Elongation is the angle between the Sun and the planet as seen from here. Drag through a full cycle and watch it go from zero, when the planet is lost in the Sun, out to 180 degrees, when it is up all night.
The marked angle sits at the Earth, because that is where the angle physically is.
At opposition, Earth lies between the Sun and an outer planet. The planet is generally closer to us, its Earth-facing side is nearly fully lit, and it remains visible through the night, making this a useful observing period.
The cycle repeats on the synodic period rather than the orbital one, because Earth is moving too. Jupiter takes 12 years to go round the Sun but only 399 days to come back to opposition, which is why its oppositions arrive about a month later each year.
Zero is conjunction, 180 degrees is opposition. Dashed lines mark the events; the solid line is the date shown on the left.
For a direct check, start in the east after sunset. An outer planet near opposition rises around sunset, reaches its highest point near midnight and sets in the west around sunrise. To compare brightness, record the same planet at similar altitudes over several weeks; one visual estimate cannot tell whether the exact opposition instant has arrived.
The years in the table are synodic periods between successive oppositions, not the time each planet takes to orbit the Sun. Earth is moving too: Jupiter needs almost 12 years to complete an orbit, yet opposition returns about every 1.09 years.
Mars is the extreme case: its opposition distance nearly doubles between a poor one and a good one, and brightness falls off as the square of distance, so a perihelic opposition of Mars is several times brighter than an aphelic one. Jupiter's hardly vary. "Mars is at opposition" is therefore far less useful than "Mars is at a perihelic opposition".
A few hours makes no difference to planning a night out. It does matter if you expect a date computed one way to match a date printed the other way, which is why the definition is written down here rather than assumed.
This page uses the geometric definition: opposition is the instant the planet and the Sun differ by exactly 180 degrees in ecliptic longitude, measured from the Earth-Moon barycentre in the J2000 frame. An almanac's opposition is the apparent one, with light-time, aberration and the ecliptic of date applied. The two differ by a few hours.
| Planet | Elongation through one synodic period | Time for one lap |
|---|---|---|
| Mars | 180° | 2.14 yr |
| Jupiter | 180° | 1.09 yr |
| Saturn | 180° | 1.04 yr |
| Uranus | 180° | 1.01 yr |
| Neptune | 180° | 1.01 yr |
At zero elongation, the planet is in the same direction as the Sun and rises and sets at nearly the same time. This is conjunction, when the planet is usually lost in the Sun's glare. At 180 degrees, the planet is opposite the Sun, rising around sunset and setting around sunrise. This is opposition. Visibility changes with the angles in between.
An opposition puts us on the line between the Sun and the planet. It says nothing about how long that line is. Both orbits are ellipses, so the distance at opposition depends on where each planet happens to be in its own.
They usually fall within a week of each other and it is tempting to treat them as one. They are not. Opposition is defined by direction, closest approach by distance, and because both planets are travelling along ellipses the two do not coincide. For Mars the gap reaches about eight days; for Jupiter and Saturn it is one or two.
Related tools: Why planets appear to move backwards · Greatest elongation · Astro Events
Orbital elements from NASA/JPL, "Keplerian Elements for Approximate Positions of the Major Planets" (Standish & Williams, 1992), valid 1800–2050. Positions are geometric — no light-time or aberration — because this is a diagram of the ecliptic plane, not a simulation of a telescope view.