There's a lot of astronomy involved in Wicca. Moon phases, planets, the zodiac constellations, solstices, equinoxes and so on - some of it is pretty easy to understand, but a lot of it is quite involved for people that up until now haven't really had an interest in astronomy. I'm one of the lucky ones - I've had the opportunity to study astronomy at university, and that makes it much easier for me to understand a lot of these astronomical concepts. So, for those of you that don't know your perihelions from your parsecs, I've written a page to explain some common Wicca-related astronomical phenomena in plain English. If there's something else you'd like to see explained here, let me know.

Blue Moon        Equinox        Moon Phases

Retrograde        Solstice        The 13 Constellations of the Zodiac


Blue Moon

Firstly, a blue moon is not the second full moon in a calendar month. This notion came about through a mistake in an issue of Sky & Telescope magazine in 1946. A true blue moon is the third moon in a series of four full moons in an astronomical season. Astronomical seasons are determined by the position of the Sun relative to the Earth, and so begin at the solstices and equinoxes - for example, astronomical summer begins at the summer solstice, astronomical autumn begins at the autumn equinox and so on. Within these seasons are, usually, three full moons that are called (in summer, for example) the 'early summer', 'mid summer' and 'late summer' moons. But, because the Earth travels in an elliptical orbit around the Sun, the periods between the solstices and equinoxes aren't exactly the same and their timing changes slightly from year to year; hence, astronomical seasons aren't all the same, varying by a few days in length and timing. Sometimes the Earth's orbit extends and moves a season far enough to encompass a fourth full moon (about every 2.72 years). In this case, the third full moon is declared the blue moon so the last moon can still be called 'late'.

This is a very similar sounding system to the one used by the Maine Farmers' Almanac - but there is one fundamental difference. The Almanac assumes an idealised Sun that travels at a set speed and generates an equinox every year between March 21 and March 22. Obviously, this isn't the case. The dates and times for the astronomical and Almanac systems are, for the most part, different - they do overlap occasionally, but they differ more often than they agree. I personally prefer the astronomical method - I'm a scientific creature and I don't like inaccuracies. I think that puts me in the minority when it comes to blue moons, but I don't mind that - I like accuracy! So, as with the zodiac, you need to decide whether you want to use an idealised model or the physical movement of the cosmos.


Working out the dates of blue moons

With the dates and times of solstices, equinoxes and full moons you can work out when a blue moon will occur. For example, in Melbourne in 2010 there is a blue moon - I only know that because I got out a pen and paper and crunched some numbers:

Step 1 -

First, work out the astronomical seasons for your area. You'll need to know your time zone relative to Coordinated Universal Time, or UTC (also called Greenwich Mean Time, or GMT), which is UTC+10 for Melbourne. Because of Melbourne's position on the Earth relative to Greenwich, England (UTC+0) we are 10 hours ahead and therefore we have to add 10 hours to all the times in the astronomical data, which are given in UTC. Note that daylight savings changes the time difference - during daylight savings Melbourne is an extra hour ahead, and our UTC designation becomes UTC+11. For those of you not living in Melbourne (and all those that rely on the news to tell us when to change clocks), daylight savings is in operation between the first Sunday in October and the first Sunday in the following April.

To define the astronomical seasons, it's easiest to write out the dates (the solstices and equinoxes) in order. I've chosen the astronomical year from 2009-2010, because in there somewhere should be a blue moon (about 2.7 years after the last one):

Season UTC Melbourne Time


December 21, 2009 @ 17:47 -

March 20, 2010 @ 17:32

December 22, 2009 @ 04:47 -

March 21, 2010 @ 04:32


March 20, 2010 @ 17:32 -

June 21, 2010 @ 11:28

March 21, 2010 @ 04:32 -

June 21, 2010 @ 21:28


June 21, 2010 @ 11:28 -

September 23, 2010 @ 03:09

June 21, 2010 @ 21:28 -

September 23, 2010 @ 13:09


September 23, 2010 @ 03:09 -

December 21, 2010 @ 23:38

September 23, 2010 @ 13:09 -

December 22, 2010@ 10:38


Step 2 -

Now that we know the dates of the seasons, we can work out how many full moons each one has. An educated guess suggests the blue moon should be towards the end of the year, so we'll try the full moons that fall in astronomical Spring (beginning September 23 @ 13:09 and ending December 22 @ 10:38, Melbourne time):

1. September 23 @ 09:17 UTC = September 23 @ 19:17 UTC+10

2. October 23 @ 01:36 UTC = October 23 @ 12:36 UTC+11

3. November 21 @ 17:27 UTC = November 22 @ 04:27 UTC+11

4. December 21 @ 08:13 UTC = December 21 @ 19:13 UTC+11

So, astronomical Spring in 2010 has four full moons instead of usual three. The three usual moons would be called 'early Spring', 'mid Spring' and 'late Spring'; the third full moon in a season with four is designated the blue moon so the other three moons can keep their names. So for Melbourne, in 2010 there is a blue moon on November 22.



