Many western astrologers use the tropical zodiac which has twelve 30 degree divisions with Aries beginning on the spring equinox ascendant or vernal point, currently at about five sixths of the way through the constellation the Fishes. (The exact figure varies from different opinions of this 'anayamsa'.)

Others, notably eastern astrolgers of the Vedic school, use the sidereal zodiac with twelve equal houses of 30 degrees with Aries beginning at the start of the constellation the Ram.

Maria Thun's elemental rhythms are based upon the same 'beginning' as the sidereal zodiac but the visible constellations are the 12 divisions of the ecliptic or zodiac. It is based on what you see, sometimes called the Astronomical zodiac, with the inclusion of the constellation Ophiucus into the Scorpion. A thirteenth constellation - bring on the DaVinci Code buffs!

The exact boundaries between the constellations is  a debated point. Brian Keats' calendar is based upon the following divisions researched by Paul Platt of the USA;