Five systems read your birthday. Three of them are the same number.
A sun sign from Greece, a day pillar from China, a day name from the Maya count, a planetary day from Babylon, a life path from modern numerology. They look like five independent opinions about you. Below is the arithmetic that shows what each one is actually counting — and where that leaves the disagreements between them.
What the five of them say
| System | Your day | What it is counting | Repeats every |
|---|
Nothing in that table is looked up in a database of birthdays. Four of the five are arithmetic on the date itself, shown in full further down. The sun sign is the exception: it comes from the conventional table of cusp dates, the same one every horoscope column uses. So we also print where the sun actually was that day, and say whether the two agree.
Three of them are one number
Astronomers count days in one unbroken run from a fixed starting point, with no months and no years in the way. That count for your birthday is:
—Julian day number
Which is why they come back together
Divide one number by 7, by 60 and by 260 and the three answers must line up again on a fixed schedule — the first day divisible by all three, which is —, or —. These are the next times your exact combination of weekday, day pillar and day name returns.
| Date | Age | Same three | Sun sign that day |
|---|
And the one that reads your spelling
A life path number is the digits of the written date added up. That makes it the only reading here keyed to how the day is written rather than to the day itself. Write the same birthday in the Julian calendar — the one Europe used until 1582, and Russia until 1918 — and it is the same day, the same sunrise, the same everything:
How each figure is worked out
- Julian day number. The standard integer day count, the same one astronomers use. Every figure below except the sun sign is arithmetic on it, so it is printed rather than hidden.
- Planetary day. (JDN + 1) mod 7, counting 0 as Sunday. Julian day 0 was a Monday, which is where the +1 comes from; the planet is the classical name that weekday still carries in the Romance languages.
- Day pillar. Stem (JDN + 9) mod 10, branch (JDN + 1) mod 12 — together a 60-day cycle. Checked against 1 October 1949, a recorded Jia-Zi day.
- Tzolk’in. Number ((JDN + 5) mod 13) + 1, name (JDN + 16) mod 20, on the Goodman-Martinez-Thompson correlation that anchors 4 Ahau 8 Cumku to Julian day 584283. Other correlations exist and would shift the name; this is the one in general use.
- Sun sign. From the conventional table of cusp dates — the one printed in every horoscope column, where Aries starts on 21 March every year. The sun does not keep to that schedule, so we also work out its true longitude for the date from the standard low-precision solar formula and say when the table has put a cusp birthday on the wrong side.
- Life path. Year, month and day each reduced by adding digits until one remains, then those three added and reduced again. Master numbers 11, 22 and 33 are not held back, because the schools disagree about when they are and picking one silently would hide a real dispute.