The Changing Sky

Some changes in the sky take generations to notice.  One such change was first noticed by the Greek astronomer  Hipparchus in the second century B.C. by examining sky charts from earlier astronomers.  He recognized that the vernal equinox slowly shifted its position on the celestial sphere towards the west.  This shift was known as the precession of the equinox.  The original name for the vernal equinox was the First Point of Aries.  If you examine your SC001 sky chart, you will see that the vernal equinox is in Pisces (and Aries is above and to the east).  Not so in the past.  Use your SC001 sky chart to locate the position of the sun (on the ecliptic) on your birthday.  Now notice which constellation of the zodiac the sun lies in.  Notice anything odd?  In most cases, you will be one full constellation off your "sign".  This is because horoscopes are not adjusting to this change in the sky.

You can do all these things in Stellarium.  Stellarium only allows years from -99999 to  99999 (but entering negative years is a bit tricky ... play with it).  You can still go back far enough to observe this.  Starting with the current date, draw in the ecliptic (of date) and the equator (of date).  Now mark the equinox (of date).  Center on the constellation Pisces.  The (celestial) equator and ecliptic intersect at the vernal equinox.  Change to year 0 and see if you can observe the shift in the vernal equinox relative to the background constellations.  Compare the equinox (of date) versus the equinox in the year 2000 (J2000).  Can you see that there are some changes in the sky that take a VERY long time to observe.


Sky charts in the future will place the vernal equinox in the constellation Aquarius.  (Was this what the 5th Dimension was singing about in their song "Age of Aquarius"?)  Come back in about 26,000 years and your SC0001 sky chart will again be accurate because that is the time it takes for the vernal equinox to migrate back to the same point in the sky.  It should be clear that your lifetime is far too short to notice any appreciable precession to the naked eye.

So what cause this change?

If you've ever watched a top spinning, it "wobbles" at the same time.  The earth does exactly the same thing as it revolves around the sun.  This 26,000 year "wobble" results in this slow westward migration of the vernal equinox.

Precession of a top animation

The exact reason this occurs comes from gravitational tugs by the moon (and sun) on the equatorial bulge on the earth which tries to alter our tilt of 23˝ degrees.  Precession is the net result, and you would need a physics class to cover this completely (which we will not).

Effects of Precession

Precession is a constant annoyance to astronomers because sky charts need periodic updating.  Your SC001 sky chart is calibrated for 2000 AD.  Every 50 years the charts are updated.  Why?  If you examine the spinning top animation above and imagine the red line represents the earth's equator, you will see that it, too, shifts position in space.  This means the entire celestial equator (which is an extension of that plane) is shifting with time.  Since all astronomical coordinates are based on the position of these lines, all the astronomical coordinates need to be re-set as well.  Fortunately, modern computers can handle this task with ease.

Another interesting effect precession has on our sky is the changing position of the celestial poles.  The animation shows that precession also alters where our axis points in space.  Currently, we are fortunate to have a moderately bright star, Polaris, within 1 degree of the actual celestial north pole.  The ancient Greeks had none, but the earlier Egyptians used the star, Thuban, in Draco as a pole star (when they were constructing the pyramids 4000 years ago).  Your sky map SC002 shows the migration of the pole as a dotted circle.  You will notice that about half a precession cycle away, a very bright star, Vega, became/becomes a great pole star (almost).   When H. G. Well wrote his classic science fiction novel - The Time Machine, he took precession into account.  The time traveler was moving through time so fast that day/night and even seasonal effects were blurred ... all the traveler would notice is precession as he raced through the centuries.

I made a movie clip showing where the north celestial pole will be over a period of about 12,000 years.  Can you tell when the clip displays our current era?  The file is pretty large .wmv format (10 MB) OR .mp4 format (7 MB).

Stellarium users can observe this by centering on Polaris and marking the north celestial pole.  Now advance to the years 4000, 8000, etc and see how precession will change our north celestial pole.  You can even mark the "precession circles" which is actually only one circle.  This circle indicates the locations of the north celestial pole as the earth wobbles on its axis.

Note:  Click on any star in the sky and look at the long list of data that appears.  You may have noticed that Stellarium displays two values of right ascension and declination (RA/DE) .... one "of date" and the other at the year 2000.  Since precession changes the location of the vernal equinox on the celestial sphere, it also changes the coordinates of ALL objects.  Astronomy books once updated the values every 50 years but computers are able to update instantly. 

Other (very) long term changes in the sky

The three motions of the earth, (rotation, revolution, and precession) all produce changes in the sky ... each on their own time scale.  Another change occurs as the sun moves around the Milky Way.  Our sun revolves around our home galaxy once every 225 million years.  As it does so, the positions of the stars shift with respect to each other ... meaning that, in time, even the familiar constellations become unrecognizable.  The point is, everything in the sky is changing even if we don't live long enough to observe these changes.

ŠJim Mihal 2004, 2014, 2019- all rights reserved