Space Matter is a weekly column that delves into space science and the mechanics of spaceflight. From the latest discoveries in the universe around us to the fits and starts of rocket test flights, you’ll find analysis, discussion and an eternal optimism about space and launching ourselves into the cosmos.
Watch out, everyone. From April 9 through May 3, Mercury is in retrograde, so we all have something to blame for our terrible luck over the next couple of weeks. But what does Mercury being in retrograde actually mean, astronomy-wise?
We’ve known about Mercury, and its retrograde motions, since ancient times. An Assyrian astronomer, dating back to the 14th century BC, referred to Mercury as “the jumping planet” in cuneiform tablets. The planet is named after the Roman messenger god due to its swift travel through our skies; the ancient Greeks associated the planet with Hermes.
Mercury takes 88 days to orbit the Sun, compared to the Earth’s 365 days. If you think about the way these orbits work, this means that occasionally, the Earth overtakes Mercury in its orbit. That’s what happens when we say Mercury is in retrograde—from the perspective of the Earth, the planet usually moves west to east through our night sky (called prograde). But when the Earth overtakes Mercury in its orbit, the little planet actually moves backwards, east to west, through the stars from our perspective. That is retrograde motion.
Again, it’s important to emphasize that Mercury moving in retrograde is only from the Earth’s point of view, due to the fact that the Earth orbits the Sun faster than Mercury does. And it’s not the only planet that goes into retrograde: every single one, from Venus to Jupiter to Neptune, retrogrades from our perspective.
How does it work with the outer planets, whose orbits are longer than the Earth’s? It’s simply the opposite—retrograde motion happens when the Earth overtakes the other planets in their orbits. The further out the planet, the more often it goes into retrograde from our perspective.
Photo courtesy of NASA and Tunc Tezel
Retrograde does have some pretty cool results, though: On the surface of Mercury, retrograde motion actually causes the Sun to have two sunrises in one day. Mercury travels very fast around the Sun (faster than any other planet in the solar system), but it rotates comparatively very slowly on its own axis. One day on Mars, one rotation of its axis, takes about 175 Earth days. But you’ll remember that it orbits the Sun in 88 days—so a day on Mercury is actually almost twice as long as a year on Mercury. This is due to its proximity to the Sun; for a long time, scientists thought that Mercury was tidally locked to the Sun—meaning that the influence of the Sun’s gravity was so strong that its rotation on its own axis was equal to its orbital period—like our Moon. The Moon is tidally locked to the Earth, so a day on the Moon is equal to a year on the Moon. Due to this phenomenon, we only ever see one side of the Moon from the Earth.
When Mercury approaches the point in its orbit where it’s closest to the Sun (called perihelion—remember, the orbit of planets is elliptical, rather than circular, and Mercury’s orbital eccentricity, or the elliptical shape of its orbit, is greater than any other planet’s), the speed that it orbits and the speed that it rotates are equal. The consequence of this? The Sun actually moves in retrograde. One day out of every year, a person standing on the surface of Mercury would see the sun rise, fall back below the horizon, and then rise again.
The bottom line is that retrograde is a natural consequence of a heliocentric solar system. Mercury in retrograde shoulders the blame for so many events because it supposedly increases the chaos in our lives—according to astrology, you’re not supposed to sign contracts or make any binding decisions while Mercury is in retrograde, and you should keep an eye out for any misunderstandings, as these will happen frequently and tend to have lasting repercussions.
Mercury will be in retrograde again in 2017—from August 13 to September 5 and then again from December 3 through 23, but it means nothing more than the fact that the little planet moves more slowly around the Sun than the Earth does. Though the concept of Mercury in retrograde has made its way into the mainstream pop culture consciousness, Mercury’s apparent motion through our skies isn’t responsible for anyone’s bad luck.
Lead image courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Top photo courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Swapna Krishna is a freelance writer, editor and giant space/sci-fi geek.