Precession of the Equinoxes

Today we tracked the Sun through the constellations, finding which constellations it goes through (the zodiac) and when it enters and leaves each one using Stellarium.  We compared these to astrological dates and found that they were about a month off.  Stellarium lets you go back in time, so we were able to go back to 200 BC and see that back then the dates for the Sun to be in each constellation matched up well with the astrological dates, and specifically that the vernal equinox was in Aries back then (now it is in Pisces).

We also found out that there is a 13th zodiacal constellation, Ophiuchus.  In the traditional ecliptic coordinate system, Ophiuchus is considered to be part of Scorpius - but the truth is that Ophiuchus has a better claim to be a zodiacal constellation than does Scorpius.  The Sun barely spends any time in Scorpius, and spends much more in Ophiuchus.  But I have to admit that Scorpius is cooler than Ophiuchus, and much easier to spot in the sky.  (Besides, if you go by where the Sun was on the date of my birth, I'm actually an Ophiuchus.  But I won't admit to that publicly.)

Finding out that the vernal equinox used to be in Aries led us to investigate the motion of the equinoxes relative to the stars.  My students found that the Celestial Equator moves relative to the stars (but the Ecliptic doesn't), and they found that it takes about 26,000 years for a full cycle of this motion (which we call precession of the equinoxes).  They determined this period using both Stellarium and my Celestial Globe program.  Once they had determined that the equinoxes move westward relative to the stars, they were able to figure out that the tropical year (vernal equinox to vernal equinox) is slightly shorter than the sidereal year.

I don't know the history of the discovery of precession very well.  I have read that Hipparcos was the first person to notice the effect.  Apparently one important factor in the discovery is that Hipparchus had access to Babylonian data from centuries before his own time.  A comparison of data over such a long time scale is essential for measuring precession because precession is so slow.  If you wait a century the equinoxes will move only 1/260th of the way around, or about 1.4 degrees.  So without a long time scale, the changes are so small that they are hopelessly difficult to measure with naked eye instrumentation.

But just because the data were available doesn't mean that the discovery will be made.  I assume that Hipparchus was very concerned about determining the length of the tropical year.  The tropical year is much harder to measure directly than the sidereal year, but it is the tropical year that we really care about (since it is the year that gives us the full cycle of seasons).  Hipparchus had every reason to assume that the two years were the same, but perhaps he had noticed that calendars based on the sidereal year failed to match up with the seasons after centuries of use.  I really don't know the history, but something must have motivated him to look for this tiny effect.

In any case, it is precession that explains the difference between the dates for the Sun entering/exiting the constellations and the horoscope/astrological dates.  The astrological "signs" were established at least by the time of Ptolemy's Tetrabiblos (which became the authoritative source on astrology, just as Ptolemy's Almagest became the authoritative source on astronomy) around AD 150. I suspect they date back further, maybe to the time of Hipparchus in about 130 BC, but I don't know that history very well either.  In the intervening two millennia since then, precession has moved the equinoxes around a bit.

In my class we won't spend much time talking about precession, except when we discuss theoretical explanations for it (a rotation of the ninth sphere, Copernicus' imperfectly synchronized motions of Earth).  But in practice it was an important issue for astronomers because one had to correct for precession to make high precision astrometrical measurements.  This issue plays a more important role in my galaxies course than it does in my Copernican Revolution course, so maybe I'll write more about it in the Spring.