Topocentric Ephemeris
A Topocentric Ephemeris study generates tables of planetary positions as seen from a specific location on Earth's surface. Unlike the geocentric perspective, topocentric calculations account for parallax effects, which are especially significant for the Moon and other bodies close to Earth.
Creating a Topocentric Ephemeris Study
You can create a new Topocentric Ephemeris study in either of these ways:
- Click the Topocentric Ephemeris button on the toolbar
- Select Ephemerides > Topocentric Ephemeris from the menu
A new study appears in the studies list with its configuration panel displayed.
Configuration
The Topocentric Ephemeris shares the same configuration parameters as the Geocentric Ephemeris:
- Study Name, Location, Zodiac, Ref. Plane
- Start Date, Start Time, End Date
- Interval and Time Unit
- Bodies and Metrics lists
The key difference is that the coordinate origin is fixed to Topocentric. The selected location determines the exact observer position on Earth's surface for all calculations.
The location you select directly affects the computed positions. Select the geographic location that matches your intended observer position.
When to Use Topocentric
Topocentric positions differ from geocentric positions most noticeably for:
- The Moon -- Parallax can shift the Moon's apparent position by up to one degree
- Near-Earth bodies -- Asteroids and other objects close to Earth show measurable parallax
For distant planets, the difference between geocentric and topocentric positions is negligible.