Astronomy Essentials

A conjunction happens when 2 worlds meet

On Tuesday, October 10, 2023, you can see a lovely conjunction in the morning sky. The dazzling planet Venus and bright star Regulus – Heart of the Lion in Leo – will meet on the sky’s dome. By coincidence, the waning crescent moon will be visible that morning also! What a glorious sight in the east before dawn!

Technically speaking, objects are said to be in conjunction when they have the same right ascension – sort of like celestial longitude – on our sky’s dome. Practically speaking, objects in conjunction will likely be visible near each other for some days. So be sure to watch for Venus and Regulus on the days before and after the October 10 conjunction, too.

The 2024 lunar calendars are here! Best Christmas gifts in the universe! Check ’em out here.

2 positions of the moon near Venus, star Regulus, and star group the Sickle all along a green ecliptic line.
Venus and Regulus meet in conjunction, near the waning crescent moon. Chart via EarthSky.

What is a conjunction?

Occasionally, two or more objects meet up with each other in our sky. Astronomers use the word conjunction to describe these meetings. The word conjunction comes from Latin, meaning to join together. Maybe you remember the old Conjunction Junction cartoons from the 1970s. In language, conjunctions relate to clauses brought together with words like and. In astronomy, conjunctions relate to two or more objects brought together in the sky.

Sometimes one of these objects is the sun, so the conjunction cannot be seen. But other conjunctions – between stars, our moon, and the planets – can be truly spectacular.

Ten vertical panels showing the daily positions of Venus and Jupiter closer and closer each day.
View at EarthSky Community Photos. | Soumyadeep Mukherjee of Dhanbad, India, captured these photos of Venus and Jupiter heading toward conjunction on March 1-2, 2023, and wrote: “… Venus and Jupiter have stolen the attention of astrophotographers! They were inching close to one another, slowly but surely. I was lucky enough to capture their relative movement for the last 10 days.” Thank you, Soumyadeep.

You can’t see an inferior conjunction

An inferior conjunction is when an object passes between us and the sun. Any object that orbits the sun closer than Earth does might pass through inferior conjunction from time to time. That is, it can, assuming its orbit lies more or less close to the ecliptic.

Usually, though, when astronomers speak of an inferior conjunction, they’re talking about Venus or Mercury, which orbit between Earth and the sun. Astronomers sometimes refer to Venus and Mercury as inferior planets. When they’re at or near inferior conjunction, we generally can’t see them. They’re hidden in the sun’s glare. Occasionally, though, Venus or Mercury at inferior conjunction can be seen to transit across the sun’s disk.

We shouldn’t forget the moon here. It passes between Earth and the sun at new moon once each month. Therefore it would be correct, if a little unusual, to say that the moon is at inferior conjunction when it’s at its new phase.

Diagram with sun, Earth, and 8 positions of Venus around its orbit showing conjunction locations.
This chart uses the orbit of Venus to show the the points of inferior and superior conjunction. Venus was last at inferior conjunction on August 13, 2023, and will be in that position again on March 22, 2025. It was last at superior conjunction on October 22, 2022, and will be there again on June 4, 2024. Illustration by John Jardine Goss/ EarthSky.

You can’t see a superior conjunction either

A superior conjunction is when an object passes behind the sun from our point of view. Look at Venus’ orbit in the diagram above. Half of its conjunctions with the sun – when they come together on our sky’s dome – are inferior conjunctions, and half are superior conjunctions. It’s fun to imagine the inferior planets on an endless cycle of passing in front of the sun, as seen from Earth, then behind it, and back again, like squirrels running around a tree.

Meanwhile, the superior planets – or planets farther from the sun than Earth – can never be at inferior conjunction. Mars, Jupiter, Saturn, Uranus and Neptune can never pass between us and the sun. So the superior planets only have superior conjunctions.

But other conjunctions can look beautiful

The most common – and most exciting – type of conjunction doesn’t involve the sun. Any time two objects pass each other on the sky’s dome, they’re said to be at conjunction. These sorts of conjunctions – maybe between two planets, or a planet and a star, or a star and the moon – happen multiple times every month. They are beautiful. The view can stop you in your tracks.

For example, if you were fortunate enough to have looked at the moon on July 21, 1969, as Neil Armstrong and Buzz Aldrin headed home from the Sea of Tranquillity, you’d have seen the moon in conjunction with Spica, the brightest star in the constellation Virgo. They were only about two degrees apart that night. That’s a bit more than the width of your index finger held out at arm’s length.

There are always a few particularly good conjunctions every year. On March 1-2, 2023, we were treated to a spectacular conjunction between bright planets Venus and Jupiter, as you can see below. Click here to see a full gallery of Venus-Jupiter conjunction photos captured by members of the EarthSky community.

Looking up at dark sky framed by bare tree branches, one bright and one dimmer dot side by side.
View at EarthSky Community Photos. | Irina Hynes in Riverside, Illinois, captured this image of the Jupiter and Venus conjunction on March 1, 2023. Thank you, Irina!

Watch for and enjoy conjunctions

People often think about the night sky as being permanent and unchanging, at least on a human scale. If you watch the skies often, though, you’ve surely noticed that’s not true. The stars don’t move relative to each other, but they do move across the sky over the course of a single night, as Earth spins under the sky. And, from one night to the next, each star rises and sets four minutes earlier each day, as Earth moves around the sun.

Once you’ve found the ecliptic, you can see where the real action is. Because they are relatively close to us, the planets and moon do move relative to each other and the stars, and quickly, from our point of view. They change their positions, appear to move closer together and farther apart, and sometimes pass by each other in the sky coming to conjunction. Of all of the pleasures of stargazing, seeing this movement of our nearest neighbors is one of the greatest.

Stay up to date with upcoming conjunctions via EarthSky’s night sky guide.

Dawn with a mountain foreground, the moon and three bright planets.
View at EarthSky Community Photos. | On March 28, 2022, we were treated to an amazing 4-part conjunction. Ji-Hoon Kim in Gongju, Chungnam, South Korea, captured this view before sunrise and wrote: “March 28, 2022, around 5:00 a.m. Korean time. The old moon, Saturn, Mars, and Venus were visible.” Thank you, Ji-Hoon!

Bottom line: A conjunction is when two objects are close together on our sky’s dome. Some conjunctions can’t be seen, but others look spectacular.

October 6, 2023
Astronomy Essentials

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