The short answer
A conjunction is when two objects in the sky appear close together as seen from Earth, such as the Moon next to Venus, or Jupiter next to Saturn. They are not actually near each other in space; they simply line up along our line of sight. Conjunctions are easy, beautiful and need no equipment, which makes them some of the best sky events for beginners.
Three meanings of “conjunction”
The word is used in a few related ways, which can make news stories confusing.
| Meaning | What it is | Can you see it? |
|---|---|---|
| Strict (astronomers’) | Two objects share the same east-west position on the sky, measured along the ecliptic or in right ascension | Usually, if both are bright and away from the Sun |
| Popular | Any close pairing of the Moon and planets, or of planets with each other or with bright stars | Yes; this is the one worth going out for |
| With the Sun | A planet lines up with the Sun, as seen from Earth | No; the planet is lost in the Sun’s glare |
In the strict sense, conjunction is a precise moment. In practice, the pair looks almost as close for a day or so either side, and the closest apparent approach, sometimes called an appulse, can happen a few hours away from the formal moment.
When a planet is in conjunction with the Sun, it’s on the near or far side of the Sun and invisible for a while. The outer planets reach the opposite position, called opposition, about halfway between conjunctions.
How close is close?
Sky distances are measured in degrees. Your hand at arm’s length is a handy ruler: a fingertip covers about one degree, a fist about ten. The full Moon is about half a degree across.
- Within a few degrees: a striking pair to the naked eye, easy to frame in a photo.
- Within about one degree: both fit in the field of view of most binoculars, and often of a low-power telescope eyepiece.
- Within a fraction of a degree: rare and spectacular. Planet-planet pairings this close happen only occasionally.
The Moon is involved in most conjunctions, because it moves quickly, about its own width every hour, and passes every planet once a month. Its position against the planets shifts noticeably over an evening, so the pairing changes as you watch.
How to watch a conjunction
- Check the time and direction. Many conjunctions happen low in the evening or morning twilight, especially those involving Venus or Mercury. You need a clear view to that horizon. See sky events for upcoming pairings and planets visible tonight for where to look.
- Start with your eyes. The arrangement against the twilight sky is the main attraction. A crescent Moon with Venus nearby is one of the most photographed sights in astronomy.
- Add binoculars. They’ll show Jupiter’s moons as tiny points, and the faint glow on the Moon’s dark side, called earthshine.
- Photograph it with a foreground. A phone on a steady surface, with a tree or skyline in frame, gives a sense of scale.
Occultations
Sometimes the alignment is exact and one object passes in front of another. That’s an occultation. Most involve the Moon covering a star or a planet. Because the Moon has no atmosphere, a star disappears behind its edge almost instantly, as if switched off, and reappears just as suddenly on the other side. A planet takes a few seconds to vanish, because it shows a small disk.
Occultations are local events. Because the Moon is relatively close to Earth, its position against the stars shifts depending on where you stand, so an occultation seen from one region can be a near miss from another. Timings differ by minutes across a country. Check a prediction for your location and start watching a few minutes early. Solar eclipses are a kind of occultation too, the Moon covering the Sun; see how to observe an eclipse safely.