⊞ Free Speculum & Ascensional Table Calculator
Umbriel builds the speculum — the table of declinations, semi-arcs, ascensional differences and oblique ascensions that traditional predictive work is done from. Free, and it leaves a cell blank when the honest answer is blank.
- What you'll needBirth date, time and place
- What you getThe full speculum for your chart: declination, ascensional difference, diurnal and nocturnal semi-arc, oblique ascension and more
- CostFree — no account required
Runs in your browser. Nothing to install, nothing to set up.
What it actually is
A speculum is the working table behind traditional predictive astrology. Before you can direct anything you need each planet's declination, its ascensional difference, its semi-arcs and its oblique ascension — and historically an astrologer computed those by hand before beginning.
It is unglamorous and it is where the actual work happens. Primary directions, the traditional houses and much of the older literature all assume a speculum has already been built; without it, most of that material is unreadable.
⚠️ Some cells are legitimately empty, and this is the part that catches software out. A body that never rises or never sets at your latitude is circumpolar and has no semi-arc — there is no answer, and a table that prints a number there has fabricated one. Umbriel leaves those blank and says why.
This only shows up at high latitudes. Nothing at a mid-northern latitude is ever circumpolar, so a chart from London or New York will never exercise the case — which is exactly why it is worth knowing the tool handles it.
How Umbriel computes it
Each body's ascensional difference, semi-arc and oblique ascension are computed from its declination and your birth latitude. ⚠️ Circumpolar bodies — those that never rise or never set at your latitude — have no semi-arc, and the table leaves those cells EMPTY rather than filling them with a number. Blank is the correct answer there, and a table that always has a figure in every cell is a table that is making some of them up. Nothing at a mid-northern latitude is ever circumpolar, so this can only be seen by testing a chart far enough north.
What's in your result
- DeclinationNorth or south of the celestial equator, for every body.
- Ascensional differenceHow far the body's rising differs from the equator's.
- Diurnal and nocturnal semi-arcThe halves of its day and night arcs — blank when circumpolar.
- Oblique ascension and descensionThe values traditional house and direction work is built on.
- Right ascension and meridian distanceThe remaining columns a full speculum carries.
Common questions
What is a speculum in astrology?
The table of positional values — declination, ascensional difference, semi-arcs, oblique ascension — that traditional predictive techniques are computed from. It is the working sheet behind primary directions.
What is a semi-arc?
Half the arc a body travels between rising and setting (the diurnal semi-arc) or between setting and rising (the nocturnal one). It is the basic unit of traditional timing.
Why is a cell in my speculum empty?
Because the body is circumpolar at your latitude — it never rises or never sets, so it has no semi-arc. Blank is the correct answer, and a table that fills it with a number is inventing one.
Do I need a birth time for a speculum?
Yes. Almost every column depends on the relationship between the bodies and your local horizon, which is what a birth time and place establish.
Do I need a speculum if I am not doing primary directions?
Probably not. It is a working table rather than a reading, and most people meet it only when a traditional text assumes one. It is here because the older material is unreadable without it.
Is the speculum calculator free?
Yes — the full table, with circumpolar cases handled honestly, and no account.
Go deeper
Each of these is a free tool in its own right.
Umbriel offers every system here as a set of prompts to think with, never as a statement of fact about who you are. Take what's useful, leave the rest.