About the project
Planetary positions, made visible
ClockAstronomical is an interactive astronomy visualization. It turns a calculated planetary-position snapshot into a clock-like zodiac wheel, then lets you pause, rewind, accelerate, or jump directly to a selected UTC instant. The goal is to make angular position and apparent motion easier to inspect without adding predictions or interpretations.
- Viewpoint
- Earth-centred
- Zodiac
- Tropical
- Available dates
- AD 1000–3000
- Calculation
- In your browser
Purpose and scope
An ephemeris describes how the coordinates of celestial bodies vary with time. ClockAstronomical presents one calculated instant at a time and can animate successive instants, making the changing longitudes easier to explore than a static table alone.
ClockAstronomical does not provide horoscopes, forecasts, personality readings, compatibility analysis, houses, an ascendant, aspects, or advice. It reports a defined coordinate calculation and leaves its meaning there. The Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are collectively called “displayed bodies” when scientific classification matters.
What ClockAstronomical calculates
For each selected instant, ClockAstronomical uses the open-source Astronomy Engine 2.1.19 library to calculate a light-time- and aberration-corrected vector from the centre of Earth to each displayed body. It transforms that vector into the true ecliptic and equinox of the selected date, then reports the apparent geocentric tropical ecliptic longitude from 0° up to, but not including, 360°.
The wheel divides that longitude circle into twelve equal 30° tropical zodiac sectors, beginning with Aries at 0°. The degree table shows the same position as a sign, a degree within that sign, and a full longitude. Two displayed decimal places are a reading aid, not a claim of 0.01° accuracy.
For Mercury through Pluto, an R means longitude was decreasing across a numerical window from half a day before to half a day after the selected instant. The Sun and Moon are always shown without a retrograde label. This method classifies motion at the displayed instant; it does not solve the exact time of a station.
The wheel is not a local sky map. ClockAstronomical does not ask for your latitude, longitude, or elevation, and it does not show altitude, azimuth, rise or set times, distance, ecliptic latitude, or visibility from your location.
Dates, time, and calendar convention
ClockAstronomical accepts instants from 1 January AD 1000 at 00:00:00.000 UTC through 31 December AD 3000 at 23:59:59.999 UTC, inclusive. Manual entries are interpreted as UTC, and shared chart links preserve the selected timestamp in the URL.
The calendar is proleptic Gregorian: modern Gregorian calendar rules are extended backward before their historical introduction. A date copied from a Julian-calendar or other historical source may therefore need conversion before it represents the same physical day. The browser time model also does not represent leap-second instants.
Accuracy and JPL verification
Availability and accuracy are separate policies. ClockAstronomical maintains a 0.1° accuracy target from 1900 through 2100. Dates before 1900 or after 2100 remain available but are labelled Extended / approximate. The target is not a warranty, certification, or substitute for a professional ephemeris.
Before choosing the long-range window, the shipped calculation pipeline was compared with NASA/JPL Horizons DE441 observer output for ecliptic-of-date longitude from the geocentre. A terrestrial-time-matched grid sampled every 90 days from the start of AD 1000 through the start of AD 3101—8,527 samples for each displayed body. The largest angular difference in that grid was 0.0610°.
A separate same-calendar-label daily comparison for AD 3000 found its largest difference in the Moon, at 0.8209°. That larger value primarily illustrates how future civil-time and Earth-rotation assumptions can cause two systems to map the same written date and time to different physical instants. ClockAstronomical therefore stops at AD 3000 and does not promise a blanket one-degree tolerance outside 1900–2100.
Thousands of passing samples are useful evidence, but they do not prove a continuous maximum over two millennia. Small longitude differences can also change a sign, displayed degree, or retrograde label near a boundary. Use a suitably configured authoritative ephemeris whenever precision is consequential.
Privacy by design
Chart inputs and astronomical calculations run in the browser and are not sent to a ClockAstronomical application backend. The site has no account system, chart database, or profile. A theme preference can be stored on your device, and a share link places the selected timestamp, renderer, and zodiac mode in the URL you choose to copy.
Loading any website still requires requests to its host. ClockAstronomical also uses Google Analytics to measure visits and usage and currently loads its typefaces from Google Fonts. The complete current data-flow description is maintained in the Privacy Policy.
Sources, versions, and corrections
ClockAstronomical’s technical explanations are based on the calculation that ships in the site, its pinned Astronomy Engine dependency, and primary astronomical sources. Worked examples should identify the UTC instant and software version so the result can be reproduced.
Reproducing a discrepancy requires the complete timestamp, the other tool’s coordinate and time settings, and the values being compared. Confirmed errors found during testing and maintenance are corrected in the code or content. Material changes to dated policy pages receive a revised effective date. The FAQ covers common comparison and reading questions.