Mars has a moon that rises in the west and sets in the east twice a day
Looking up from the surface of Mars reveals an uncanny celestial motion. Phobos, the planet’s innermost moon, circles Mars in just 7 hours and 39 minutes—far faster than Mars's own 24.6-hour rotation. Because it overtakes the planet's spin, Phobos appears to rise in the west and plunge below the eastern horizon. Traveling across the sky in around four hours, it completes this backward journey twice every single Martian day.
An Upside-Down Sky
On Earth, human intuition about the heavens is anchored in a single, unyielding rule: everything rises in the east and sets in the west. The Sun, the Moon, the distant planets, and the constellations all follow this uniform trajectory because Earth rotates toward the east. As our planet spins on its axis once every twenty-four hours, it carries observers through space, creating the optical impression that the entire cosmos is drifting in the opposite direction. For billions of years of terrestrial history, no natural celestial body visible to the naked eye has ever violated this daily pattern.
On Mars, however, that basic intuition completely unravels. An observer standing in the rust-colored dust of the Martian equator watching the night sky would witness a small, jagged moon break above the western horizon. Rather than lingering for twelve hours, this moon marches rapidly across the stars from west to east, cutting through the sky in a mere four hours before dipping beneath the eastern horizon. Roughly seven hours later, the same moon climbs back out of the west to repeat the journey, completing this apparent backward trek more than twice every single Martian day.
The Mechanics of Orbital Overtaking
The celestial body responsible for this counterintuitive display is Phobos, the innermost and larger of Mars's two tiny moons. The backward motion is not an optical illusion caused by an eccentric tilt or an atypical orbital direction. Like Earth's Moon and the vast majority of bodies in the Solar System, Phobos orbits its host planet in a prograde direction—counterclockwise when viewed from above the planet's north pole, matching the direction of Mars's own axial spin. The reversal in apparent motion comes down entirely to relative velocity.
Mars takes approximately 24 hours and 37 minutes to complete one full rotation, a period known as a sol. For a moon to appear stationary in the Martian sky, hovering permanently over a single longitude in what is known as an areostationary orbit, it would need to orbit at an altitude of about 17,000 kilometers above the surface. Phobos, however, skims just 6,000 kilometers above the Martian terrain, making it closer to its primary planet than any other known moon in the Solar System. At this proximity, gravitational mechanics dictate an orbital period of just 7 hours and 39 minutes. Because Phobos circles Mars more than three times faster than Mars rotates underneath it, the moon constantly overtakes the planet's spin, creating the visual reality of a westward rising and an eastward setting.
A Night-Sky Contrast with Deimos
The unusual behavior of Phobos becomes even starker when contrasted with Mars's other moon, Deimos. Positioned much farther out at an altitude of more than 20,000 kilometers, Deimos requires roughly 30.3 hours to complete a single circuit around Mars. Because its orbital period exceeds the length of a Martian sol, Deimos behaves much more like Earth's Moon, rising reliably in the east and setting in the west.
Yet because Deimos moves only slightly slower than the rotation of Mars, it crawls across the Martian sky with agonizing slowness. After rising in the east, Deimos takes approximately two and a half Martian days—more than sixty hours—to reach the western horizon. While Deimos slowly drifts overhead, Phobos dashes past it repeatedly, overtaking its sibling moon multiple times in opposite directions before Deimos can even complete a single transit from east to west.
Discovery Against the Glare of Mars
Despite its dramatic motion, Phobos remained hidden from astronomers throughout the centuries of early telescope observations. Its diminutive size and extreme proximity to Mars made it extraordinarily difficult to resolve against the blinding glare of the red planet. In August 1877, American astronomer Asaph Hall set out to systematically search for Martian satellites using the Great Equatorial refractor at the United States Naval Observatory in Washington, D.C.
After nights of fruitless searching and persistent eye strain, Hall was ready to abandon the effort, but his wife, Angeline Stickney, urged him to return to the telescope for one more attempt. The following night, August 12, 1877, Hall spotted Deimos. Six nights later, on August 18, he detected Phobos much closer to the planetary limb. Hall named the moons after the mythological twin sons of Ares, the Greek god of war: Phobos, representing fear and panic, and Deimos, representing dread. Decades later, the largest impact crater on Phobos, spanning roughly nine kilometers across, was formally named Stickney in honor of Angeline's decisive encouragement.
A Fragile, Heavily Battered World
Spacecraft flybys, beginning with Mariner 9 in 1971 and continuing through orbiters like Viking, Mars Global Surveyor, and Mars Express, revealed that Phobos looks nothing like Earth's spherical satellite. It is an irregularly shaped, heavily cratered body measuring roughly 27 by 22 by 18 kilometers. The immense Stickney crater dominates one entire side of the moon, an impact so violent relative to Phobos's size that it came dangerously close to shattering the moon entirely.
Phobos also displays strange parallel grooves and troughs running across its surface. Early hypotheses suggested these were stress fractures produced by the impact that formed Stickney. More recent analyses indicate they may be chains of secondary craters carved out by ejecta thrown from impacts on the Martian surface below, or surface expressions of structural deformation. Furthermore, Phobos has an exceptionally low mean density of around 1.87 grams per cubic centimeter, indicating a porous interior composed of carbon-rich rock and possibly water ice—a structural collection of rubble loosely bound by gravity rather than a solid monolith.
The Origin Puzzle and an Inevitable Demise
The physical nature of Phobos has generated an ongoing scientific debate regarding where it came from. Its dark, carbonaceous surface spectrum and low density strongly resemble C-type or D-type asteroids commonly found in the outer asteroid belt, suggesting Mars gravitationally captured a stray space rock early in its history. However, dynamicists struggle to explain how capture could result in such a nearly circular orbit lying almost perfectly in Mars's equatorial plane. An alternative hypothesis proposes that Phobos formed from a debris ring created when a giant impactor struck early Mars, though reconciling this with the moon's primitive, asteroid-like composition remains an active area of investigation.
Regardless of how Phobos arrived in its orbit, its current trajectory guarantees its destruction. Because Phobos orbits below the synchronous orbital radius, tidal forces work against it. The gravity of Phobos creates a slight tidal bulge on Mars that lags behind the fast-moving moon. This gravitational lag acts as a continuous brake, draining orbital energy and causing Phobos to spiral inward toward Mars at a rate of roughly two meters every century. In approximately 30 to 50 million years, Phobos will cross the Martian Roche limit, where tidal stresses will overcome the moon's weak internal gravity, tearing it apart to produce a dramatic ring system around Mars, with the remaining fragments raining down onto the planet.
Key takeaways
•Phobos rises in the west and sets in the east because its rapid 7-hour-and-39-minute orbit outpaces the 24.6-hour rotation of Mars, reversing its apparent direction across the sky.
•Because it orbits beneath the areostationary threshold, Phobos traverses the Martian sky in roughly four hours and completes this journey more than twice per Martian sol.
•Asaph Hall discovered Phobos in 1877 at the U.S. Naval Observatory, and the moon's largest feature, Stickney crater, was named in honor of his wife, who encouraged the search.
•Tidal friction causes Phobos to spiral inward by roughly two meters per century, sealing its fate to either shatter into a ring system or collide with Mars in 30 to 50 million years.