Planetary Opposition and Conjunction

FAS Astronomers Blog, Volume 30, Number 13.

You might have seen some articles recently (September 2022) about Jupiter’s bright opposition. You also might remember Mars’ opposition of 2018, which was the closest since 2003. But, what you ask, is an opposition? Let’s find out.

The eight (or maybe nine?) planets of our solar system orbit the Sun in a counterclockwise direction when viewed from the north. They all have very predictable orbits ranging from Mercury, the fastest at 88 days, out to Neptune at 165 years. A planet’s orbit is referred to as its sidereal period, which is the time required to complete one orbit about the Sun.

However, the perceived motion of the planets when viewed from the Earth is a bit more complicated because both the planets and the Earth are simultaneously moving about the Sun. The position of the planets relative the Earth and in our night sky depends on both motions.

Elongation is a term that describes the observed angle between a planet and the Sun when viewed from the Earth. It can range from 0^o when the planet is in the same direction as the Sun to 180^o when the planet is opposite the Sun.

Planets also have a synodic period, which is the time required to complete one full cycle in the night sky and return to the same position relative to the Earth. It is technically the time between any two successive oppositions or similar conjunctions (as defined below).

Superior Planets

The superior planets are those farther from the Sun than the Earth (Mars, Jupiter, Saturn, Uranus, and Neptune). Astronomers describe the position of these planets relative to the Earth using the following terms.

• Opposition is when a planet is directly opposite the Earth from the Sun (180^o elongation).
• Conjunction is when a planet is directly in line with the Sun from the Earth (0^o elongation).
• Quadrature is when a planet is at 90^o

At opposition, the superior planets are generally at their closest approach to the Earth. They appear relatively bright in the night sky, and it is a great time to view them through your telescope. At conjunction, they are generally at their farthest distance from the Earth. In this case, they are dimmer in the sky and the viewing isn’t quite as good. As a result, astronomers get somewhat excided when a planet is near opposition, but pretty much ignore their conjunctions (and quadrature).

Jupiter just passed its closest opposition in sixty years on September 26, 2022. But don’t worry if you missed it, Jupiter will remain bright in the evening sky for the next month or two.

The outer planets move slowly from west to east against the background of stars. Jupiter completes one cycle every 12 years or so. It moves from one constellation of the ecliptic to another about once a year. Saturn takes a little longer, 29 ½ years, to move through the ecliptic. Uranus and Neptune take even longer at 84 and 165 years respectively. As an example, you might remember the conjunction of Jupiter and Saturn a couple years ago when they were both in the same relative position in the night sky. Now Jupiter has moved much farther to the east than Saturn.

The superior planets have a synodic period, but it isn’t that interesting. It is equal to the sidereal period of the Earth (its orbit of 1 year) plus a time to account for the movement of the planet over a year’s time. So, all the outer planets have a synodic period of just over one year, with Jupiter the longest at 399 days (365 + 34 days) and Neptune the shortest at 368 days (365 + 3 days).

The motion of Mars is similar, faster, but with a couple slight twists. Mars takes around two years (26 months) to complete one orbit. Therefore, it alternates between opposition and conjunction a little over once a year. It also has the longest synodic period of the superior planets at 780 days (365 + 415 days).

Mars has an elliptical orbit.

• At its closest point (Perihelion) Mars is 128 million miles from the Sun (1.38 AU).
• At its farthest point (Aphelion) Mars is 155 million miles from the Sun (1.67 AU).

So, sometimes its opposition is relatively close, while other times it is relatively distant.

• Aphelic Opposition is when Mars is as far as 63 million miles from the Earth.
• Perihelic Opposition is when Mars is as close as 35 million miles away (this occurs every 15 years or so, and most recently in 2003 and again in 2018).

Because the Earth is moving faster than Mars, it catches up and passes Mars every two years or so. As a result, for a short time, Mars appears to go through a retrograde motion as it moves from east to west against the background of stars. Back in the days when folks thought the solar system was geocentric (the Earth was at the center), this motion caused them all kinds of headaches. They just couldn’t understand it. Now, with our modern view of a heliocentric solar system (with the Sun at the center), it makes sense. The outer planets also exhibit a retrograde motion, but not to the same extent as Mars.

By the way, Mars will follow a retrograde path during November 2022 through mid-January 2023 as it moves through the constellation Taurus. In mid-January it will reverse course and again pass-through Taurus and then move on toward Gemini. It will also be in opposition on December 8, 2022, and it will have a close conjunction with an almost full Moon on the night of December 7, 2022. So, you might look for a bright reddish “star”, which will be visible most of the night during late November and early December.

Inferior Planets

The two planets closer to the Sun from the Earth are called inferior planets (Mercury and Venus). They are never opposite the Earth from the Sun, and as such, do not undergo opposition. These planets instead experience two conjunctions. They have elongations that peak between 18^o to 28^o for Mercury and 45^o to 47^o for Venus and they undergo phases similar to that of the Moon.

• Superior Conjunction is when the planet is on the opposite side of the Sun from the Earth (0^o elongation). They appear as a full disk like a full Moon.
• Inferior Conjunction is when the planet is on the same side of the Sun as the Earth (0^o elongation). They disappear like a new Moon.
• Greatest Elongation is when the planet is at its greatest elongation. This varies based on the distance of the planet from the Sun and the eccentricity of its orbit. They appear as a half circle like a first or third quarter Moon.

The inferior planets follow a distinct pattern in the sky relative to the Earth as they move through their synodic period. Venus traces out five different 583.92-day synodic periods every eight years. The synodic period of Mercury is shorter at 115.9 days.

• Both Mercury and Venus pass behind the Sun (superior conjunction). They appear as small full disks.
• They then move to the left/east (relative to the Sun) and begin to rise in the western horizon before sunset.
• As they move more to the left/east, they appear higher in the sky, and they start to go through waning phases.
• Eventually they reach their greatest eastern elongation, appear as a half circle, and are near their peak height in the evening sky.
• They continue to move closer to the Earth and now are lower in the sky and appear more of a crescent.
• They then move in front of the Sun (inferior conjunction). Just before and after, they appear as a large and thin crescent.
• As they move to the right/west (relative to the Sun), they start to rise in eastern horizon before sunrise.
• They soon reach their greatest western elongation, appear as a half circle, and are again near their peak height in the morning sky.
• As they move farther from the Earth, they become dimmer and go through waxing phases until they again reach superior conjunction.

Although, both planets are full disks at superior conjunction and thin crescents near inferior conjunction, they are much brighter near inferior conjunction because their relative size is larger due to their much closer distance to the Earth.

And yes, the below is a bit confusing.

• When they are east of the Sun, they appear in the western sky before sunset. They appear above the Sun in the evening, follow the setting Sun, and set after the Sun.
• When they are west of the Sun, they appear in the eastern sky before sunrise. They appear above the Sun in the morning, precede the rising Sun, and rise before the Sun.