Time is the hard part

Walk a birth time from a hospital clock to the timescale the planets are computed in, and name the three questions to ask of any recorded time.

Lesson 9 of 10 in Astronomy for Astrologers · free to read · no account, no email

Your clock is a political object

The positions in a chart are astronomy. The time you feed in is history, law and paperwork, and that is where nearly all real-world error lives.

A birth time is recorded in local civil time: an offset from Greenwich that a government chose, possibly shifted by an hour for daylight saving that a parliament voted for. Neither of those is a fact about the sky. Standard time zones only began spreading in the 1880s; before that, towns kept their own local mean time, set by their own meridian, and a birth in 1875 may be recorded in a time that differed from its neighbour city by minutes.

And the rules belong to the place, not to the country as it exists now. Borders moved. Zones were redrawn, sometimes several times in a decade, sometimes for a war. Half-hour and even three-quarter-hour offsets exist and have existed. So the question is never “what is the offset for this country” but “what offset was actually in force at this spot, on this date”.

Umbriel resolves that from the historical zone record for the location rather than from the country's current rule, because those two answers disagree for a very large number of twentieth-century births.

Two timescales, and why they are not one

The last step of that ladder is the one almost nobody is taught, and it is the one that separates a careful engine from a plausible one.

Universal Time is the Earth's rotation. It is what your Midheaven and Ascendant are computed from, because those depend on how far round the planet has turned.

Terrestrial Time is uniform — a steady tick, no wobble. Planetary theories are written in it, because gravity does not care what the Earth's crust is doing.

The Earth's rotation is not steady. It is slowing, irregularly, as tides drag on it and as mass shifts inside it. The accumulated difference between the two timescales is called ΔT, and it is measured, not modelled: about 69 seconds today, near zero around 1900, roughly 26 minutes in the year 1000, and something like seven hours three thousand years ago.

Now notice what an error in ΔT does, because it is counter-intuitive. It moves the planets, because they are computed from TT, and leaves the houses alone, because those come from UT. So a mishandled ΔT does not rotate the chart — it slides the planets through it, and the symptom is planets sitting in the wrong rooms. Seven hours puts the Moon nearly four degrees out. For an ancient chart that is the difference between two signs.

Three questions to ask of any birth time

You will be handed times by people who are guessing, by certificates that were filled in later, and by relatives who remember a season rather than an hour. Ask these three, every time.

What zone was actually in force there, then? Not the country's current rule. Wartime offsets, double summer time, occupations and redrawn borders all show up in twentieth-century birth data, and a missed one is a whole hour or two.

Was daylight saving on, and was it applied? This is the single most common error in amateur charts. One hour is about fifteen degrees of Ascendant — half a sign, and frequently one house. Worse, some certificates record the standard-time equivalent and some record what the clock said, and the two are an hour apart with nothing to distinguish them.

Is this the birth, or the paperwork? Times get rounded to the nearest five, ten or fifteen minutes as a matter of habit; some are written down at the end of a shift. A time ending in a zero or a five deserves a raised eyebrow and a range.

None of this is pedantry, because the previous lesson gave you the exchange rate: four minutes is a degree. An hour of unnoticed daylight saving is fifteen degrees of everything that depends on the horizon. The professional answer is not to be anxious about it; it is to know the size of the doubt, say it, and read what survives it.

What that looks like

Britain, summer 1942. Double summer time was in force: clocks were two hours ahead of Greenwich rather than the usual one. A birth recorded at 4pm is really 2pm UT. An astrologer who assumes a single hour of summer time gets the Ascendant thirty degrees out — a whole sign — and gets the Midheaven wrong with it. Every planet is in the right sign, the whole reading is plausible, and every house placement is wrong. This is why the zone history, not the country, is the thing to look up.

What to take away

• A recorded birth time is civil time — a government's offset, possibly plus daylight saving — and the offset belongs to the place on that date, not to the country now.
• Universal Time is the Earth's rotation and sets the angles; Terrestrial Time is uniform and the planets are computed from it.
• ΔT is the measured gap between them: ~69 seconds now, ~26 minutes in the year 1000. An error in it slides the planets through the houses without rotating the chart.
• Ask three things of every birth time: what zone was in force, whether daylight saving applied, and whether the time is the birth or the paperwork.

The common mistake

Looking up a country's present time zone and applying it to a historical birth. Zones were redrawn, wars added hours, and daylight saving rules were local and erratic — and the error is not subtle: one unnoticed hour is about half a sign of Ascendant.

Try it on your own chart

Find out what your own birth location's offset actually was on your birth date, and whether daylight saving was in force. Then cast your chart with the hour shifted by one and compare: note which planets change house. That set of planets is exactly what a missed daylight-saving hour would have cost you, and it is usually more than people expect.

Check yourself

The same questions the lesson asks in the app. Open one to see the answer.

Keep going

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