Current snapshot
One timestamp and one set of geocentric tropical longitudes, not a future table.
Celestial coordinates · time series
Ephemeris
Understand ephemerides, ecliptic longitude, coordinate frames, time standards, retrograde notation, interpolation, and the limits of a current snapshot.
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Direct answer
An ephemeris is a table or service giving calculated celestial positions at specified times. To interpret one correctly, identify its time standard, coordinate origin, reference frame, observer location, zodiac or coordinate system, and precision.
Quick facts
Essential metadata
Time standard, coordinate system, reference frame, observer origin, and calculation method.
Retrograde
Apparent backward motion in a chosen coordinate as observed over time, not reversal of a planet’s orbit.
Verified current positions · refreshes hourly
The Active Transits Sky Map
Current geocentric longitudes. No local rise, set, altitude, direction, weather, or future visibility is inferred.
Angular comparisons use ecliptic longitude. They do not establish apparent separation, altitude, brightness, darkness, or local visibility.
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What an ephemeris contains
Traditional printed ephemerides list daily or periodic positions for the Sun, Moon, planets, and selected points. Digital services can calculate arbitrary times and more coordinates. Columns may include longitude, latitude, right ascension, declination, distance, speed, phase, rise/set, or events. Never assume two tables use the same definitions because their rows look similar.
Time and reference choices
UTC, Terrestrial Time, Julian dates, and other time standards serve different purposes. Positions can be geocentric, topocentric, or heliocentric; apparent or geometric; tropical or sidereal; equatorial or ecliptic. Small discrepancies may be legitimate consequences of these choices rather than errors. The metadata is part of the result.
Reading sign and degree
In a tropical longitude table, normalize the value from 0 to 360 degrees, divide it into twelve equal sectors, and express the remainder as degrees within the sign. For example, a longitude in the 90-120 degree sector is labeled Cancer. This notation does not state altitude, azimuth, distance, brightness, or physical location in a zodiac constellation.
Retrograde and stations
Retrograde notation means the displayed geocentric coordinate is decreasing over time. A station is the transition between direct and retrograde motion in that coordinate. Exact station times require comparing positions or velocities across time; a single snapshot can report the current flag but cannot calculate when the next station will occur.
Interpolation and precision
Fast lunar motion and nonlinear motion near stations make casual interpolation risky. Match the precision of the source and purpose, and do not add decimal places that the method cannot support. For observing, topocentric effects can matter; for future events, use a service designed to compute the event rather than projecting an hourly snapshot forward.
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Sources and live-data notes
- U.S. Naval Observatory: Astronomical Data ↗Authoritative astronomical data services and event tables.
- U.S. Naval Observatory: Rise, Set, and Twilight Definitions ↗Definitions for location-dependent rise, set, and twilight calculations.
- NASA: Skywatching ↗Current observing education and monthly highlights; visibility still depends on location, time, horizon, and weather.
- The Great Almanac live APISame-origin gateway for the validated hourly current-position snapshot; the upstream source is identified in the response metadata.
Continue through the Sky & Time Almanac
Cluster hubSky & Time AlmanacAll observation guides, live instruments, and source boundaries.
Observed sky · hourlyLive Sky TodaySee the exact Active Transits sky map from the webapp, using hourly geocentric longitudes for the Sun, Moon, planets, nodes, Chiron, and Lilith.
Observing calendar · verify locallyCelestial EventsLearn the major kinds of celestial events, how to verify an observing opportunity, and what a current longitude feed can and cannot establish.
Lunar almanac · observed nowMoon TodayRead the Moon’s current geocentric longitude, zodiac degree, approximate phase family, and the limits of non-location-aware lunar data.
Seasonal astronomy · one instantEquinoxes and SolsticesLearn what equinoxes and solstices measure, why their calendar dates shift, and why daylight and season do not behave identically everywhere.
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Common questions
Why do two ephemerides disagree slightly?
They may use different time standards, reference frames, observer origins, zodiac definitions, corrections, or numerical precision. Compare metadata first.
Does retrograde mean a planet reverses its orbit?
No. It is apparent backward motion in a coordinate as seen from the chosen observing frame.
Can I derive future positions from the current API response?
Not reliably. The endpoint is a verified current snapshot, not an orbital propagator or future ephemeris service.
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