What Is a Planetary Ephemeris?

An ephemeris connects a moment in time with the calculated position of a celestial body. A traditional printed ephemeris lays out many moments in rows and columns. ClockAstronomical takes a different approach: it turns one selected UTC instant into a wheel and a compact table, then recalculates the display as you move through time.

That makes ClockAstronomical useful as a planetary ephemeris online, but with an important qualification. It is an interactive, single-instant visualization, not an official or certified ephemeris, a downloadable data set, or a calendar of astronomical events. The JPL Horizons system is an example of a customizable professional ephemeris service.

What an ephemeris tells you

The U.S. Naval Observatory describes an ephemeris as a table or data file of coordinates over time, or an algorithm that can generate such a table. An ephemeris must specify more than a body name and a number. The instant, observer, coordinate system, reference plane, and corrections all affect what the number means. JPL’s Horizons manual distinguishes Earth-centered, Sun-centered, fixed-epoch, and of-date frames.

ClockAstronomical uses one clearly defined view:

• the observer is the center of Earth, so the results are geocentric;
• the coordinate is apparent ecliptic longitude of date;
• the zodiac is tropical, with 0° Aries at the equinox-based origin;
• longitude increases from 0° up to, but not including, 360°;
• the selected date and time are interpreted as UTC.

Longitude is only one angular coordinate. It omits ecliptic latitude and distance, so the wheel is a one-dimensional projection rather than a complete three-dimensional location.

Those choices are why zodiac planetary positions from ClockAstronomical should not be compared directly with a heliocentric orbit diagram, a location-based sky map, or a sidereal table. They answer different questions.

From longitude to sign and degree

The twelve signs divide the 360° circle into equal 30° sectors. Converting a normalized longitude is straightforward:

1. Divide the longitude by 30 and take the whole-number part to identify the sign, starting with Aries as zero.
2. Subtract the sign’s starting longitude to get the degree within that sign.

For example, 23.21° is 23.21° Aries. A longitude of 247.58° is 7.58° Sagittarius because Sagittarius begins at 240°. A longitude of 300.48° is 0.48° Aquarius because Aquarius begins at 300°.

This is the bridge between planetary positions with longitude and planetary positions by sign and degree. The sign is not an extra value looked up from another service; it is a label for one 30° interval of the same longitude.

A worked ClockAstronomical snapshot

The table below reproduces ClockAstronomical’s displayed output for 21 December 2020 at 18:00:00 UTC, generated at ClockAstronomical commit 6c4fcf5 with Astronomy Engine 2.1.19. It is not a claim of independent certification. NASA documents 21 December as the date of the unusually close 2020 Jupiter–Saturn conjunction. ClockAstronomical places both markers at nearly the same longitude, but it does not detect or label conjunctions.

Body	Sign	Degree	Longitude	Retrograde
Sun	Capricorn	0.34°	270.34°	—
Moon	Pisces	27.71°	357.71°	—
Mercury	Capricorn	1.25°	271.25°	—
Venus	Sagittarius	7.58°	247.58°	—
Mars	Aries	23.21°	23.21°	—
Jupiter	Aquarius	0.48°	300.48°	—
Saturn	Aquarius	0.48°	300.48°	—
Uranus	Taurus	6.96°	36.96°	R
Neptune	Pisces	18.32°	348.32°	—
Pluto	Capricorn	23.86°	293.86°	—

The Sun and Moon are included because people commonly want Sun, Moon, and planetary positions in one view. More precisely, ClockAstronomical displays the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Thus search shorthand such as Sun-through-Pluto positions or Sun, Moon, Mercury, Venus, Mars positions describes display groupings, not a scientific reclassification of the Sun, Moon, or Pluto as major planets.

The two decimal places make the table readable; they do not promise 0.01° accuracy.

Reading the different chart labels

Several search phrases describe overlapping ways of presenting the same underlying fields:

• A planetary position chart or planet position chart may be a wheel, table, or both. ClockAstronomical provides both for one instant.
• A planetary positions chart online emphasizes that the calculation is available in a browser. ClockAstronomical performs its calculations locally rather than fetching a precomputed table.
• Planet positions in zodiac signs, planet positions with degrees, and planetary positions with degrees refer to progressively more detailed readings of the same result.
• A planetary sign and degree calculator or a view of planets by zodiac sign and degree converts each 0–360° longitude into its 30° sign sector.
• Planetary longitudes by date and planet degrees by date focus on the numeric table instead of the wheel.

An astrology ephemeris calculator often presents astronomical coordinates using zodiac vocabulary. ClockAstronomical can show planetary positions in the zodiac, but it does not add houses, aspects, an ascendant, predictions, or interpretations. Similarly, the phrases tropical planetary positions and western astrology planetary positions describe the coordinate convention used here, not advice about what a position means.

How ClockAstronomical calculates the snapshot

ClockAstronomical uses the open-source Astronomy Engine library. For each displayed body, the application obtains a light-time- and aberration-corrected geocentric vector, rotates it into true ecliptic-of-date coordinates, takes the longitude, and normalizes it into the range from 0° to less than 360°. The wheel and table are two renderings of that same result.

The interactive planetary ephemeris then compares the body’s longitude half a day before and half a day after the selected instant. A decreasing longitude produces an R marker for Mercury through Pluto. This is a before-and-after numerical classification, not an exact station finder; the retrograde guide explains the boundary case.

ClockAstronomical accepts supported UTC instants from AD 1000 through 3000. Its stated 0.1° accuracy target applies only from 1900 through 2100. Results outside that target range remain available but are visibly marked Extended / approximate. Displayed output is not guaranteed or certified reference data, especially over the extended range.

Use the right view for the question

Use the date-and-time guide when you have a particular UTC instant. Use the interactive zodiac wheel guide when you want to watch the displayed positions change. If two tools disagree, first check their time zone, observer, tropical or sidereal setting, and reference frame before treating the difference as an error.

Primary sources and methodology

• JPL Horizons system manual for ephemeris scope, reference frames, and observer-centered ecliptic coordinates.
• U.S. Naval Observatory: ephemerides and reference systems for the table/file/algorithm definition.
• Astronomy Engine project documentation for the calculation library and its validation methodology.
• NASA: The Great Conjunction of Jupiter and Saturn for the context of the dated example.