Here’s what I’m working on right now for my book project, tentatively titled something like The Time Traveler’s Pocket Reference Guide. I’ve had some input from helpful astronomers at the US Naval Observatory [transcript of conversation so far], and I’m using the unbelievably kick-ass software Starry Night to simulate astronomical observations from ancient times.

Basically, the book is a pocket reference and survival guide for time travelers. One of the first problems confronting a traveler to the distant past is to find out when they are, starting with broad epochs. Hopefully, the traveler will be able to do this with only naked-eye observations, and reference to charts that can fit in a small book (that also will be crammed with blueprints for the internal combustion engine, maps of major trade routes, phrase books for important languages, and the hottest stocks to buy in 1900.) Here’s what I’ve come up with so far:

HOW TO DETERMINE THE APPROXIMATE CENTURY FROM THE STARS:
(Still to come: how to determine a more precise year, if you find that you're within
the scope of recorded history.  This will likely come from Dr. Thomas Corbin's suggestion
to use a table of synodic periods for Jupiter, Saturn, and Uranus.)
1. Determine whether you are on the northern or southern hemisphere by reference
to a simple constellation chart [star chart to be included].  Locate the
constellations that lie along the sun, moon, and planet's path across the
sky.  These constellations are the familiar Zodiac constellations, and are
useful for reference, especially if you can collaborate with a local astronomer.
2. Locate the four points of the compass by finding Polaris (northern hemisphere)
or by finding Sigma Octantis (southern hemisphere), then determining the
locations of east and west on the horizon.  You may wish to erect markers or
standing stones to help remember these locations, as many ancient peoples have done.
3. Each day, monitor sunrise to find the day at which the sun rises most exactly
to the east.  This is the vernal equinox, the day on which day and night are
exactly equal everywhere in the world.  There are two equinoxes
each year; the vernal equinox and the autumnal equinox.  Since the chart
below references only the vernal equinox, you'll need to determine
which equinox you're looking at.
* In the northern hemisphere, the sun will rise a small amount further to
the North each day as the vernal equinox passes, and each day will get slightly
longer and warmer.  The opposite is true of the autumnal equinox.
* In the southern hemisphere, the sun will rise a small amount further to to
the South each day as the vernal equinox passes, and each day will get slightly
shorter and cooler.  The opposite is true of the vernal equinox.
4. Note in what constellation the sun rises on the day of the vernal equinox, and
refer to the chart below to determine the your approximate
temporal location.
LOCATION OF SUN
AT VERNAL EQUINOX     	YEAR
Pisces		 2000 CE -     1 CE
Aries		    1 BCE -  2000 BCE
Taurus		 2000 BCE -  4000 BCE
Gemini		 4000 BCE -  6500 BCE
Cancer		 6500 BCE -  8000 BCE
Leo		 8000 BCE - 10500 BCE
Virgo		10500 BCE - 12500 BCE
Libra		12500 BCE - 15000 BCE
Scorpio		15000 BCE - 17000 BCE
Sagittarius	17000 BCE - 19500 BCE
Capricorn	19500 BCE - 21500 BCE
Aquarius	21500 BCE - 24000 BCE
This cycle repeats, so it's possible that if the sun rises in Aquarius, you're
either temporally located in 24,000 BCE or 48,000 BCE.  Add 26,000 years to each date
to determine other possibilities for your location.
Further back in time than 50,000 BCE, you may not be able to recognize
constellations.  If this is the case, you'll need to pay less attention to the
astronomical date and more attention to the geologic and evolutionary progression
of the Earth itself.
This method will not work if you are very close to either the North pole
or the South pole.  If this is the case, however, it's assumed that you'll have
other, more pressing issues to worry about than the exact year.  You may look for
Polaris to determine if you are in immediate danger of being hunted by a polar bear:
polar bears are only found in the southern hemisphere, in Antarctica.