Every year or so a headline promises a rare "planetary alignment", sometimes with a suggestion that the combined gravity will do something dramatic. The reality is gentler and, once you look at it from above, rather obvious.
Two kinds of alignment
When astronomers talk about an alignment they usually mean one of two things. The first is a conjunction: two or more planets appearing close together in Earth's sky. Because all the planets travel along the ecliptic, they pass one another regularly. Jupiter and Saturn's "Great Conjunction" of December 2020 put the two giants a tenth of a degree apart, their closest since 1623.
The second is a planet parade: several planets visible in the sky at once, strung along the ecliptic. This looks like a line from Earth only because we are seeing the flat plane of the Solar System edge-on.
Try it on the map
Set the map to 21 December 2020 and look at Jupiter and Saturn. From above they are not touching; they are simply in the same direction as seen from Earth, with Saturn about 730 million km further out. Now drag the slider through 2025 and watch the "parade" of January and February that year: Venus, Mars, Jupiter and Saturn were on the same side of the Sun as Earth, so all four were in the evening sky. From above, they are spread over a quarter of the sky.
Why a perfect line never happens
The planets orbit at different speeds, from Mercury's 88 days to Neptune's 165 years. For all eight to lie on one straight line from the Sun would require their positions to coincide to within a fraction of a degree, and the odds of that are so remote it has not happened in the Solar System's history and will not before the Sun dies. Even the loose alignments in the news are typically spread over 30° or more.
And the gravity?
The Moon's tidal pull on Earth is more than 10,000 times stronger than that of all the other planets combined, even when they gang up on one side. An alignment changes the tides by less than a millimetre. Enjoy the view; there is nothing to brace for.
