Wednesday, December 16, 2020

let's talk about alignments

Be careful of the articles you read about the alignment of planets and other solar system bodies. Journalists with little or no scientific background might casually use terms which have astronomical import.

I've made some infographics to explain all the interesting celestial alignments. We're viewing the planets of the solar system from above the Sun. Note that the orbits are not scale and were drawn as perfect circles. Note the planet sizes and the central star are not to scale. Not even close.

UPDATED! I replaced the infographic for inferior and superior conjunctions...


A planetary opposition occurs when an "outer" planet is opposite the Earth from the Sun. A line could be drawn from the Sun, through the Earth, and onward to the planet in question. An Earth sandwich.

infographic - opposition

Outer planets refer to Mars, Jupiter, and beyond, of course. All this can apply to minor planets, dwarf planets, asteroids, comets, etc. 

Oppositions are significant because the Earth is as close as it can get to the outer planet, at least in terms of celestial alignment. A notable special case is Mars where it follows a highly elliptical orbit making some oppositions much better than others.

In general, an opposition with a distant outer planet, Jupiter and beyond, happens about every year. That's because our planet zips around the Sun faster. The outer planets moving slower you can think of as being in nearly the same position one year later. For Jupiter, it's actually close to 400 days. Mars? 780 days! Mars is movin' fast and it takes a while to catch up! Beep beep.

If you substitute the Moon for the red planet position above and the alignments are perfect in three dimensions, the Earth will block some of the sunlight reaching the Moon. This we refer to as a lunar eclipse. Curiously, no one refers to the circumstance as "the Moon is at opposition."

solar conjunction

When a planet is on the opposite side of the Sun from the Earth, with the Sun in the middle, we call this a conjunction. Specifically, a solar conjunction. 

infographic - conjunction with Sun

Personally, I don't get excited about these events. I don't add the particulars to my astronomy planning calendar. Partly because there is nothing to see.

Given the small planet could be behind the large Sun, the Sun would literally block the view from Earth. But the more immediate consideration is that the planet, even when not perfectly aligned with the Sun, is extremely close to the Sun. When the Sun up in our daytime sky, there's no way to see the planet given the bright blue sky and the intense glare of our star. 

In fact, there's a danger element here. Do not attempt to view planets near to the Sun, before, during, or after conjunction. Certainly do not use magnification, i.e. binoculars or telescope. The risk of damaging your eyes is too great.

All that said, the "moment" of conjunction with the Sun is significant astronomically even though we cannot directly observe it. It means that the planet which was in the evening sky a short while ago is now transitioning to the morning sky.

Again a solar conjunction is a moment in time and we can take two things away from these events:

  1. the planet is currently not visible
  2. it will show up though, soon, in the morning sky...
We explicitly refer to outer planets here. And the alignment with the Sun is referred to simply as a conjunction. There are special cases though for the "inner" planets.

inferior and superior solar conjunction

The "inner" planets will periodically align with the Sun as viewed from the Earth. Actually, it will happen two times during each inner planet's orbit...

infographic - inferior and superior conjunctions

The inner planets refer to Mercury and Venus, both closer to the Sun than the Earth.

The illustration shows the tan planet between the Earth and the Sun. This is referred to as inferior conjunction or inferior solar conjunction.

The beige planet is on the other side of the Sun but still lined up with the Earth. When an inner planet is on the far side of the Sun from the Earth, we call this superior conjunction. [ed: Graphic updated.]

Like in the previous scenario, an inner planet on the far side of the Sun will be invisible due to bright, dangerous glare of the Sun.

If someone says the inner planet is "opposite" the Sun, that choice of wording might be misconstrued as "opposition." That's why I've been trying to avoid the usage.

Now something neat can happen when an inner planet is at inferior conjunction and everything is aligning perfectly: you can get a solar transit.

Transits of Mercury are somewhat common but difficult to observe as Mercury is so small. I observed and imaged the 2016 May 9 event. The last one occurred on 2019 Nov 11. I watched it on the internet. The next is 2049 May 7.

Transits of Venus are rare. The last one was 2012 Jun 5. I was fortunate and viewed and imaged it. A really fun day.

It should go without saying that solar transits are dangerous to observe. You must use proper filters for your eyes, camera lenses, binoculars, and telescope.

One more thought experiment. If you swap the Moon for the tan planet in the diagram above, put the Moon between the Sun and the Earth, and the alignment in 3D space is perfect, you get a solar eclipse

Then there's the sub-types of solar eclipses: partial, annular, and total. Some argue a total solar eclipse, where the Sun is completely blocked by the Moon, should be called an occultation.

An occultation is was one object blocks the view of another. The most common usage of this is when an asteroid occults a star or the Moon occults another celestial object, say a planet or a double star.


An appulse describes when two celestial objects are relatively close to one another from our perspective on the Earth.

infographic - appulse

The diagram above shows that the red planet is a bit to the right of the beige one. While they are not perfectly aligned, and not technically in conjunction, they may well be attractively close in the sky. That often makes for good photo opportunities.

I do not know if there's a strict definition of an appulse which specifies how close the objects need to be to one another. It seems that people perk up when objects are with 5 or 6 degrees, which is the approximately field of view for binoculars. When objects get to about 1 degree in separation, that means they are possibly viewed in a telescope at low power. Less than a degree? Every one loses their mind.

planetary conjunction

A conjunction of planets may occur when two planets line up well as viewed from the third rock from the Sun.

infographic - planetary conjunction

A planetary conjunction can occur with inner and outer planets. 

Venus-Jupiter conjunctions are arguably the easiest to observe with the naked eye as they are normally the two brightest planets in the sky.

Jupiter-Saturn conjunctions happen about every 20 years. Given Saturn moves 1/3rd of it's orbit in that time, the two planets will appear in different parts of the sky. That means sometimes the Sun will be in the way.

Technically, a conjunction in astronomy has a very strict definition. The objects are at the same Right Ascension or longitude along that ecliptic.

Again, you can get occultations happening where a large foreground object blocks or hides the background distant object. This can happen with planets, the Moon, and even moons of planets. But you need that perfect alignment in all three dimensions.

Hope all this helps.

See the RASC Observer's Handbook for more information on alignments, dates of oppositions and conjunctions, lunar and solar eclipses, and various occultations.


There are other alignments or arrangements that astronomers may refer to from time to time (e.g. quadranture) but I've highlighted key ones relevant now...

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