He replied "Midwinter Moon."

"No, you're wrong.  This is Eagle Moon.  Look!  There is an eagle now passing behind you."

When the old man turned to look, his throat was cut.

The murderer remarked, "Did you expect to see an eagle at this season?"

The old man was familiar with the patterns and signs his tribe used to mark the passage of seasons.  He knew the time.  Nevertheless, a fatal twinge of doubt led him to look away in search of verification, and he fell victim to a cunning distractor.

The moral of the tale is simple:  Always be certain what time it is.

How do you "know" the time of day or year?

Celestial Routines

Much of the early history of astronomy is the story of humans' quest to identify and master the patterns and rhythms of the sky's denizens in order to know the time.

If you have lived most of your life in a place that floods the night sky with artificial illumination, it is likely that you have seldom -- if ever -- seen the faint natural light of the celestial bodies our ancestors relied upon to mark the passage of time. 

Just as modern lighting has erased the stars from the night sky, the advent of standardized civil clocks and calendars has effectively erased our collective memory of the sky's intimate role in defining time's natural structure.

It will be helpful to begin by reacquainting you with some of the principal classes of celestial bodies and their basic motions.

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• Fixed stars.  The night sky is covered with points of light.  Most, but not all, of them remain fixed in position relative to one another.

From our vantage point on Earth, all the fixed stars in the sky appear to move together as though they are attached to a huge inverted bowl that is spinning around a fixed point. 

Leaving the shutter of a camera open for several minutes captures the trails of the fixed stars as they make their daily counter-clockwise trip around the north polar axis.

The star in the center with the smallest trail is Polaris, an otherwise average-looking star that happens to be situated almost directly above the earth's axis of rotation.

(Global positioning hint:  If you're ever lost somewhere in the northern hemisphere, locate Polaris and measure its angular height above your horizon to find your latitude.)

The fixed stars move constantly and steadily from East to West across the sky.  In one 24-hour period, they make slightly more than one complete turn (361°).  As a result, each star rises about four minutes earlier each day.  Sirius -- the brightest of all the fixed stars that can be seen from Earth -- rises at sunset in January.  Every two weeks, it rises roughly an hour earlier.  By August, it is rising at sunrise.  When during the year is a good time to look for Sirius in the night sky?

• Constellations.  Although the fixed stars are scattered randomly on the sky, humans have used their imaginations to discern patterns in the arrangements of the brighter stars.  These patterns -- which vary from culture to culture and from time to time -- are called constellations and help to map the sky.

• The Zodiac.  A dozen constellations form a celestial highway for the Moon, the Sun and the planets -- heavenly bodies that appear to move against the background of the fixed stars.  These constellations are called the "zodiac" because many of them represent real or fabulous animals.

• The Moon.  The Moon is constantly moving from West to East through the zodiac.  It completes one circuit of the zodiac in a little under four weeks.  It makes about thirteen of these circuits in one year.

The Moon always shows the same face toward Earth, but it does not shine by its own light.  The amount of the Moon's visible surface that is illuminated changes in a regular cycle every 29 to 30 days.

When the Moon is "new," it rises with the sun and is out all day long.  Even so, we usually cannot see it because it is positioned too near to the Sun in the bright daytime sky.  It also has its dark side pointing our way, so it reflects little light to us. 

To get our first glimpse of the returning Moon, we have to look for a thin sliver of light low on the western horizon just after sunset a day or two after new moon. 

Every night, the Moon sets about one hour later than the night before.  And every night, more and more of its surface is illuminated. 

Roughly two weeks after New Moon, it is rising at sunset and setting at sunrise.  Its entire surface is covered with light. 

Over the next two weeks, the Moon's lit surface shrinks until it is only a sliver that can be seen on the eastern horizon shortly before sunrise.

• The Sun.  The Sun rises every day on the eastern horizon -- sort of.

As this montage of photographs shows, sunrise occurs at different points along the horizon during the year.  In fact, the Sun only rises due east on two days of the year:  the first day of spring (vernal equinox, ~ March 21) and the first day of fall (autumnal equinox, ~ September 21). 

In the center of the montage, you can see the Sun rising (between the two silos) on the vernal equinox.   After the vernal equinox, the Sun's rising point inches northward a little each day (toward the left in the photo). 

