Planetary Position Calculator
Note: this uses a simplified circular-orbit approximation, not a precision ephemeris.
Roughly Where a Planet Sits Along Its Orbit
Precision ephemerides — the tables NASA JPL Horizons or planetarium software rely on — account for orbital eccentricity, inclination, and gravitational perturbations from every other body in the solar system. This calculator does something simpler and more transparent: it treats each planet's orbit as a perfect circle traveled at constant angular speed, and reports how far around that circle the planet has moved since a fixed reference date. It's a mean-motion approximation, not a sky position, and it says so plainly in its own output.
The Formula
The J2000.0 epoch (January 1, 2000) is the standard reference point in modern astronomy; each planet's mean longitude at that moment, combined with its known orbital period, is enough to extrapolate forward or backward to any other date under the circular-orbit assumption.
Where an Approximation Like This Is Useful
- Quick mental models — getting a rough sense of whether Mars is on the near side or far side of the Sun from Earth right now, without needing ephemeris software.
- Teaching orbital mechanics — the circular-orbit simplification isolates the core relationship between period, time elapsed, and angular position, before introducing the complications of eccentric orbits.
- Sanity-checking simulations — a fast mean-motion estimate is a useful cross-check against the output of a more detailed orbital simulator.
Orbital Periods Used in This Model
| Planet | Orbital period (days) | Orbital period (years) |
|---|---|---|
| Mercury | 87.9691 | 0.24 |
| Venus | 224.701 | 0.62 |
| Earth | 365.256 | 1.00 |
| Mars | 686.980 | 1.88 |
| Jupiter | 4,332.589 | 11.86 |
| Saturn | 10,759.22 | 29.46 |
| Uranus | 30,688.5 | 84.02 |
| Neptune | 60,182.0 | 164.77 |
How to Use This Calculator
- Choose a planet from the dropdown (Mercury through Neptune).
- Enter the date you want the position for.
- Select Calculate to see the planet's approximate orbital angle and what fraction of its current orbit it has completed.
Related Calculations
See how that orbital period itself is derived from Kepler's third law with the Orbital Period Calculator, or check the Orbital Velocity Calculator for how fast a planet moves along its path.