Sidereal Astrology Calculator

Enter your birth details above and this calculator casts your chart in the sidereal zodiac, the one measured against the fixed stars. This is Western astrology read sidereally, on the Fagan-Bradley ayanamsa. It is not Vedic astrology, which uses a sidereal zodiac too but is a separate tradition with its own techniques. You get the full chart: every planet by sign, degree and house, the Ascendant and Midheaven, the aspects between them, and a written interpretation of each placement.

It also does something most sidereal calculators do not. Beside every placement whose sign has moved, it shows you the tropical reading you would have had, and explains what each of the two signs says about that particular planet. You are not left holding a chart full of unfamiliar signs with no way to connect it to the one you already know.

Sidereal and Tropical: What Actually Differs

Both zodiacs describe the same sky. They disagree about where to start counting.

The tropical zodiac starts at the spring equinox, so a tropical sign tells you where a planet sits in the cycle of the seasons. This is the zodiac behind almost every horoscope you have read.

The sidereal zodiac starts from the fixed stars, so a sidereal sign tells you which constellation a planet actually stood against at the moment you were born.

The two lined up around seventeen hundred years ago. Since then the equinox has drifted backwards against the stars at roughly fifty seconds of arc a year, a movement called precession, and the gap has opened to about twenty-four and a half degrees. Neither zodiac is the corrected version of the other. They measure different things, and both have long working traditions behind them.

What Changes in Your Chart, and What Does Not

This is the part most explanations skip, and it decides how much of your chart you actually need to re-learn.

Your houses do not change. A planet in your eleventh house is in your eleventh house in both zodiacs. The house cusps shift backwards by exactly the same amount the planets do, so nothing crosses from one house into another.

Your aspects do not change. An aspect is the angular distance between two planets. Both planets move by the same amount, so that distance is untouched. Every conjunction, square, trine and opposition survives at exactly the same orb.

The shape of your chart does not change. The wheel looks the same. A stellium is still a stellium, a grand trine is still a grand trine, and the emphasis by hemisphere and quadrant is identical.

What changes is the sign. Because the gap is about twenty-four and a half degrees, a planet sitting in the first twenty-four degrees of a tropical sign lands in the sign before it. A planet in the last few degrees stays where it is. On a typical chart most placements move and two or three do not.

Anything read from the sign moves with it. That includes the sign interpretation, the rulership of each house, and the traditional dignities, so a planet in its own sign tropically may not be in its own sign here. The report shows you every one of these changes as it happens.

Which Sidereal Sign Am I?

The quickest way to know is to run the chart, but the rule is simple enough to check in your head. Find the degree of your tropical Sun. If it falls in the first twenty-four degrees of its sign, your sidereal Sun is one sign earlier. If it falls in the last six, it stays put.

A Sun at ten degrees of Leo reads as Cancer here. A Sun at twenty-eight degrees of Leo remains Leo. Anyone born within a few days of a sign boundary is the interesting case, and the report gives the exact degree so there is no guessing.

The same rule applies to your Moon, your rising sign and everything else in the chart. The rising sign is usually the change people notice first, because it is the one quoted alongside the Sun.

The Comparison, Placement by Placement

The report opens with a table listing only the placements whose sign has changed. Anything that stayed put is left out, because a line telling you nothing happened is a line in the way.

Underneath each of those placements, beside its interpretation, sits a short note reading it both ways. Not a general statement about zodiacs, but what those two specific signs mean for that specific planet. What Scorpio asks of Mars is not what it asks of Venus, and the note says so.

The point is to let you hold both readings at once and decide for yourself which one describes you. That is a judgement no calculator can make on your behalf, which is why the comparison is built into the report and not left as an exercise.

The Ayanamsa and the Synetic Vernal Point

The gap between the two zodiacs is called the ayanamsa. Different schools measure it slightly differently, and this calculator uses Fagan-Bradley, the standard in Western sidereal astrology, named for Cyril Fagan and Donald Bradley.

Your report prints the exact ayanamsa for your birth date, since it widens every year. It also prints the Synetic Vernal Point, the same figure stated the other way round: the position in the tropical zodiac where sidereal zero degrees Aries currently falls. Sidereal astrologers usually quote the SVP, and the report explains what it is and why it moves.

Houses Are Calculated in Campanus

The report defaults to the Campanus house system, which Western sidereal astrologers have generally preferred for the clean geometry of how it divides the sky. If you would rather read your chart in Placidus, Whole Signs, Koch, Regiomontanus or any of the others, you can switch from the settings on the report itself and it recalculates immediately.

Changing the house system does move planets between houses, which is worth knowing, because it is one of the few settings on the page that genuinely alters the reading.

What Your Report Includes

• A full sidereal chart wheel with the aspect grid and declination graph
• Every planet, the angles, Chiron, Black Moon Lilith, the lunar nodes and four asteroids, by sign, degree and house
• A written interpretation for every placement, by sign and by house
• A comparison table of every placement whose sign differs from the tropical reading
• A note under each of those explaining what the two zodiacs each say about that planet
• The ayanamsa and the Synetic Vernal Point for your birth date, both explained
• Aspects, chart patterns, elements, qualities, hemisphere and quadrant emphasis
• Your chart ruler, house rulers and traditional dignities
• A link to read the same birth data as a tropical chart

Frequently Asked Questions

Read alongside your Natal Chart Report, which covers the same birth data in the tropical zodiac.