Two viewpoints on one sky
A geocentric planetary position describes a line from the centre of Earth to a body. A topocentric position begins at an observer on Earth’s surface. Both may be expressed as zodiac longitude and ecliptic latitude, but the two lines are not generally parallel. The effect is parallax: a nearby object changes apparent direction when the viewpoint changes. This tool compares ten planetary body centres at the same UTC instant, using the same tropical ecliptic of date for both columns.
Location does not alter the Earth-centred column. It does change the surface-observer column. The longitude shift may be positive or negative, because the observer’s relation to the body changes as Earth rotates. A topocentric Moon can therefore be east or west of its geocentric degree at different times or places. A small longitude shift does not mean the whole positional difference is zero: ecliptic latitude is shown separately.
Why the Moon is the useful test case
The Moon is much closer to Earth than the Sun or planets, so moving the starting point from Earth’s centre to its surface changes the Moon’s apparent direction most strongly. At the editable Manila example, the calculator gives both longitudes and their signed arcminute difference. Change the location while keeping the UTC instant fixed to see the geocentric Moon remain in place as the observer-centred Moon moves. The exact size and sign belong to that particular time and location; one fixed “Moon correction” cannot be applied to every chart.
Calculation sequence
The entered civil date and clock time are converted to UTC by subtracting the supplied offset in hours east of Greenwich. Astronomy Engine then computes an apparent Earth-centred EQJ vector for each body. It separately computes an observer-centred EQJ vector for the entered latitude, longitude, and elevation. Each vector is converted to true ecliptic coordinates of date; the displayed tropical longitudes share the same equinox, so the subtraction compares viewpoints rather than mixing epochs. The difference is wrapped into the range from −180° to +180° and displayed in arcminutes.
Birth-time and place limits
An exact natal comparison requires the date, a recorded local clock time, a historical UTC offset, and coordinates. The Moon and the rotating observer both change position while an uncertain hour passes. Entering an arbitrary noon and reading its Moon shift as the birth value would give false precision. If only a date is known, use a chosen moment as an illustration and wait for a reliable birth record before assigning natal degrees. Check the actual civil offset, including any daylight saving rule, for the birthplace and year. The optional elevation defaults to sea level and has a smaller effect than a mistaken clock hour or location.
How to compare other software
Compare the same UTC instant, place, zodiac, ecliptic frame, and apparent versus geometric correction settings. Astrology applications may differ in ephemeris, nutation model, precision, or whether a Moon position is topocentric. This site uses the bundled Astronomy Engine. A table here should be read as a transparent independent calculation, not as a numerical reproduction of another service’s output. Symbolic interpretation of a shift is a separate astrological choice.