the towers. The observer's location is probably
somewhere in these structures, positioned to view the sunrise over the eastern horizon.

If the towers at each end of the row mark the two solstices, then the observer would be positioned due west
of the center tower. This is because the sun rises due east at equinox, half way between the June and December solstice.

Use Google Earth and position the cursor over the center tower.
The latitude is displayed in the information bar:

The observer would be located on the same latitude, west of the tower. Drawing a line at that latitude from
the center tower produces this result:

Sunrise at equinox, as viewed from the observing position indicated, would be over the center tower.

The location for the observer shown above supports the work of Clive Ruggles, University of Leicester. Working at the site
he reported that a long enclosed corridor led to this point with no other apparent purpose then to provide access to the observatory.

To determine if the two end towers align with the June and December solstice we need to determine the azimuth
(compass direction) of these towers from the observer's position.

Using Google Earth, determine the location (lat and long) of the two end towers, these towers are at the North
and South ends of the row. Use the same procedure to determine the observers coordinates.

Observer's Coordinates	latitude: 9 33 40.27 S longitude: 78 13 46.63 W
North Tower Coordinates	latitude: 9 33 37.13 S longitude: 78 13 38.72 W
South Tower Coordinates	latitude: 9 33 43.3 S longitude: 78 13 38.9 W

I determined the angles by entering the coordinates into the online calculator at this site:

Azimuth from Observer to North Tower: 68.09 degrees
Azimuth from Observer to South Tower: 111.58 degrees

Astronomy software "Starry Night" is capable of displaying the sky from anywhere on earth at anytime.
I set Starry Night to the latitude of the observer, changed the date to December solstice (Dec 21) 400 BC.
You may have noticed that the towers are on a high ridge meaning that the sun would not be visible at the
horizon. We will have to calculate how high above the horizon the sun will be when it first becomes visible.

Using Google Earth and positioning the cursor over the center tower, we find its elevation is
206 meters above sea level, moving to the observer we find an elevation of 176 meters.
The tower is 30 meters above the observer. (206m - 176m = 30m)

To determine the angle we need to know the horizontal distance to the tower from the observer.
Again, Google Earth provides that information. Choose "Tools - Measure" from the menu.
Draw a line from the center tower to the observer, as in the image below.

Google Earth indicates the distance as 246.69 meters, I'll round that to 247 meters.
The triangle looks like this:

Assuming this is a right angle triangle we can use some trigonometry to calculate the angle "a".
Tan a = 30/247
Tan a = .121
a = 6.9 degrees

We are ready to test the thesis - "Is this a solar observatory designed to mark December and June solstice?"

We know the alignments of the two end towers with the observer. We will use Starry night to recreate sunrises
at solstice and see if they occur over the end towers.

If you have Starry Night, set the location for Chankillo using the coordinates for the observer.
Set the date to approximate the June solstice - June 21, 400BC
Adjust the time so the sun is 6.9 degrees above the East horizon.
Point the cursor at the sun and record the Azimuth.

Summer Solstice Azimuth from Starry Night	66 degrees
Winter Solstice Azimuth from Starry Night	112 degrees

This image displays the results, the actual solstice sunrise, both December and June, appear close to the end towers.
The geometry of the site agrees with the results from Starry Night.

It would seem reasonable to state that the 13 towers at Chankillo were constructed as a solar observatory!

There are thousands of archaeological sites around the world, some may have an undiscovered astronomical purpose.
Working with Google Earth, Starry Night and other software you may be able to contribute knowledge about these sites.