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How we calculate the sky and your chart

Every position on Fatesight is calculated from astronomy for the moment you see it or the moment you were born, never drawn by hand or copied from a table. This page says how, and how we check it, at the end, has our test results and checks you can repeat yourself.

What is calculated and what is interpreted

Calculated: every planet's position, sign change and retrograde date, the Moon's phases, eclipses, rising signs, houses and Saturn return dates. These are astronomy. The code gives one answer for a given moment and place, and you can check it against other people's tables.

Interpreted: what a placement, a card or a number means. Those readings follow named traditions, from Ptolemy's Tetrabiblos to Waite's tarot, and each page's "How this page is made" box says which. Tarot cards are drawn for the day and your question, so the same question on the same day draws the same cards.

The library

We compute where the Sun, the Moon and the planets are with astronomy-engine, an open-source astronomy library that works out each body's position from its orbit. Its code is public under the MIT licence, so anyone can read and check it. The same library calculates our pages, the free calculators, your birth chart and your readings, so a position is the same wherever you see it.

Time

The planets keep to a steady astronomical clock, while our clocks follow the Earth's slightly uneven turning. We convert clock time (UTC) to astronomical time with delta T, the gap between the two, as NASA data gives it. A birth time is turned into UTC with the time zone rules of its place on its date, from the time zone database computers use worldwide.

Positions and signs

A position is where a body appears from the centre of the Earth along the ecliptic, the Sun's path through the sky, allowing for the time its light takes to reach us. The signs are the tropical zodiac's: twelve slices of 30° counted from the point where the Sun crosses the celestial equator in March. So the Sun in Aries means the Sun is in the first 30° from that point, whatever stars stand behind it.

Moon phases

The phase comes from the angle between the Moon and the Sun as seen from Earth, so the share of the Moon that's lit, and tonight's phase, are exact for the moment the page loads. The new, first quarter, full and last quarter Moons are found to the minute and given in UTC, with your own clock beside them where your browser allows.

Sign changes, retrogrades and eclipses

To find when a planet enters a sign or turns retrograde or direct, we take its position day by day, then narrow each change down by halving the interval until it's found to the hour or better. Eclipses come from the library's own eclipse search.

Your sign and your birth chart

Your Sun sign is the zodiac sign the Sun was in on your birthday; a birthday on the boundary of two signs is shown as a cusp. Your birth chart uses your birth date, time and place: the planets' positions in the twelve signs, your rising sign (the sign rising on the eastern horizon) and the twelve houses. Houses use the Placidus system, the most widely used in Western astrology. Very far north or south, where Placidus can't be calculated, we use the Porphyry system, as Swiss Ephemeris does, and say so.

Chinese New Year

The Chinese zodiac's year starts at Chinese New Year, between January 21 and February 20. We calculate it by the calendar's own rule, from the new Moons and the Sun's position on Beijing's clock: the lunar month that holds the winter solstice is the eleventh, and the year begins two months later, with the calendar's rule for leap months. A test checks our dates against the published ones.

Known limits

• Your birth time. The rising sign moves through all twelve signs in a day, about one every two hours, so a birth time that's off can change your rising sign and your houses. Without a birth time we can't place them, and on a day the Moon changed sign we show both signs.
• Far north and south. Inside the polar circles, Placidus houses can't be drawn, so those charts use Porphyry's.
• Dates. Our free pages give their dates in UTC unless they show your own clock, so a change late in the evening in the Americas can fall on the next day.

What we read from them

The positions are measured; their meanings come from the astrological and palmistry traditions, from Ptolemy's Tetrabiblos onward. How it works has the rest, and our editorial standards say how our pages are made.

How we check it

We test our calculations against reference ephemerides, the tables of positions astronomers use, and these are the results. Differences are in arcseconds: an arcsecond is 1/3,600 of a degree, and the Moon's disc is about 1,900 arcseconds across.

Against NASA data

Spot checks against the positions NASA publishes for the Sun, the Moon and the planets (values retrieved September 28, 2026):

Against Swiss Ephemeris and NASA data

The Sun, the Moon and the planets in 300 birth charts from 1900 to 2030, at places all over the world, compared with Swiss Ephemeris and with NASA data:

Calculated with astronomy-engine 2.1.19. Our test suite works these figures out again from the reference values on every change, and fails if any position goes past its limit. This table was last updated on October 25, 2026.

Checks you can repeat

You don't have to take our word for any of this. Here are our dates beside tables anyone can open. Ours are worked out by the same code that draws our pages, and our tests fail if the two ever drift more than two minutes apart.

Eclipses, against NASA's tables

Open NASA's solar eclipse table and lunar eclipse table and find these dates. NASA lists the greatest eclipse in dynamical time (TD), which runs about 70 seconds ahead of the UTC we show.

Equinoxes and solstices, against the US Naval Observatory

Open the Naval Observatory's table of seasons for 2027. These are the moments the Sun enters Aries, Cancer, Libra and Capricorn.

Moon phases, against the US Naval Observatory

Open the Naval Observatory's table of Moon phases for 2027. These are the first eight of the year.