As planets in our solar system journey around the sun, there are times when they seem to align in the night sky. But does this mean all eight planets have ever perfectly aligned?
The answer hinges on your interpretation of “align” when it comes to the planets of our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Firstly, it’s important to understand that the orbits of these planets are tilted at various angles relative to the sun’s equator. This means that even when planets appear to form a straight line in the sky, they are not actually aligned in a straight line in three-dimensional space, explains Arthur Kosowsky, an astrophysicist at the University of Pittsburgh.
Nikhita Madhanpall, an astrophysicist at Wits University in South Africa, points out that the idea of planetary alignment is more about how things look from our viewpoint on Earth rather than any real physical alignment in space.
A planetary conjunction occurs when two or more planets seem to be close together from our perspective on Earth. It’s crucial to note that these planets are not actually near each other. Even when planets seem to align for an observer on Earth, they are still vastly distant from one another in space, as noted by The Planetary Society.
The criteria for considering planets as aligned is not clearly defined, says Wayne Barkhouse, an astrophysicist at the University of North Dakota. Any definition would involve “angular degrees,” which astronomers use to measure the apparent distance between celestial objects in the sky.
If you were to measure the distance around the entire horizon, it would total 360 degrees. To put the vastness of the horizon into perspective, the full moon appears only half a degree across, according to Las Cumbres Observatory in Goleta, California.
In “Mathematical Astronomy Morsels” (Willmann-Bell, 1997), Jean Meeus, a Belgian meteorologist and amateur astronomer, calculated that the three innermost planets — Mercury, Venus, and Earth — “line up within 3.6 degrees on average every 39.6 years,” Barkhouse mentions.
Aligning more planets requires more time. Meeus found that all eight planets will align within 3.6 degrees, for instance, every 396 billion years. This means such an alignment has never happened and never will, considering the sun is expected to become a white dwarf in about 6 billion years. As it transforms into a red giant and expands, it will likely engulf Mercury, Venus, and possibly Earth, leaving only five planets in our solar system.
The odds of all eight planets aligning within 1 degree of sky are even slimmer, occurring, on average, every 13.4 trillion years, Barkhouse explains. This is in stark contrast to the universe’s age, which is about 13.8 billion years.
If you define the planets as aligned when they are within the same 180-degree-wide area of the sky, the next occurrence will be on May 6, 2492, according to Christopher Baird, an associate professor of physics at West Texas A&M University. The last time the eight planets were within 30 degrees of each other was on Jan. 1, 1665, and it will happen again on March 20, 2673, as per the National Solar Observatory’s facility at Sacramento Peak, California.
Madhanpall emphasizes that planetary alignments have almost no significant physical effects on Earth. The only real impact is the beautiful sight they provide in the sky, Barkhouse adds. There’s no risk of increased earthquakes or any similar phenomena. The change in gravitational force experienced by Earth due to any planetary alignment is minimal.