Parade of Planets

Gazing into the night sky can impart a viewer with a sobering sense of perspective. The billions of stars out there. The millions of years the light took to travel from distant galaxies. It’s easy to be overwhelmed with a feeling of unimportance, as if we’re a speck of sand on a beach of unimaginable size.

But our perspective from Earth can also create illusionary gifts, geometries unable to be seen by anyone but us.

When we gaze at the firmament, stars in constellations might seem to be tight companions, but, in reality, they are extraordinarily far from each other. The stars in the Big Dipper, for example, are all many light years away from each other, yet they appear to create a ladle in one section of our night sky. This viewpoint arises thanks to the celestial sphere, an imaginary sphere of arbitrarily large radius centered on the Earth. Thanks to the vast distances of the stars, everything we see in the cosmos appears to be the same distance away from our eyes, as if cast onto the inner surface of the celestial sphere. The limitations of our optical biology can make it difficult to determine the actual distances of stars, but the situation can conjure some rather enjoyable shapes and convergences.

Visualization of a celestial sphere - graphic by ChristianReady
A visualization of the Big Dipper as seen from Earth - created by Kevin M. Gill
A visualization of the stars of the Big Dipper in 3D space - Encyclopedia Britannica

We can see non-star items in the sky, too. Reinforcing the smallness of a being on Earth, the other planets in our solar system, despite their closeness, appear to be about the same size as stars. Thirteen-hundred Earths could fit inside Jupiter and, on a cosmic scale, the planet is just two doors down, yet it looks the same in the night sky as objects at unfathomable distances. Because all the planets orbit the Sun and are relatively close to us, they dance across the celestial sphere more vigorously than distant stars.

Sometimes, as the planets all race around their sun-centered tracks, the orbs appear rather close to each other from our perspective. When two planets nestle next to each other in our sky, we call it a conjunction. Longtime readers of the newsletter will recall the Great Conjunction of 2020, in which Saturn and Jupiter seemed destined to smash into each other. The orbits of the two largest planets in the solar system align – from our perspective – in 20-year periods.

The Great Conjunction of 2020 - photo by Marek Nikodem

Conjunctions don’t need to involve planets. The Moon and other satellites can get involved, in addition to other cosmic bodies.

The Moon, Venus, and Jupiter in conjunction in 2009 - European Southern Observatory
Comet NEOWISE in conjunction with Talitha Borealis - photo by Bautsch

Conjunctions occur when two objects have the same right ascension or ecliptic longitude – the celestial sphere version of longitude. If bodies match in ascension and declination – the celestial sphere version of latitude – we have a syzygy, the alignment necessary for eclipses. When a nearer object appears larger than a farther one, we call it an occultation; conversely, when the nearer item appears smaller than the farther one, we get a transit. These alignments are relatively rare, and even more infrequent when we take the Sun and the Moon out of the equation.

Sometimes, we can spy planets in our viewport even if the ascensions aren’t particularly close. In general, conjunctions can be viewed as a clustered subset of planetary alignment. Thinking about the planets aligning is pleasing, but gets relatively little love in the astronomical sciences. Why? This notion of “alignment” is a rather generous definition of the word. A syzygy of all the major bodies of our solar system would be an incredibly satisfying phenomenon, but something like the following image will never occur:

The planets in a straight line - Lunar and Planetary Institute

The planets do not neatly orbit the Sun in line with Earth. Instead, they swing above and below the ecliptic by varying degrees. So, even if Uranus, for example, lined up behind Saturn, the two might not be aligned in the vertical plane (as imagined in the photo above). Even beyond this constriction, it is beyond lottery-rare for the planets to queue in a way that even resembles the photo above. According to astronomer Jean Meeus, all eight planets align within 3.6 degrees of the sky every 396 billion years. The number rises to 13.4 trillion years if one reduces the separation to 1.0 degrees. So, while a close-to-straight line is not necessarily theoretically impossible, by the time we reach the 396 billion year mark, not all eight planets will even exist, as the Sun will have grown to the point of consuming its inner children.

According to Christopher Baird, a physics professor at West Texas A&M University, the closest the planets will come to alignment will occur on 6 May 2492, when they will look like the following image in the sky:

May 6, 2492 alignment - graphic by Christopher Baird

Not even close to a straight line (which would look closer to a vertical line).

Planetary alignment refers more broadly to the bodies appearing in the same 180-degree arc of the sky at the same time. This definition might seem a bit wide – after all, it’s half the sky! – but the alignment in a scope of this size is still uncommon.

So, when a group of planets decide to get together in the same patch of the firmament, skywatchers get excited. Occasionally, they’ll line up as if they are in a caravan, forming a parade of planets.

On the morning of 3 June 2024, six planets will form a nice train in the predawn hours:

An approximation of the planetary alignment on 3 June 2024 from the Northern Hemisphere looking east - graphic by Kyle Stout

Unfortunately, the parade will not appear as striking as the bright line above.

Uranus and Neptune will not be visible to the naked eye. Further, Mercury and Jupiter will be close to the horizon. To see the latter two, one will need an unobstructed view of the horizon and impeccable timing, while the entire parade will require a telescope or, perhaps, binoculars to see.

Fortunately, this parade is not the only one in the near future. The same six planets will emerge for a line on 28 August 2024 and again in January 2025. Things get even better next February, as Venus joins the mix, meaning we can see all seven official, non-Earth planets in one patch of sky.

Further into the future, in September 2040, five planets able to be spied with the naked eye will align, along with a crescent moon!

If you have the time and ability to watch the upcoming alignment, go out about an hour before dawn for the best chance to see all six planets. Download a star/sky app to show you the spots to seek, especially for that pesky Uranus.

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