How the calculations work

Planet positions

The Sun, Moon and planets come from astronomy-engine, an open-source astronomy library (MIT licence). For each body we take its apparent position as seen from the centre of the Earth, measured along the ecliptic of date: the same quantity an ephemeris lists. In our tests it agrees with NASA JPL Horizons to within 12 arcseconds. See the accuracy page.

Signs and degrees

We use the tropical zodiac, which most Western astrologers use: 0° Aries is the March equinox point, and each sign spans exactly 30°. Your sign is the longitude divided by 30; the remainder is your degree within the sign. Minutes are cut off rather than rounded, as chart software prints them, so 28.49° shows as 28°29′, never 28°30′.

Vedic (sidereal) astrology measures from the stars instead, currently about 24° behind the tropical zodiac. A sidereal calculator will often give the sign before ours. Neither is a mistake; they are different systems.

Your birth time and time zone

Birth places come from GeoNames (CC BY 4.0), each with its official time zone. Your browser’s built-in time-zone database then gives the offset from UTC that applied at your birth place on your birth date, including daylight saving and historical changes. Two clock times need a rule:

• A time that never happened, because the clocks jumped forward, is read as standard time.
• A time that happened twice, because the clocks went back, is read as the first one (still on daylight time).

The result tells you when either rule was used. If your town is not in the list, you can enter the UTC offset yourself.

Unknown birth time

Without a birth time we calculate the whole day, from 00:00 to 23:59 at your birth place. If the body stayed in one sign, that is your sign. If it changed sign, you see both signs and the exact time of the change. With no birth place either, the window covers your date in every time zone on Earth. More on unknown birth times.

Cusp warnings

When a result is within 1° of the next or previous sign, it carries a cusp warning that says how much your birth time would have to change to move it across. For the Moon that is about two hours per degree; for slow planets the birth time cannot change the sign at all.

Rising sign and midheaven

From your birth instant and longitude we get local sidereal time, which gives the right ascension of the midheaven. Projecting that onto the ecliptic with the true tilt of the Earth’s axis gives the midheaven; adding your latitude gives the ascendant, the point of the ecliptic rising in the east. Inside the polar circles the ascendant can jump across signs, and the result says so.

North node and Lilith

The north node is the mean lunar node and Black Moon Lilith is the mean lunar apogee, both from the standard series in Jean Meeus’s Astronomical Algorithms. The “true” node can differ from the mean by up to about 1.5°, and “true” Lilith by up to about 30°, so check which one another site uses before comparing.

Saturn return

We find Saturn’s longitude at your birth, then step day by day through the five years around ages 29.5, 58.9 and 88.4 to find every moment Saturn is back at exactly that longitude, and narrow each to the minute. A retrograde loop near your natal degree gives three exact hits; all are listed.

Chinese zodiac

Each lunar new year is the new moon, in China Standard Time (UTC+8), that falls between 21 January and 20 February. We compute those dates for 1900–2100 and check them against a published list. The animal follows a 12-year cycle and the element a 10-year cycle, both counted from 1984, a Wood Rat year.

Sources and credits

• Calculations: astronomy-engine by Don Cross (MIT licence).
• Reference positions: NASA JPL Horizons.
• Node and Lilith series: Jean Meeus, Astronomical Algorithms, 2nd edition.
• Places and time zones: GeoNames (CC BY 4.0), cities with a population of 15,000 or more.
• Photography: NASA (Mariner 10 and Cassini frames) and Unsplash contributors via the Unsplash License.