The Celtic Wheel Horologium

East West Zenith Nadir

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The Celtic year was quartered by the solar events with which we are familiar—the solstices (“sun-standing”) and the equinoxes (“equal-night”)—and then it was quartered again at the midpoints between the solar events, the “cross-quarter days.”  The result was that the summer solstice, the longest day of the year, was midsummer’s day, not the first day of summer as we observe it.  The same goes for the winter solstice, the shortest day of the year and the middle of winter in the old system.  (It never made sense to me that as soon as summer begins, the days start getting shorter, or that it has already been cold for some time by the “first day of winter.”)

The names of most of the old holidays are no longer household words (except Yule), and most people, if they have heard the old names at all, are likely to associate them with paganism.  Modern “pagans” who claim these holidays as their own do nothing to dispel this idea.  But they live on in traditions such as Candlemas/Groundhog Day, May Day, and All Saint’s Day/Halloween.

The true cross-quarter dates are calculated based on the position of the earth in its elliptical orbit (or the sun’s “ecliptic longitude” as seen from earth) and are actually a few days later than the traditional dates of Imbolc, Beltane, Lughnasadh, and Samhain.  The major solar events occur at the 0°, 90°, 180°, and 270° points with respect to the vernal equinox; the cross-quarter dates are placed at the 45° points between them, which may be up to 12 hours away from the midpoint between the calendar dates.

The Celtic Wheel Horologium indicates the current date with respect to the solstices, equinoxes, and cross-quarter days, as well as the positions of the sun, moon, and stars.  The view is that of an observer looking down at the solar system from above the earth’s north pole.  The vertical blue line is the meridian, the line passing from the north to the south celestial poles through the zenith of a terrestrial observer.  Times and dates are local based on the user’s system time.

Most of the diagram makes one clockwise revolution during the course of a 24-hour day, rising in the east and setting in the west; the positions of the various elements are indicated by the hour numbers on the perimeter.  The spokes of the wheel indicate the solstices, equinoxes, and cross-quarter days.  The current date is indicated by the large blue pointer.  The black pointer above the vernal equinox indicates sidereal time, or “star time,” the point of reference for the earth’s rotation with respect to the fixed stars.  The inner white pointer indicates clock time, which phases in and out of alignment with sidereal time over the course of a year due to the earth’s rotation around the sun.  The inner yellow pointer indicates true solar time, or “sundial time,” which differs from clock time to varying degrees due to daylight saving time, and the effects of the earth’s elliptical orbit and axial tilt (“the equation of time”).  The outer pointers are minute hands corresponding to the inner pointers.  Sidereal time gains on clock time by a little less than four minutes per day.

Astronomical time is based on the meridian overhead, not midnight, so the actual position of the sun in the sky is opposite that of solar time.  (The vernal equinox crosses the meridian at sidereal time 00:00, while the sun crosses it at solar noon.)  The position of the moon is indicated as well; it will be adjacent to the sun when new, and opposite when full.  The names of the zodiac constellations indicate their true sidereal positions in the sky.  The inner eccentric circle indicates the contrast between the summer solstice, when the sun is highest, and the winter solstice, when the sun is lowest.

Accurate sidereal and solar times require knowledge of the observer’s longitude.  Time zone is used by default as a rough estimation, but in some parts of the world it can be two hours off.  If your location uses permanent daylight saving time, check the box.
Entering your actual longitude will result in accurate figures.  Use a negative number for West, or positive number(s) and check West.
Look it up on Google Maps; right-click your location and select “What’s here”; latitude and longitude are shown in the card at the bottom.
If you save and open the document with a text editor, you may enter a default longitude where it says:
// enter your longitude here if desired, positive for East and negative for West:
var myLongitude = -121.71;

The Old English month names:

JanuaryAfter Yule Month  AprilEaster Month  JulyAfter Litha Month  OctoberWinter Full Month
FebruaryHearth Cake Month MayThree Milking Month AugustWeed Month NovemberBlood Month
MarchHretha Month JuneEre Litha Month SeptemberHoly Month DecemberEre Yule Month

Calculations are based on the “higher accuracy” equations in Astronomical Algorithms, second edition, by Jean Meeus, and the full VSOP87 data set.

For more detailed sun and moon information, times, and dates, see the Solar and Lunar Data page.

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