General Astronomy/Zodiac Dates/2007

A year is defined as the time it takes for a planet to complete one orbit of the Sun. We observe the effect as the change of the seasons and the movement of the constellations. Over the course of a year, the Sun moves through a great circle on the celestial sphere, tracing out the same path year after year. This path is called the ecliptic. The ecliptic is not only the path of the Sun in the sky, it also marks the plane of the Earth's orbit of the Sun. The planets orbit the Sun in different planes but near to the ecliptic.

It takes Earth 365 days, 5 hours, 48 minutes to complete one orbit of the Sun, so a year on Earth is 365 days, 5 hours, 48 minutes (365.2422 days) long. In our everyday speech, when we speak of a "year," we are often referring to an Earth year. When most people, in most contexts, are asked, "How old are you?", the standard response is to give the age in Earth years. Integer multiples of a solar year on Earth have many applications in our culture, in which birthdays and other anniversaries (such as wedding anniversaries) are popularly celebrated. In the Gregorian calendar, every year begins at midnight on January 1, then ends twelve months later at 11:59:59 p.m. on December 31. However, the number of days in a year, 365.2422, is not a whole number. Instead of waiting until 5:48 a.m. on January 1, we account for the 0.2422 of a day by adding an extra day at the end of February (February 29, also referred to as Leap Day) every four years. That way, a four-year period has 1,461 days, not 1,460, or 1,461 / 4 = 365.25 days per year.

Throughout the 365-day orbit, we pass through twelve constellations. If we define each of twelve phases to be the time we spend in each constellation, then each phase lasts roughly one month. The constellations are Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, and Capricorn. We culturally mark our seasons (spring, summer, autumn, winter) using the transitions between these phases. In the northern hemisphere, the spring equinox happens in March, and it coincides with the transition between Pisces and Aries, and we define "spring" to begin on this moment and end at the summer solstice in June, which is the transition point between Gemini and Cancer. The word "summer" is defined as the period between the summer solstice and the fall equinox, the transition point in September at which Virgo ends and Libra begins. "Fall" or "autumn" is then defined as the period between the fall equinox and the winter solstice, the transition in December between Sagittarius and Capricorn. Finally, "winter" is the period between the winter solstice and the spring equinox, and the cycle repeats.

In pop culture, astrology is the practice of discerning or predicting information about human affairs and terrestrial events by studying the apparent positions of celestial objects. The Earth's movement through the constellations plays a major role in a very popular form of astrology. Supposedly, the constellation (zodiac sign) you are born under can predict your personality, your behavior, and compatibility with others. If you asked people what their zodiac sign is, you may hear, "I'm a Capricorn," "I'm a Scorpio," etc. Culturally, each of the twelve zodiac signs has personalities assigned to them. For instance, in an Allure article, Libra (September 23 — October 22) is described as follows: "Balance, harmony, and justice define Libra energy. As a cardinal air sign, Libra is represented by the scales (interestingly, the only inanimate object of the zodiac), an association that reflects Libra's fixation on establishing equilibrium. Libra is obsessed with symmetry and strives to create equilibrium in all areas of life — especially when it comes to matters of the heart." Astrology is popular and fun, but it is not supported by scientific evidence. Astrology has been rejected by the scientific community as having no explanatory power for describing the universe. Scientific testing has found no evidence to support the premises or purported effects outlined in astrological traditions.

Where astrology has made falsifiable predictions, it has been falsified. The most famous test was headed by Shawn Carlson and included a committee of scientists and a committee of astrologers. It led to the conclusion that natal astrology performed no better than chance. Astrology has not demonstrated its effectiveness in controlled studies and has no scientific validity, and is thus regarded as pseudoscience. There is no proposed mechanism of action by which the positions and motions of stars and planets could affect people and events on Earth in the way astrologers say they do that does not contradict well-understood, basic aspects of biology and physics. Even so, many people believe in astrology, and confirmation bias can often reinforce existing beliefs in its predictive power. For the rest of this article, we will be discussing science-backed astronomy, not astrology.

The table below shows the dates and times, first in Eastern Time (ET) and then in Universal Time Coordinated (UTC), for the transitions between constellations in the year 2007, the seventh year of the 21st century and the third millennium A.D. This year marked Earth's 2,007th orbit around the Sun in the Anno Domini era. The 24-hour clock format is used here.

Constellation	Start Time (ET)	Start Time (UTC)
Aquarius	January 20, 2007, 06:00:50	January 20, 2007, 11:00:50
Pisces	February 18, 2007, 20:08:56	February 19, 2007, 01:08:56
Aries	March 20, 2007, 20:07:25	March 21, 2007, 00:07:25
Taurus	April 20, 2007, 07:07:03	April 20, 2007, 11:07:03
Gemini	May 21, 2007, 06:11:56	May 21, 2007, 10:11:56
Cancer	June 21, 2007, 14:06:26	June 21, 2007, 18:06:26
Leo	July 23, 2007, 01:00:11	July 23, 2007, 05:00:11
Virgo	August 23, 2007, 08:07:59	August 23, 2007, 12:07:59
Libra	September 23, 2007, 05:51:14	September 23, 2007, 09:51:14
Scorpio	October 23, 2007, 15:15:24	October 23, 2007, 19:15:24
Sagittarius	November 22, 2007, 11:49:54	November 22, 2007, 16:49:54
Capricorn	December 22, 2007, 01:07:49	December 22, 2007, 06:07:49

Calculating the Earth year

One way astronomers approximate the length of an Earth year is to take the length of time between one crossing of a well-defined fixed point in Earth's orbit and the next crossing of that same point. This is easily done using constellations. Using multi-year tables of equinox and solstice times, such as the table on Wikipedia's page for "Solstice," you can get a very good approximation of the length of a year.

For instance, in 2016, the Earth entered the Scorpio phase on October 22, 2016, at 7:45:34 p.m. (19:45:34) Eastern Daylight Time (23:45:34 UTC). Twelve months later, in October 2017, the Scorpio phase began on October 23, 2017, at 01:26:42 (05:26:42 UTC). The difference between these two times is 365 days, 5 hours, 41 minutes, 8 seconds, missing the true value of 365 days, 5 hours, 48 minutes by only seven minutes. Last year, in 2023, the Aquarius phase was reached on January 20, 2023, at 3:29:34 a.m. Eastern Standard Time (08:29:34 UTC). This year, in 2024, it was reached on January 20, 2024, at 09:07:20 (14:07:20 UTC). Again, using Timeanddate's "date duration" calculator quickly shows that the difference between the times is 365 days, 5 hours, 37 minutes, 46 seconds. The error is a little bigger this time, eleven minutes, but that's still minuscule! Yes, every year is a little bit different, and there are some natural, year-to-year fluctuations, but by taking the average over a large sample of years over a long period of time (say, 20 years or 50 years), astronomers can get closer to the target value of 365.2422 days.