Around June 21 (summer solstice), the Sun stops its northward march and begins to retrace its steps.  On the left, you can see it rising on the summer solstice, several degrees north of east.  The Sun will rise due east a few months later in September and then begin to inch southward at sunrise (toward the right in the photo). 

On the right, you can see the Sun rising on the morning of the winter solstice (~ December 21).  Some early observers likened the Sun's annual zigzag along the horizon to the action of a weaver.

• The planets.  There are a handful of stars that wander, each at its own pace, through the field of fixed stars.  The word "planet" comes from the Greek word planetes meaning wanderer.

Celestial Surprises

• Comets.  Comets get their name from the Greek word kometes (long-haired star).  They appear in the sky at unpredictable times.  Unlike the Sun, Moon and planets, whose movements are always constrained by the boundaries of the zodiac, comets appear to be free to travel through any constellation they please.  For these and other reasons, comets were long thought to be atmospheric phenomena like a thunderstorm or halo around the Moon.

Halley's comet (684) as  illustrated in the Nuremberg Chronicles, 1493.
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Comets were believed to portend important events.  The Bayeux tapestry (1073-1083) shows men (on left) pointing to a comet (Halley's, 1066) sent to foretell the fall of King Harold of England at the Battle of Hastings.
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In his painting, The Adoration of the Magi (1303-1306), Giotto di Bondone depicts the Star of Bethlehem as a comet (Halley's, 1301).
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Different comet types -- including those shaped like swords -- illustrated in Description de l'univers, 1683.
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Comet Hale-Bopp, 1997.

• Meteors.  Because meteors are momentary flashes of bright light, they are sometimes called "falling" or "shooting" stars, but they are not stars at all.  In general, the appearance of a meteor is completely unpredictable.  You may see one streak across any part of the sky at any time of year, at any time of night.  Occasionally, people would be surprised to see a swarm of thousands of meteors appear all across the sky over a brief period of time.  The periodicity of these awe-inspiring events -- called "meteor showers" -- was unrecognized until the late nineteenth century.

The  spectacular 1833 Leonid meteor shower as depicted by the Dakota Sioux.
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The 1833 Leonid meteor shower depicted by Europeans.
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The 2001 Leonid meteor shower.
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• Eclipses.  Solar eclipses occur whenever the Moon's shadow falls upon the surface of the Earth.  The special circumstances that make this possible are completely predictable and recur two to five time each year.  Only a very small area of the Earth is covered by the Moon's shadow during any given eclipse, so only a relatively small number of people are lucky enough to see the event.  An individual who remains in the same location for his or her entire life will be lucky to witness even one total solar eclipse. 

The  solar eclipse of 1869 as depicted by the Dakota Sioux.
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The 1869 solar eclipse depicted by solar astronomer, Charles Young of Princeton University.
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Solar eclipse, April 14, 1992.

• Newgrange (near Dublin, Ireland; ca. 3300 BCE):

The midwinter sunrise illuminates a decorated stone at the end of the narrow 80 ft passageway built into a man-made mound.
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Entrance to Newgrange.  Midwinter sunrise light enters the upper opening, or roofbox.
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Interior passageway at Newgrange.
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Sunlight shining through the roofbox at Newgrange.
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• Stonehenge (near Salisbury, England; constructed in different stages from 3100-1100 BCE)

Some who have studied the arrangement of standing stones at Stonehenge have suggested it was built and used to predict important celestial alignments:  eclipses, solstices, and lunar standstills.  Many others disagree.
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• Maes Howe (Orkney Islands, Scotland;  2500 BCE)

At sunset in midwinter, sunlight enters a narrow passage at Maes Howe and illuminates the back wall.  Seen from inside the tunnel opening, the setting midwinter Sun appears to blink on and off as it passes behind an irregular ridge of distant mountains.  The effect is like watching multiple sunsets on one day.  Midsummer moonset can also be viewed from this vantage point. 

Given what we know today about the heavens, the importance we assign to certain celestial phenomena, and the particular celestial events to which we devote special time and attention, it is natural for us to look at these ancient monuments and imagine how WE would have used them.

Archaeoastronomers have to exercise a great deal of caution when "decoding" prehistoric structures like Maes Howe or Stonehenge.  The people who built them are no longer available to tell us why they created the site the way they did, what purpose they hoped it would serve, or how it actually worked out in practice.  They left no written record of their plans or intentions.  Are the special celestial alignments that we see in these monuments today part of the builders' original plan, or are they simply coincidental?  How can we be sure?