English: The Earth's seasons can be understood as "phases of the Sun", similar to how the Moon goes through its cycle of phases. This graph shows the seasons in the Northern Hemisphere, with the x-axis being time and the y-axis being intensity of Sun energy (blue) and temperature (pink). (For the Southern Hemisphere, this cycle is shifted by 180 degrees.) The top row of single letters mark the Zodiac signs of:

Aries, Taurus, Gemini /
Cancer, Leo, Virgo /
Libra, Scorpio, Sagittarius /
Capricorn, Aquarius, Pisces.

The bottom row of single letters are the months of the year starting with March. The triangles on top point to the "quarter days", with the "cross-quarter days" indicated at the bottom. (Other names have been given to these days. This image lists only an example.)

The Earth's orbit around the Sun forms a circle, and a circle's motion projected in time forms a sine wave. Because of the Earth's axial tilt, this cycle corresponds to large variation in the amount of Sun energy (in blue) at a location on the Earth's surface. There is a thermal lag in heating up the land and seas, so the temperature cycle (in pink) is shifted in a delay of roughly one month (the amount of delay depends on location, and is not always symmetric).

The first division of this cycle is between the peak and valley, with the top point of the Sun energy cycle being called "midsummer" (also called "Litha"). The bottom point is "Midwinter" (also called "Yule"). Half of the year is above the midline of Sun energy and half of the year is below. Intersection with the y-axis marks the Equinoxes, which divides the colder and hotter halves of the year (or more accurately Sun energy, because temperatures are subject to seasonal lag).

The second division splits the cycle into four, marking the four seasons throughout each year: Spring, Summer, Fall, and Winter. The Spanish word for Spring is 'Primavera', with prima- indicating that this is the first season. This season brings the growth of new life, as plants emerge after the dead of winter has passed. With the cycle divided into quarter-phases, Spring is the First Quarter, Summer is Full, Autumn is the Last Quarter, and Winter is New. The Midwinter Yule is the time of year of Christmas, and this has been thought of as the "rebirth of the Sun", marking the start of a new year.

For the third division of the year, there are different approaches. The most widely used is the month. The month is based on the natural lunar cycle with phases that are very easy to observe. In one year, the Moon goes through slightly more than 12⅓ lunations. So a natural way to divide the year is by these 12 moons (12 "moonths", 12 months), with each of the four seasons having three months. The Zodiac system divides the year into 12 houses. For the Tropical Zodiac, the cardinal points are fixed to Vernal Equinox, Summer Solstice, Autumnal Equinox, and Winter Solstice.

As the temperature cycle is delayed by seasonal lag, the months of the year are also shifted from the starting points that the Tropical Zodiac is fixed to. This calendar shift amounts to basically one-third of a month. So in the Gregorian Calendar, the new year does not start exactly at the bottom of the Sun cycle, but about 10 days after this minimum. The first sign of the Zodiac is Aries, starting at the Vernal Equinox. The month of March was named by the Romans who had used this as the first month of their year. It was literally the first month when the weather warmed up enough where soldiers could be marched off into war. The month is named after Mars, the god of war. With temperatures lagging by about a month and the calendar months shifted by roughly one-third of a month, the "cross quarter days" happen at the start of November, February, May and August. These days roughly coincide with the four transition points of the temperature cycle, with Imbolc being around the coldest day of the year and Lammas being around the hottest day of the year. The coldest days typically happen in late January or February, with the hottest happening around late July or August. The sign of Leo is symbolized as a lion and associated with the heat of the Sun.

There is further meaning in the symbols of the Zodiac. Aries is drawn as a split of horns which signifies the crossing point of Sun energy that happens at the equinox. Easter is the Christian holiday tied to this equinox, and is related to the quarter day of Ostara, meaning Eastern Star. The focus is on the East, as the Sun rises in the East marks the beginning of the day, the equinox starting the season of Spring marks the beginning of new life in a year. Cancer is symbolized as a crab, an animal that walks sideways in the time of year when the Sun "walks sideways" after its peak of Sun energy. The word 'equinox' means equal night, with the daylight and nighttime split equally. In the month of September, this balancing of day and night is symbolized by the scales of Libra marking the Vernal Equinox. Both Cancer and Capricorn mark a physical location on the Earth known as the tropics. The word tropic meaning 'to turn', where the Sun's north-south elevation from one to the next stops and then turns back. The word 'solstice' means that the Sun stands still. The Tropic of Cancer is the most northerly line that the Sun reaches directly overhead, while the Tropic of Capricorn is the most southerly. "Cancer" is a deeply misunderstood concept in modern times. People equate it to "disease", but the essence of Cancer is not a disease at all. Cancer is the peak of the growth cycle. This is a normal, healthy and necessary aspect of existence. It only becomes a disease when the Cancer growth effect is left unchecked. It is the failure of Cancer to stop and turn back (tropic) that is the actual nature of the disease called "Cancer". Cancer is an integral part of the harmonious cycle of growth and decay. A more proper terminology for the disease would be "cell growth out of balance in the aspect of Cancer". And it is easy to imagine that this is exactly what the disease was known as originally, yet over the generations the original meaning was lost, and doctors just started calling it "Cancer" for short. There is a disease that is the exact opposite of runaway growth. Cellular senescence is a condition where the cells fail to divide. Here the growth cycle is out of balance in the aspect of Capricorn. But no one with this disease says "I have Capricorn". It is simply referred to as "old age". Imbalance on this side of the circle is well understood as being part of the natural cycle of life and death. (Those with interests in both zodiacal seasonal cycles and biology can take note that the first four houses have the letters ATGC.)

Scorpio can be understood in terms of the cross-quarter day Samhain as the time around which the temperature cycle crosses the midpoint. The Sun is stung by a scorpion and dies. This transition into the underworld is marked by All-Saints Day and Halloween (All-Hallows Eve, the evening preceding All-Saints Day), both of which can be understood as tributes to death. The Mexican festival of Día de Muertos, held at this same time of year, has no ambiguity about this connection. The season is marked by leaves falling and crops being harvested. Falling leaves is the iconic marker of the entire season that is named Fall. Scorpio is followed by Sagittarius, whose bow and arrow can be understood as another symbol for which death is inflicted. Aquarius happens in the dead of Winter, and the word 'Winter' is derived from old words meaning 'wet'/'water'. Aquarius is the water bearer, and this anticipates the coming of Spring rains which bring new life. Taurus is signified by a bull which is the beast of burden used since ancient times to till the soil for sowing seed, and it is in this season of Spring when seedlings spring out of the ground. Gemini is symbolized as twins, which can be understood as symbolic of abundant birth, happening during the time of year known for an abundance of flowers coming into bloom.

The beginning of this description mentioned the similarity to the phases of the Moon. It can also be noted that the Moon experiences a similar "seasonal lag" where its surface temperatures are shifted in phase from the Sun energy (insolation) it receives as the Moon rotates once a month. This is discussed very little because there is no life on the Moon, but it is a concern for robotic probes and astronauts when they are there.