What is a "planetary alignment"?

A planetary alignment is when several planets appear to line up in the night sky from our vantage point on Earth. 

Although still billions of kilometers apart from each other in space, multiple planets can appear close to each other in the sky because of where they are in their orbits. 

Think of it like watching boats sailing across a wide bay. From the shore, there are brief moments when a few of them appear to be near each other, even though they're actually sailing separate routes spread across miles of open water. 

Even though the boats are far apart from each other, from your perspective, they’re clustered. The same is true of planetary alignments.

Why do alignments happen?

All of the planets in the Solar System orbit the Sun on roughly the same flat plane, called the ecliptic. Because of this shared plane, the planets always appear to move along a similar path across our sky. 

As the planets complete their orbits — some quickly, like Mercury (88 days), and some slowly, like Saturn (29 years) — there are windows when several of them cluster on the same side of the Sun and within a relatively small arc of the sky as seen from Earth. These windows can last anywhere from a few days to several weeks.

Not all alignments are created equal 

There are a few different kinds of alignments you might hear about in the news. 

A planetary conjunction – two planets appearing very close together in the sky – is the most basic and therefore the most common. A conjunction between the Moon and a planet happens about once a month on average. Pairings between planets happen less frequently. The “Great Conjunction” between Jupiter and Saturn occurs roughly every 20 years and is a real treat for observers who get to see both spectacular planets through a telescope at once. 

When multiple planets spread across the sky at once, it’s called a planetary parade, or parade of planets. This isn’t a technical term in astronomy, but it helps convey that something special is happening. These tend to happen every few years, with fewer-planet parades happening more often.

Some alignments are more impressive than others, depending on which planets are involved. Bright, easily visible planets like Venus, Jupiter, Mars, and Saturn make for a much more striking show than dim, distant Uranus or Neptune, which are generally best seen with binoculars or a telescope.

An occultation is the ultimate planetary alignment. This happens when one planetary body passes in front of another one, blocking the smaller one's view from Earth. The most common kind of occultation happens when the Moon passes in front of a planet, since it appears so large in Earth’s night sky.

Extremely rarely, a planet will occult another planet. This is so rare because planets appear very small in Earth’s skies, so their alignment has to be extraordinarily precise for them to actually produce an occultation. The next time this happens will be on Nov. 22, 2065, when Venus will occult Jupiter.

How to see a planetary alignment

You’ll often hear about an upcoming planetary alignment in the local news. Part of what makes planetary alignments so popular is that they’re easy to see. You don’t need specialized equipment or scientific knowledge – you can just go outside at night and look for dots in the sky that tend to be brighter than the stars. During a planetary alignment, multiple planets will appear in a line tracing a slight arc across the sky, rather than a straight line.

Planets will also appear as steadier points of light that don't twinkle the way stars do. This is because, unlike stars, planets are close enough to appear as tiny disks rather than pinpoints, which smooths out the atmospheric interference that causes twinkling.

For planetary alignments involving Venus or Mercury, you’ll have to go outside just after sunset or just before sunrise to see them. This is because these two planets are on the same side of the sky as the Sun, and tend to set and rise either right before or after the Sun.