6 planets lined up in an arc? Is January 21 really the day of a unique parade of planets

You have probably already seen the loud headlines in the media or publicity about “all the planets will line up in one arc” or “a unique parade of planets” on January 21. You may have even read what lies beneath those headlines. Today, we will find out if this phenomenon is as amazing as it is presented and try to understand what is so special about the date of January 21.

Six planets and the Moon over the Melchsee-Frutt mountains, Switzerland, January 15, 2025.
Photo: @MrPhoton, Astrobin.

How the planets move

Basic knowledge about the components of the solar system is usually acquired in the first grades of school. Since 2006, when Pluto was officially “downgraded” to a dwarf planet, there have been 8 large planets left in our system. As we move away from the Sun, we can sequentially cross the orbits of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Simplified images of the Solar System show 8 planets in circular orbits around the Sun. And it looks as if all their trajectories are located in the same plane. “To a first approximation”, this approach can be considered correct. But in reality, each orbit is tilted at a slight angle relative to the plane of the Earth’s orbit. In addition, the orbits of all the planets are not circular, but elliptical – slightly elongated.

The Solar System

In high school, during geometry lessons, among the most challenging stereometric problems are those where you need to consider the section of a sphere by a plane. From those problems, it is clear that such a section is a circle. And the practice of solving such problems will be useful to us now. Yes, the school curriculum has finally come in handy!

Although this concept may seem very outdated, astronomers still use the term “celestial sphere” today, but they put a slightly different meaning into it than they did hundreds of years ago. The celestial sphere is an imaginary sphere of arbitrary (but impliedly quite large) radius centered at the point of observation, on which all the celestial bodies are projected.

The visible path of the Sun on the celestial sphere.
Source: https://digfir-published.macmillanusa.com/universe10e/asset/img_ch2/c02_fig14.html

For the time being, let’s assume that the Sun and all the planets are located in the same plane and that we are surrounded by a huge imaginary celestial sphere. Then the projections of all the planets and the central star will be located on the cross-section of the celestial sphere in this plane. And as we know, it is a circle, with us in the very center of it.

Now let’s look around in our minds. We will see a certain arc of this circle. And, of course, it is along this arc that all the currently visible planets will be located. Even if we take into account the inclination of the planets’ orbital planes (which is no more than 7°), their position will not deviate much from the circle described above.

That’s right: the planets do not move randomly across the sky. You will never see any of them in, say, the Ursa Major or Cassiopeia. ALL the planets are ALWAYS located along a single line that circles the celestial sphere, and…

…and its name is ecliptic!

In astronomy, an ecliptic is a circle that the Sun moves in the celestial sphere. To put it simply, it is the same circle that we get when we cross the celestial sphere with the plane of the Earth’s orbit. At any given time, there is a part of this circle in the sky – a certain arc.

Humanity tends to measure everything by its standards, so it is not surprising that the inclination of the orbital planes of all the bodies of the Solar System is also usually indicated relative to the ecliptic. Sometimes the planets cross the ecliptic, but the rest of the time they do not move away from it by more than a few degrees.

The location of the planets along the ecliptic on January 21 at 3:30 p.m. UTC (10:30 a.m. UTC-5).
Source: virtual planetarium SkySafari.

Thus, if there are at least a few planets in the sky at a given moment, then by mentally drawing an arc through them, you will have an idea of how the ecliptic is moving. So, we found out that all the planets visible at any time are lined up along an arc, which is part of the Sun’s trajectory. By the way, the plane of the Moon’s orbit also does not deviate much from the ecliptic plane. As a result, we can observe conjunctions of the Moon with the planets or even coverage.

Why January 21? And is it possible to see the planets on any other day?

The news that on January 21, 2025, “6 planets will line up in one arc” spread so quickly that it is almost impossible to track down its original source. We assume it came from an article on Star Walk.

The text emphasizes that January 21 is an “average” date, and for each location, the optimal day of observation is slightly different (varies by 3 days), since in different cities the planet will be visible in the smallest sector of the sky at slightly different times. However, the article does not define the term “sky sector”. And what astronomers understand by this term is not appropriate in terms of meaning, since the coordinates of the planets in the celestial sphere are almost the same for all Earthly observers, and the optimal time is determined only by the time of day, so it should vary by no more than 12-13 hours.

One way or another, the evening sky is rich in planets now. Moving along the ecliptic from west to east, you can see Venus and Saturn, followed by Neptune, then Uranus and Jupiter, and finally Mars, which remains bright after the recent opposition on January 16. But it’s worth noting that only 4 of these planets are visible to the naked eye. To see Uranus and Neptune, you’ll need binoculars or a telescope.

Is the configuration in which 6 planets are visible at the same time unique to January 21? In short, no. Most of the planets move quite slowly across the sky, so you’re unlikely to see a difference between January 21 and January 22. For example, on New Year’s Eve, if you were observing in the evening, you could also see all of these planets in the sky at the same time, provided you were facing northeast and southwest. At that time, the angle between the directions to the westernmost planet (Venus) and the easternmost planet (Mars) was 154.5°.

If you look at the position of the planets on January 21, Mars remains the easternmost planet, but Saturn is the lowest in the west, and the angle between them is 127.7°. So maybe the recommended date is due to a decrease in this angle? It turns out not, as the angular distance between the planets will continue to decrease.

Evening sky on December 31, 2024, at 5:00 p.m. UTC (12:00 p.m. UTC-5) – all planets except Mercury are in the sky at the same time. Source: virtual planetarium SkySafari.

Another date mentioned in some posts about the “alignment” and “parade of planets” is February 28, announcing that “7 planets will line up in the sky”. What exactly is in store for us? In the image below, you can see the location of the planets in the evening sky on this day. Mercury will additionally be visible low on the western horizon. However, note that Saturn is quite close to the Sun (10°) and is located even lower than Mercury. Therefore, it will set behind the horizon just 50 minutes after our daytime dawn. This makes the Lord of the Rings a rather difficult object to observe. With a magnitude of +1.1m, Saturn will melt into the evening sky. The same goes for Neptune: due to its low brightness and small angular distance to the Sun, it will be almost impossible to observe. Thus, although at the end of February there will indeed be 7 planets in the sky at the same time, not all of them will be available for observation.

Western evening sky on February 28, 2025. Source: virtual planetarium SkySafari.

Let’s briefly summarize. The arrangement of the planets along one arc is their normal location, none of the planets move far from the ecliptic. If at some point it turned out otherwise, you would know for sure that it was the apocalypse of the Solar System. In January, in the evenings, it is indeed possible to observe 6 planets (except for Mercury) simultaneously, and it is not necessary to be tied to the 21st. But in early February, the conditions for observing Saturn deteriorate as it approaches conjunction with the Sun. How rare is it to see 6-7 planets in the sky at the same time? It does not happen often, but it is not a once-in-a-lifetime event. For example, at the end of August 2025, before sunrise, all planets except Mars will be visible.

We recommend paying attention to the southwestern part of the sky on February 1, where Venus, the Moon, Saturn, and Neptune will be nearby. You can start observing at approximately 3:40 p.m. UTC (10:40 a.m. UTC-5), but the exact time depends on your location. And, of course, Uranus, Jupiter, and Mars will also be waiting for you in the eastern and southeastern parts of the sky.

Southwestern evening sky on February 1, 2025. Source: virtual planetarium SkySafari.
Advertising