An equinox occurs when the Earth's axis is exactly side-on to the Sun, causing the day and night to be of almost equal length (i.e. sunrise and sunset are almost exactly 12 hours apart). For a moment during the day, the centre of the Sun is positioned exactly over the Earth's equator; this is the point at which the Sun crosses the celestial equator. The word 'equinox' comes from the Latin aequus ('equal') and nol ('night'). Technically, day and night at an equinox are not equal - there is a difference of a few minutes due to atmospheric scattering of sunlight and also the latitude at which you happen to be on that day.

Image from Click for larger image.

The celestial sphere, showing the solstices and equinoxes.

(In relation to the Southern Hemisphere)


Moon Phases

The phases of the Moon refer to the amount of the lunar surface that is lit by the Sun when viewed from Earth. For obvious reasons, this usually means how much of the Moon is visible at night. There are nine lunar phases, as follows:

Dark Moon: not visible.

New Moon: not visible, or the very first visible crescent.

Waxing Crescent: Left 1-49% visible.

First Quarter: Left 50% visible.

Waxing Gibbous: Left 51-99% visible.

Full Moon: Fully visible.

Waning Gibbous: Right 99-51% visible.

Third Quarter: Right 50% visible.

Waning Gibbous: Right 49-1% visible.

The above values are for the Southern Hemisphere - for the Northern Hemisphere, swap 'right' and 'left'.

It takes about 29.53 days for the Moon to complete a single cycle - say, from full moon to full moon.


Image from Click for larger image.

How the positions of the Earth, Moon and Sun generate the

phases of the Moon, as seen from Earth (Northern Hemisphere).



Because we look at all the planets from Earth as they (and we) revolve around the Sun, sometimes they appear to be moving backwards in the sky. This movement is called apparent retrograde motion (retrograde from the Latin retro - backwards, and gradus - step) and describes the planet apparently moving in the opposite direction to the rest of the objects in the sky. Planets don't really move backwards and forwards along their orbits, it can just look like that from Earth.



A solstice occurs when the Earth's axis is pointed exactly towards or away from the Sun, causing the Sun to reach its highest point in the sky during summer (the summer solstice) and its lowest point in the sky during winter (the winter solstice). The word 'solstice' comes from the Latin sol ('sun') and sistere ('to stand still'), because of the fact its height in the ecliptic appears to be fixed for a short period. The day on which the summer solstice falls is the day of the year when the period between sunrise and sunset is longest, and the day of the winter solstice is the day that period is shortest. For a diagram, see Equinox above.


The 13 Constellations of the Zodiac

First, a little history. The word zodiac is rooted in Greek and basically means 'circle of little animals'. It was the Europeans that attached this term to a yearly cycle they developed, associated with the passage of the Sun through twelve constellations (mostly named after animals) in the sky. This cycle is the basis for the 'if you were born between this date and that date you're this sign' thing that the newspaper horoscopes are rooted in. At this point in history (this is still well before Christ), people still thought the solar system revolved around the Earth - this is called the 'geocentric' ('Earth-centred') model . This theory would have encompassed the rest of the universe too, had they known more existed.

Anyway, the constellations of the zodiac are all contained within a band of sky that extends 8° above and below the path of the Sun across the heavens (called the ecliptic). This band encompasses all the movements of the Moon, the Sun and the five 'classical' planets (Mercury, Venus, Mars, Jupiter and Saturn - the only ones that can be seen with the naked eye). This division of the sky into equal sections using these twelve constellations was the first attempt (at some point in the 1st millennium B.C.E.) to assign a latitude/longitude system to the celestial sphere like the one we have now for the Earth. There are some differences between Earth and Zodiac latitude/longitude, though - the equator of the zodiac latitude/longitude is the path of the Sun, not the actual equator of the Earth - the zodiac and Earth equators are different by 23°, due to the tilt of the Earth on its axis; and the latitudes of the zodiac system also only go up to 90°. So the zodiac, originally, was meant to be a scientific way to map out the sky and the positions of the stars and planets.

To be included in the zodiac, a constellation must lie within that 16° band of sky we call the ecliptic. Traditionally, there are 12 constellations contained within this band - Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces (these are obviously where we get the names for our star signs). But from an astronomical point of view, there are actually 13 constellations within the ecliptic - the 12 I've just mentioned plus another one called Ophiuchus. Ophiuchus doesn't appear in the traditional, 12-constellation zodiac because when it was first devised (back in the 1st millennium B.C.E.), Ophiuchus wasn't actually in the ecliptic - the changing positions of the Sun, Earth and Ophiuchus itself over the millennia have changed the positions of things in the celestial sphere, and today the Sun passes through Ophiuchus as well as the 12 classical constellations. So, if we are to use the zodiac for any practical magickal work, we need to use the current sky and its 13 constellations.

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