Oppositions
When each outer planet is opposite the Sun: closest, brightest, and above the horizon the whole night. The one date per apparition worth planning around.
Times are UTC throughout, because an instant is the same everywhere and a local rendering would hide which day an event actually falls on for you. Nothing here knows or asks where you are. Every computed instant carries an uncertainty, and most of those are measurements rather than estimates: every build checks the lunar phases against NASA's own published table, the equinoxes, solstices and Earth's apsides against the US Naval Observatory's, and the planetary positions the remaining events are derived from against JPL Horizons. The Moon's perigee and apogee are the one family with no external table to check against, and their cards say so.
5 events
Neptune at opposition
ComputedNeptune lies opposite the Sun, rising as the Sun sets and visible all night. It is near its closest to Earth for this apparition and at its brightest.
How exact: Within a few hours. The platform’s planetary positions are checked on every build against JPL Horizons and agree to under five arcminutes, which at the rate a planet separates from the Sun is a few hours in the date. Opposition is a broad event in any case: the planet is barely changed in brightness or size for a week either side.
- Distance from Earth
- 28.875 au
- Approximate magnitude
- 7.8
Where this date comes from
Derived on this platform from published position series. The algorithm and its version are recorded, and the stated uncertainty is the real one.
Method: The instant the planet's geocentric ecliptic longitude differs from the Sun's by exactly 180°, both referred to the mean equinox of date, from the JPL approximate planetary elements. (planet-solar-longitude-opposition, version 1.0.0)
Uncertainty: Within a few hours. The platform’s planetary positions are checked on every build against JPL Horizons and agree to under five arcminutes, which at the rate a planet separates from the Sun is a few hours in the date. Opposition is a broad event in any case: the planet is barely changed in brightness or size for a week either side.
Where it applies: Observable from anywhere the planet rises, which for these planets is most of the inhabited world.
Saturn at opposition
ComputedSaturn lies opposite the Sun, rising as the Sun sets and visible all night. It is near its closest to Earth for this apparition and at its brightest.
How exact: Within a few hours. The platform’s planetary positions are checked on every build against JPL Horizons and agree to under five arcminutes, which at the rate a planet separates from the Sun is a few hours in the date. Opposition is a broad event in any case: the planet is barely changed in brightness or size for a week either side.
- Distance from Earth
- 8.434 au
- Approximate magnitude
- 0.6
Where this date comes from
Derived on this platform from published position series. The algorithm and its version are recorded, and the stated uncertainty is the real one.
Method: The instant the planet's geocentric ecliptic longitude differs from the Sun's by exactly 180°, both referred to the mean equinox of date, from the JPL approximate planetary elements. (planet-solar-longitude-opposition, version 1.0.0)
Uncertainty: Within a few hours. The platform’s planetary positions are checked on every build against JPL Horizons and agree to under five arcminutes, which at the rate a planet separates from the Sun is a few hours in the date. Opposition is a broad event in any case: the planet is barely changed in brightness or size for a week either side.
Where it applies: Observable from anywhere the planet rises, which for these planets is most of the inhabited world.
Uranus at opposition
ComputedUranus lies opposite the Sun, rising as the Sun sets and visible all night. It is near its closest to Earth for this apparition and at its brightest.
How exact: Within a few hours. The platform’s planetary positions are checked on every build against JPL Horizons and agree to under five arcminutes, which at the rate a planet separates from the Sun is a few hours in the date. Opposition is a broad event in any case: the planet is barely changed in brightness or size for a week either side.
- Distance from Earth
- 18.439 au
- Approximate magnitude
- 5.6
Where this date comes from
Derived on this platform from published position series. The algorithm and its version are recorded, and the stated uncertainty is the real one.
Method: The instant the planet's geocentric ecliptic longitude differs from the Sun's by exactly 180°, both referred to the mean equinox of date, from the JPL approximate planetary elements. (planet-solar-longitude-opposition, version 1.0.0)
Uncertainty: Within a few hours. The platform’s planetary positions are checked on every build against JPL Horizons and agree to under five arcminutes, which at the rate a planet separates from the Sun is a few hours in the date. Opposition is a broad event in any case: the planet is barely changed in brightness or size for a week either side.
Where it applies: Observable from anywhere the planet rises, which for these planets is most of the inhabited world.
Jupiter at opposition
ComputedJupiter lies opposite the Sun, rising as the Sun sets and visible all night. It is near its closest to Earth for this apparition and at its brightest.
How exact: Within a few hours. The platform’s planetary positions are checked on every build against JPL Horizons and agree to under five arcminutes, which at the rate a planet separates from the Sun is a few hours in the date. Opposition is a broad event in any case: the planet is barely changed in brightness or size for a week either side.
- Distance from Earth
- 4.359 au
- Approximate magnitude
- -2.6
Where this date comes from
Derived on this platform from published position series. The algorithm and its version are recorded, and the stated uncertainty is the real one.
Method: The instant the planet's geocentric ecliptic longitude differs from the Sun's by exactly 180°, both referred to the mean equinox of date, from the JPL approximate planetary elements. (planet-solar-longitude-opposition, version 1.0.0)
Uncertainty: Within a few hours. The platform’s planetary positions are checked on every build against JPL Horizons and agree to under five arcminutes, which at the rate a planet separates from the Sun is a few hours in the date. Opposition is a broad event in any case: the planet is barely changed in brightness or size for a week either side.
Where it applies: Observable from anywhere the planet rises, which for these planets is most of the inhabited world.
Mars at opposition
ComputedMars lies opposite the Sun, rising as the Sun sets and visible all night. It is near its closest to Earth for this apparition and at its brightest.
How exact: Within a few hours. The platform’s planetary positions are checked on every build against JPL Horizons and agree to under five arcminutes, which at the rate a planet separates from the Sun is a few hours in the date. Opposition is a broad event in any case: the planet is barely changed in brightness or size for a week either side.
- Distance from Earth
- 0.678 au
- Approximate magnitude
- -1.2
Where this date comes from
Derived on this platform from published position series. The algorithm and its version are recorded, and the stated uncertainty is the real one.
Method: The instant the planet's geocentric ecliptic longitude differs from the Sun's by exactly 180°, both referred to the mean equinox of date, from the JPL approximate planetary elements. (planet-solar-longitude-opposition, version 1.0.0)
Uncertainty: Within a few hours. The platform’s planetary positions are checked on every build against JPL Horizons and agree to under five arcminutes, which at the rate a planet separates from the Sun is a few hours in the date. Opposition is a broad event in any case: the planet is barely changed in brightness or size for a week either side.
Where it applies: Observable from anywhere the planet rises, which for these planets is most of the inhabited world.
Four kinds of date
- Computed
- Derived on this platform from published position series. The algorithm and its version are recorded, and the stated uncertainty is the real one.
- Published prediction
- Taken from an authority's own published prediction. AsteriaStar reproduces it and links the document; it does not recompute it.
- Annual forecast
- Recurs every year at approximately the same point in Earth's orbit. The date shown is the expected one; the exact hour and the strength vary from year to year.
- Planned
- A date somebody intends to meet. Planned dates move, often by months. The timestamp of the last confirmation is shown so you can judge how much the date is worth.
Take this away as a calendar file: subscribe or download the next year of events, or read it as JSON from the Open Data API. Both carry the basis, the method or source, and the uncertainty for every entry.