Updated 4/26/08
The Twilight Calendar

The two most known and watched constellations by prehistoric hunter/gather/agriculturists are Pleiades and the Big Dipper. By observing stars above the North Star at twilight, ancients knew the time of year within a day or two and thus their temporal location within the yearly cycle.

So what and when is twilight? Civil Twilight is when the sun is 6 degrees below the horizon. Nautical Twilight is 12 degrees and Astronomical 18--the time at which the sky will get no darker during the night. Nautical Twilight is the time when the stars come out--the time when both the horizon and stars can be seen in order to make nautical observations. (For the darkness of the night sky worldwide and in the Western hemisphere)

For those who want to keep track of Moon phases, Sunset times,
Nautical Twilight, etc. there are some nifty shareware programs:

Mac -- HourWorld.com | PC -- GeoClock.com

also check out Tucows.com for related and other shareware.

One way to track the yearly cycle is to watch the sun move along the horizon, day to day, at either sunup or sunset. Daily observations need to be made from the same location, from the same spot every time. This is a good calendar method for a village--for stationary people. If we tell you that summer begins when the sun reaches the cleft in the horizon, you'll need to know which horizon we're talking about. In short, the horizon calendar is good for only one spot on earth.

The Twilight Calender is ideal for people who are nomadic--those lacking a hereditary horizon. The Twilight Calendar is available and accurate for every location, every spot in the northern hemisphere. Nautical Twilight can be marked by the appearance of the stars (4th magnitude and therefore dim) in the handle of the Little Dipper--which occurs roughly an hour after sunset. (see table below for exact times)

Each dot on the perimeter of the Twilight Calendar equals two days. The entire year is visible by glancing at the northern night sky. The example above is set for 36 degrees north latitude and relates particularly to Navajo usage. The indicated corn planting time as well as the ceremonial season reveal a direct relationship with Pleiades.

To use the Twilight Calendar observe the North Star at twilight and take note of the stars/constellations directly above it. For example, when the leading two stars of the Big Dipper are above the North Star at twilight it's time to begin planting corn. See the diagram above. When the middle star in the Dipper's handle is above at twilight it's time to stop all planting.

As you become more familiar with the northern sky at twilight it's a simple matter to assign important dates of any culture to the Twilight Calendar--resetting it as it were. We left a wealth of stars out of the diagram so the concept could be clearly presented. New Years Eve, the Fourth of July? Piece of cake. You can find a star that signifies your birthday, anniversary, or any other meaningful date. When that star reappears above the Pole Star you'll know the exact time of year.

Where's the North Star? It's obviously in the northern sky and up the same number of degrees as your latitude. To give you an idea, the continental United States lies between about 30 degree latitude at the southern extreme to nearly 50 degree latitude at the northern border--so viewed from the US the North Star will be between 30 and 50 degrees above the northern horizon. Orientate the diagram above to the correct time of year and you'll know what the northern sky should look like. Once you've spotted one or more of the constellations it'll be easy to identify the North Star.

The Navajo Planting Season coincides with Pleiades being absent from the night sky (May 8 to June 5). Navajos plant after Pleiades has set with the sun and before it is visible in the morning sky. "Plant while Pleiades can't see." As a general rule-of-thumb, plant while any part of the Big Dipper is above the North Star at twilight--roughly a month, May 7 to June 9. This has worked well on the Colorado Plateau in the Four Corners region of the United States for the past 1000 years.

Nautical Twilight Times for 36 degrees North Latitude
(Notice during Daylight Savings Time, times are offset one hour)

The Color of Night

Nearly everyone has been fascinated by the colors of the western skies at sunset, but have you ever noticed how the colors of the eastern sky change as night falls? How about the northern and southern skies? Have you watched as the whole sky changes from bright light blue to darkest night?

The end of a bright cloudless day is the best time to see different blankets of color in the atmosphere. If you let your eyes adjust to darkness before you look, you can see different layers of air all across the sky. Each layer has a distinct color and brightness. The colors (in bands above the horizon) should look like this as the sun sets:

Don't try to see all these colors in just one viewing. It will take many nights before you can spot all of them. But with practice you'll be able to see these bands more distinctly. It will be easier to see them in summer or autumn. Some experts say the best purples can be seen on the first clear evening after a string of spring rains. Painters, musicians, jugglers, travelers, stargazers, and other lighthearted people try to see every sunset they can. Some of them have special techniques for taking the whole scene into view. Here's one. Hold a small mirror out at arm's length. This way you can see other parts of the sky change as the sun goes down. Where does the sky get darkest first?

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