Fame, fortune, love and loss … it’s all written in the 12 “star signs” if you believe in astrology.
But you might be surprised to know the star signs in your newspaper don’t match up with the actual constellations in the sky with the same or similar names.
“There was never a one-to-one correspondence between the astronomical constellations which drew a picture in the sky, and the … star sign, which was associated with certain properties,” amateur astronomer Ian Musgrave said.
Both astronomy and astrology use the word zodiac but they mean very different things.
What is the zodiac?
In astronomy, the zodiac is a group of constellations that sit behind the path that the Sun, Moon and planets travel across the sky, as seen from Earth — 8 degrees either side of an imaginary line known as the ecliptic.
These constellations are named after figures, animals and objects from Greek mythology — although many have much more ancient origins.
The ecliptic line directly passes through 13 constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus, Aquarius and Pisces. It also passes within a hair’s-breadth of the constellation of Cetus.
Zodiac constellations sit behind the path taken by the Sun as seen from Earth.
Most Western astrology systems neatly divide the sky into 12 slabs of 30 degrees and designate a star sign based on when the Sun appears in those regions at midday throughout the year.
For a start, the neat star sign slabs don’t exactly reflect the size, boundaries or positions of the real constellations.
Some constellations such as Pisces, are much bigger than others, such as Cancer. So the Sun spends different amounts of time travelling through each actual constellation.
Each star sign can also contain several constellations — including sometimes more than one zodiac constellation, or small parts of constellations that are not part of the astrological zodiac such as Ophiuchus and Cetus.
“[The] star sign contains a large chunk of an astronomical constellation with a zodiacal name and little bits of other astronomical constellations,” Dr Musgrave said.
Shifting equinox a problem for star signs
To make matters even more confusing, most newspaper astrology columns are based on the position of the Sun in relation to the equinox that occurs in March, Dr Musgrave said.
This so-called vernal equinox marks the point in time when the Sun crosses the celestial equator and the beginning of spring in the northern hemisphere.
When the ancient astronomer Ptolemy mapped 48 constellations in the 2nd century, the vernal equinox occurred in Aries.
Thanks to the slow wobble of the Earth’s axis, the location of this equinox has shifted westward with respect to the background stars over time.
So it now appears in the constellation of Pisces. And in about 2600 it will be in the constellation of Aquarius.
That’s almost a whole star sign. We don’t notice this shift on Earth because it is so slow, and the date of the equinox stays the same due to the addition of leap years into our calendars.
But it means each star sign now contains the constellation that was formerly to the east.
And in some cases, the star sign doesn’t even contain the constellation whose name it bears.
So Capricorns, the Sun is now in Sagittarius in the real world — handy to know if you’re underwhelmed by your new year’s predictions.