Positional Astronomy

Conjunction, Opposition, Quadrature

The chart above depicts the solar system as if you were looking down on the Earth's north pole.

From this viewpoint, all planets orbit the Sun in a counter-clockwise direction.

Inferior Planet - Has an orbit that is closer than the Earth-Sun distance. (Mercury and Venus)

Superior Planet - Has an orbit that is farther than the Earth-Sun distance. (Mars, Jupiter, Saturn, Uranus, Neptune)

Opposition - A planetary position in which the Sun, the Earth and a superior planet are aligned in that order and in a straight line.
This is also the best time to observe a planet because:
  • it is at its closest approach to Earth.
  • the planet rises just as the Sun is setting, allowing observation any time during the night.

Quadrature - A planetary position in which the Sun, the Earth and a superior planet form a right angle with the Earth at the vertex.
At sunset during eastern quadrature, a superior planet is overhead and is overhead at sunrise during western quadrature.

Conjunction - A planetary position in which the Earth, the Sun and the planet are aligned in that order and in a straight line.
For an inferior planet this is called a superior conjunction.
When the order is the Earth, the inferior planet and the Sun, it is an inferior conjunction.
A conjunction is the worst time to observe a planet because it is lost in the Sun's brilliance.

Greatest Elongation - Point at which an inferior planet has the greatest visual distance from the Sun. (Mercury 28º   Venus 47º)
During Greatest Eastern Elongation an inferior planet is visible at sunset and is visible before sunrise during Greatest Western Elongation.

Synodic Period
As was mentioned in the Synodic Period Calculator, this is the time period required for a planet to return to the same point in its orbit relative to the Earth and Sun. For example, it can be the time between two conjunctions, the time between two oppositions, etc.

Intuitively, you would think that this takes place in greater than one but less than two orbits of the inner planet and generally speaking you would be right.
For example, the planet Neptune orbits the Sun in 60,190.03 days (164.79 years) and the Earth requires 365.25 days (1 year).
Inputting this information into the synodic period calculator, we find that Neptune's synodic period is 367.48 days or 1.0061 years.

Looking at the image above, in the time it takes the Earth to make slightly more than one orbit (about 362º), the planet Neptune moves from point A to point B (about 2º).

Moving closer to the Sun, we see the planet Jupiter has an orbital period of 4,332.59 days (11.8618 years) and using the calculator, we see that Jupiter's synodic period is 398.88 days (1.0921 years).

Looking at the above graphic, we see that as Jupiter moves about 33º from point A to point B (the time of one synodic period), the Earth completes slightly more than one orbit (about 393º).

So far, the planets we have examined have orbits in which the Earth only has to travel a little more than one revolution from one synodic period to the next.

Investigating the orbit of Mars reveals an interesting phenomenon. Its synodic period is 779.92 days (2.135 years) so for the first time, we have encountered a synodic period in which the outer planet makes slightly more than one revolution and the inner planet (Earth) makes more than two revolutions. (see illustration below)

If you look at the "Year One" graphic, you might think that the Earth has already caught up and passed Mars. However, at the start (leftmost chart), Mars is considered to be 360º ahead of the Earth. So, by Year One, Earth has traveled 360º and Mars has traveled about 551º.

It might seem somewhat contradictory that a faster planet can make two orbits without overtaking the slower planet. In fact, the inner planet and outer planet can make numerous orbits between synodic periods. For example look at this graphic:

The inner planet, Earth, makes one orbit per year and a fictional outer planet makes ¾ orbits per year or one orbit in 1⅓ years. Entering 1 and 1⅓ into the calculator yields a synodic period of four years. In that time the slower, outer planet makes three orbits while the faster, inner planet completes four.

The general rule when dealing with all synodic period calculations, is that the outer planet will make one less orbit than the inner planet.


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