“I had an astrologer explain my Vedic birth chart and there is absolutely nothing that is the same.” Someone wrote that on an astrology forum. On a forum in Quebec, someone else put the reason in French: sidereal astrology is the one truly aligned with the constellations. On that point, they’re right. It also explains why you can be an Aries on one site and a Pisces on another, with exactly the same birth date.
Both sites may well have done their math right. They simply use two different zodiacs. Once you know which is which, the gap makes sense, and you know which result to keep.
Why am I an Aries on one site and a Pisces on another?
A zodiac sign is a 30-degree slice of the sky. There are twelve, and together they go all the way around. The whole question is where you place the start of the first sign, Aries.
There are two answers. The tropical zodiac sets that start on a date in the year, the spring equinox. The sidereal zodiac pins it to the stars. Many centuries ago, the two starting points nearly overlapped. They’ve drifted apart since, and that gap is what moves your sign from one site to the next.
The tropical zodiac: the seasons
It’s the zodiac of Western astrology, and the one AstroLuma uses. Its starting point is the moment of the year when day and night are the same length, at the beginning of spring in the Northern Hemisphere. From there, the sky is divided into twelve equal parts.
That division never moves. Aries always begins at the equinox, whatever constellation happens to be behind it at the time. The signs borrowed their names from the constellations, and they describe something else: the cycle of the year, with the surge of spring, the fullness of summer, the pulling back of autumn and the bareness of winter. Humanistic astrology reads a psychological meaning in that cycle, and that’s the meaning it interprets in your chart.
The sidereal zodiac: the stars
The sidereal zodiac follows the constellations themselves. It’s the zodiac of Indian astrology, Vedic astrology, and of a few Western astrologers.
The gap comes from a very slow movement of the Earth. Its axis wobbles like a spinning top that’s slowing down, and the spring point slips back against the stars by about one degree every 72 years. This is called the precession of the equinoxes. At the start of the Common Era, the two zodiacs were almost aligned. Today they’re about 24 degrees apart.
In practice, if your Sun sits in the first 24 degrees of its tropical sign, it moves back one sign in sidereal. That’s the case for most people. With the Sun early in tropical Aries, sidereal gives Pisces; early in Leo, it gives Cancer.
What about the 13th sign, Ophiuchus?
The question comes back in the news every so often: is there a 13th sign, and has your sign changed?
Ophiuchus, the Serpent Bearer, is a constellation the Sun crosses each year, between Scorpio and Sagittarius. Constellations come in very different sizes, and some spill over into their neighbors. Signs are twelve equal slices of the sky. So the tropical zodiac has only twelve signs, and so does classical sidereal astrology. Your sign hasn’t changed because of Ophiuchus.
Which one is right?
Both are consistent, each with its own logic. Sidereal looks at where the stars are. Tropical looks at the moment in the cycle of the year, and that cycle is what psychological astrology has interpreted since Dane Rudhyar.
AstroLuma works in the tropical zodiac, like the humanistic school our readings draw on. Our free birth chart calculator calculates your chart in tropical: that’s the chart used for all our readings.
Other reasons your sign changes
The zodiac explains most of the gaps, though not all of them.
Many sites give your sign from a list of dates, like “Aries from March 21 to April 19.” But the Sun changes sign at a different time each year. If you were born on a day the Sun changed sign, only a calculation with the year, time and place can settle it.
Your Moon and your rising sign depend far more on the time. The Moon changes sign roughly every two and a half days, the rising sign roughly every two hours. An approximate birth time or the wrong time zone is enough to shift them. The rising sign in astrology explains why the time matters so much.
Frequently asked questions
Is sidereal astrology more accurate, since it’s aligned with the constellations?
It’s closer to the stars, that’s true. The tropical zodiac measures the cycle of the seasons, and it’s the one Western psychological astrology interprets. Each is right within its own system.
Why is my Vedic birth chart nothing like my Western one?
Vedic astrology works in the sidereal zodiac, about 24 degrees behind the tropical one, so many of your planets move back a sign. In the tropical zodiac, your sign is the same as at your birth, and it will stay that way.
Should I add Ophiuchus, the 13th sign?
Ophiuchus is a constellation, and the zodiac has twelve equal signs. Neither of the two major astrologies adds it.
How do I know which zodiac a site uses?
Look for the words “sidereal,” “Vedic” or “Lahiri” in its settings or its explanation page. With no mention, it’s almost always tropical. If you want to compare your Sun sign with your Moon and your rising sign, your Sun sign is not enough shows what each one adds.
Your tropical chart, with your Sun, Moon and rising sign in their exact places, is waiting on the free birth chart calculator. And if you’d like us to read the whole thing for you:
✦ Get the reading of your chart, calculated to the degree — the Adult birth chart · $29.99 USD
