FREE Birth Chart Calculator
An astrology calendar is arithmetic first: where a celestial body is, at what angle to another, and at which second that angle goes exact. Arithmetic can be checked, and we think you should be able to check ours. Below is exactly what everything in this section is computed with, what each term in the tables means, and how to compare our figures against independent sources.
What the calculations are based on
We use an open astronomical calculation: the method can be verified independently.
The numerical ephemerides from NASA's Jet Propulsion Laboratory – the same source used to navigate interplanetary spacecraft. Positions come from numerical integration, not analytic series.
Measured from the spring equinox point. Longitudes are apparent, with aberration and nutation applied. The nodes are true, not mean: they oscillate, and we show the oscillation rather than a smoothed curve. The sidereal (Lahiri) zodiac is available through a switch on the ephemeris pages.
We show Eastern Time by default. You can switch it: in another time zone an event sometimes falls on the neighbouring day.
The moment of each event is refined step by step down to the second.
A century and a half is computed in advance and stored as finished files. A rebuild always covers everything, so no stretch of the calendar can be left on stale values.
For a birth we apply whatever time zone the chosen city was actually on for that date.
Why this matters. An hour of difference is about half a degree of Moon. Near a sign boundary that is enough to move the Moon into the neighbouring sign. Take someone born at 2:30 a.m. on June 15, 1970, in Indianapolis: back then Indiana kept standard time all summer, so the correct hour is not the one today's rule would give you. That is why the forms in this section ask for a city and not for your time zone: we work the zone out ourselves, for the date that matters.
How to check it
Below are reference events and independent sources to compare against.
| Event | Our figure (UT) | Our figure (ET) | Where to check |
|---|---|---|---|
| Total solar eclipse, August 11, 1999 The last total eclipse of the 20th century, watched across Europe. Listed in the NASA eclipse catalog. | 11.08.1999 11:03:05 | 11.08.1999 07:03:05 | NASA eclipse catalog, Astrodienst |
| Total solar eclipse, August 21, 2017 The Great American Eclipse – the most thoroughly documented eclipse of modern times. | 21.08.2017 18:25:31 | 21.08.2017 14:25:31 | NASA eclipse catalog, Astrodienst |
| Jupiter–Saturn conjunction, December 21, 2020 The Great Conjunction, the closest since 1623. Every observatory published it. | 21.12.2020 18:20:37 | 21.12.2020 13:20:37 | NASA eclipse catalog, Astrodienst |
| Spring equinox 2026 The moment the Sun enters 0° Aries. Published by the time services. | 20.03.2026 14:45:58 | 20.03.2026 10:45:58 | NASA eclipse catalog, Astrodienst |
If you check and see a difference of several hours, it is almost certainly the time zone: most services publish in UT. A difference of tens of seconds on an eclipse is normal: different sources anchor the time to a different moment – greatest eclipse, greatest duration on the central line, or maximum for one specific place on Earth. A difference of more than an arcminute in the actual degrees, though, is worth flagging to us – that should not happen.
What the terms mean
Aspect and orb
An aspect is an angle between two bodies that counts as significant. Major: conjunction 0°, sextile 60°, square 90°, trine 120°, opposition 180°. Minor: 30°, 45°, 135°, 150°.
The calendar gives the moment the angle goes exact – that is, when the orb is zero. We deliberately don't publish lines like "this aspect is in effect from the 3rd to the 7th": orbs differ between schools, and that is interpretation rather than calculation. For personal transits we allow orbs of up to 2°.
Ingress – entering a sign
The moment a body's longitude crosses a sign boundary: 0°, 30°, 60° and so on. A retrograde planet can cross the same boundary several times – the calendar shows every crossing, not only the first.
Station and retrograde motion
A retrograde is an optical effect: from Earth a planet appears to move backward through the zodiac. A station is the moment apparent speed passes through zero. We take stations from the speed calculation, not from rounded daily tables, which is why the time is given to the minute.
Shadow period
Before it turns retrograde, a planet moves direct through the stretch of zodiac it will later retrace – that is the shadow before. After it turns direct again, it covers the same stretch a second time – the shadow after. The boundaries come from the station degrees: the shadow before begins at the degree where the planet will station a second time, the first time it crosses that point.
For shadow boundaries we give the time to the hour: the underlying positions don't let us promise better than that.
Void of Course Moon
The gap between the Moon's last exact aspect to a planet in its current sign and its entry into the next one. During that gap the Moon is attached to nothing – hence the practical rule: not the best time to start things.
Note that different schools include a different set of bodies and aspects. We take the Moon's major aspects to the bodies from the Sun to Pluto – the most common convention. That is exactly why sites and apps give different windows: it is not an error, it is a different definition.
Eclipse: magnitude and saros
Magnitude is the fraction of the disc covered at maximum. Below one means partial or annular, above one means total. The saros is the number of a series of eclipses that repeats every 18 years, 11 days and 8 hours.
Ayanamsa and the sidereal zodiac
The ayanamsa is the gap between the tropical and sidereal zodiacs, accumulated through precession. The ephemeris pages offer the Lahiri sidereal variant, the one most used in jyotish.
Why sites disagree about dates
Three reasons, in order of how often they come up:
- Time zone. A gap of exactly 4 or 5 hours means UT versus Eastern Time. A night-time event moves to the neighbouring day.
- Different definitions. Void of course, orbs, mean node versus true node – the same arithmetic, different conventions.
- Rounding. Calendars assembled from ready-made daily tables are off by tens of minutes on the fast bodies.
Data for independent checks
You can download the data for the nearest years for your own personal checks – for example,
/astrology/data/2026-08.json.
The file holds the planetary positions and the month's events with exact times.
Earlier years live in the calendar archive.
Copying, publication, and other reuse are governed by the
astronomical data usage terms.
What is not here, and why
Houses and the Ascendant in the calendar
Houses depend on the place and the exact minute of birth, so a calendar cannot have them. On this site they show up where they belong – in the birth chart.
Predictions in the tables
The calendar gives the exact moment of an event, and the note beside it helps you understand what it means astrologically. Forecasts live on the horoscopes pages, and they carry a byline.
Precision we don't have
Exact aspects are given to the second; shadow boundaries to the hour. The precision is stated next to each type of event.