Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge

Further Notes on Cross-Quarter Days

You will no doubt have noticed that our current dates of celebration of these holidays are not exactly on quarter or cross-quarter days. This is because these holidays were fixed in the calendar during the late-Middle Ages, whereas in the more distant past their coming was often marked by observing (or at least estimating) the occurrence of a particular station of the Sun along the ecliptic (i.e., observing the arrival of the Solstice or Equinox). Sites like Stonehenge in the UK are believed to be examples of such "observatories". The 4-fold divisions of the Solar ritual year on the cross-quarter days are visible in many ancient megalithic monuments, as well as being encoded in the Celtic Calendar of Coligny from the 2nd century AD. The Maya in Central America, and the Native Americans at the Cahokia mounds near St. Louis created similar "solar observatories" in their city/temple complexes, which were laid out on strict astronomical lines. The approximate dates we use today reflect half-remembered astronomical traditions that are older than our familiar calendars.

The Celtic Solar Calendar and traditional Japanese Luni-Solar Calendars used the cross-quarter days to mark the start of the various seasons, unlike the current tradition in the West where we say speak of the Equinoxes and Solstices proper as the first day of their respective season. The Celtic and traditional Japanese usages are actually more astronomically correct, at least for the latitudes of those societies. For example, in the traditional Japanese calendar the first day of spring (Risshun) is on the first cross-quarter day (Feb 3 or 4 - time of the traditional Setsubun festival which used to mark the beginning of the new year), Summer begins on May 6 (Rikka), autumn on August 8 (Risshuu), and winter on November 7 (Ritou).

In 2007, the approximate times of the cross-quarter days are as follows (all times are UTC):

   1st CQD  2007 Feb 4  05:18 UTC
   2nd CQD  2007 May 5  21:21 UTC
   3rd CQD  2007 Aug 7  21:31 UTC
   4th CQD  2007 Nov 7  19:24 UTC

These were computed using data provided by the JPL Horizons On-Line Ephemeris System to compute the ecliptic longitude of the Sun as seen from Earth (geocentric). Times are rounded to the nearest minute.

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