Planetary Alignments and Planet Parades

[NASA]

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Planetary Alignments and Planet Parades

A sky chart showing Mars, Jupiter, Saturn, and Venus in a “planet parade.”

Credits:
NASA/JPL-Caltech

On most nights, weather permitting, you can spot at least one bright planet in the night sky. While two or three planets are commonly visible in the hours around sunset, occasionally four or five bright planets can be seen simultaneously with the naked eye. These events, often called “planet parades” or “planetary alignments,” can generate significant public interest. Though not exceedingly rare, they’re worth observing since they don’t happen every year.

Why Planets Appear Along a Line in The Sky

“Planet parade” isn’t a technical term in astronomy, and “planetary alignment” can refer to several different phenomena. As the planets of our solar system orbit the Sun, they occasionally line up in space in events called oppositions and conjunctions. A planetary alignment can also refer to apparent lineups in our sky with other planets, the Moon, or bright stars.

The planets of our solar system always appear along a line on the sky. This line, referred to as the ecliptic, represents the plane in which the planets orbit, seen from our position within the plane itself.

NASA/Preston Dyches

When it comes to this second type of planetary alignment, it’s important to understand that planets always appear along a line or arc across the sky. This occurs because the planets orbit our Sun in a relatively flat, disc-shaped plane. From Earth, we’re looking into that solar system plane from within. We see the racetrack of the planets from the perspective of one of the racers ourselves. When viewed edge-on, this disc appears as a line, which we call the ecliptic or ecliptic plane.

So, while planet alignment itself isn’t unusual, what makes these events special is the opportunity to observe multiple planets simultaneously with the naked eye.

Will the Planets Actually be Visible?

Before preparing to observe a planet parade, we have to consider how high the planets will appear above the horizon. For most observers to see a planet with the naked eye, it needs to be at least a few degrees above the horizon, and10 degrees or higher is best. This is crucial because Earth’s atmosphere near the ground dims celestial objects as they rise or set. Even bright planets become difficult or impossible to spot when they’re too low, as their light gets scattered and absorbed on its path to your eye. Buildings, trees, and other obstructions often block the view near the horizon as well.

This visibility challenge is particularly notable after sunset or before sunrise, where the sky is still glowing. If a planet appears very low within the sunset glow, it is very difficult to observe.

The Planets You Can See, and Those You Can’t

Five planets are visible without optical aid: Mercury, Venus, Mars, Jupiter, and Saturn. Ancient civilizations recognized these worlds as bright lights that wandered across the starscape, while the background stars remained fixed in place. In fact, the word “planet” comes to us from the Greek word for “wanderer.”

The solar system includes two additional major planets, Uranus and Neptune, plus numerous dwarf planets like Pluto and Ceres. Uranus and Neptune orbit in the dim, cold depths of the outer solar system. Neptune absolutely requires a telescope to observe. While Uranus is technically bright enough to detect with good eyesight, it’s quite faint and requires dark skies and precise knowledge of its location among similarly faint stars, so a telescope is recommended. As we’ll discuss in the next section, planet parades necessarily must be observed in twilight before dawn or after sunset, and this is not a good time to try observing extremely faint objects like Uranus and Neptune.

Thus, claims about rare six- or seven-planet alignments which include Uranus and Neptune should be viewed with the understanding that these two distant planets will not be visible to the unaided eye.

What Makes Multi-Planet Lineups Special

Lineups of four or five planet naked-eye planets with optimal visibility typically occur every few years. Mars, Jupiter, and Saturn are frequently seen in the night sky, but the addition of Venus and Mercury make four- and five-planet lineups particularly noteworthy. Both orbit closer to the Sun than Earth, with smaller, faster orbits than the other planets. Venus is visible for only a couple of months at a time when it reaches its greatest separation from the Sun (called elongation), appearing just after sunset or before sunrise. Mercury, completing its orbit in just 88 days, is visible for only a couple of weeks (or even a few days) at a time just after sunset or just before sunrise.

Planet parades aren’t single-day events, as the planets move too slowly for that. Generally, multi-planet viewing opportunities last for weeks to a month or more. Even five-planet events last for several days as Mercury briefly emerges from and returns to the Sun’s glare.

In summary, while they aren’t once-in-a-lifetime events, planetary parades afford an uncommon opportunity to look up and appreciate our place in our solar system, with diverse worlds arrayed across the sky before our very eyes.

Other Planet Lineups

Other recent and near-future multi-planet viewing opportunities:

• January 2016 – Four planets visible at once before sunrise
• Late April to Late August 2022 – Four planets visible at once before sunrise
• Mid-June to Early July 2022 – Five planets visible at once before sunrise
• January to mid-February 2025 – Four planets visible at once after sunset
• Late August 2025 – Four planets visible at once before sunrise
• Late October 2028 – Five planets visible at once before sunrise
• Late February 2034 – Five planets visible at once after sunset (Venus and Mercury challenging to observe)

About the January/February 2025 Planet Parade

The current four-planet lineup concludes by mid-February, as Saturn sinks increasingly lower in the sky each night after sunset. By mid-to-late February, Saturn appears less than 10 degrees above the horizon as sunset fades, making it difficult to observe for most people. While Mercury briefly joins Saturn in the post-sunset glow at the end of February, both planets will be too low and faint for most observers to spot.

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