
Phobos
The Speed Racer
The moon that rises in the west and crosses the sky THREE times per Martian day
🌙 Mind-Blowing Fact
Phobos orbits Mars so fast it rises in the WEST, races across the sky in 4 hours, and you see it rise and set THREE times per day!
What is Time on Phobos?
What is Time on Phobos?
Phobos is Mars's inner moon and holds the record for the fastest orbit of any moon in the solar system relative to its planet's surface. This creates the most bizarre time experience anywhere.
The Backwards Moon
- Phobos orbits faster than Mars rotates - it completes an orbit in 7 hours 39 minutes
- Mars's day is 24.6 hours - so Phobos orbits Mars MORE THAN 3 TIMES per Martian day
- Result: Phobos rises in the WEST and sets in the EAST
- You see Phobos rise and set 3 times per day!
- Phobos rises in the west: Takes ~30 minutes to clear the horizon
- Races across the sky: 4 hours and 15 minutes from horizon to horizon
- Sets in the east: Disappears in 30 minutes
- Below horizon: About 6 hours later, it rises in the west again!
- Mars appears MASSIVE - taking up 40-45° of the sky
- Mars is always there - Phobos is tidally locked, so Mars never moves from its spot
- You see Mars rotate beneath you - One full Mars rotation takes 24.6 hours
- Mars changes phase - From "Full Mars" to crescent and back
The Rapid Transit
A Phobos "day" (from one Martian sunrise on Phobos to the next) lasts only 11 hours:
Mars Fills the Sky
Because Phobos orbits so close to Mars (only 5,989 km above the surface):
The Doomed Moon
Phobos is slowly spiraling into Mars:
- Getting closer by about 1.8 cm per year
- In 50 million years, Phobos will either crash into Mars or break apart into rings
- Time is literally running out for Phobos
The Temporal Paradox
Living on Phobos creates the strangest time experience in the solar system:
- Your "day" (sunrise to sunrise of the SUN) = 11 hours
- Your "orbit" = 7.66 hours
- Your "month" (time to orbit Mars) = 7.66 hours
- Mars's day = 24.6 hours
- You experience 3 "orbits" per Mars day
- The sun rises in the east, but Mars-rise is in the west (if you could see it from different sides)
A Day in the Life
Three Sunrises Before Lunch
You wake up in Stickney Base, built into Phobos's largest crater. Your alarm shows 07:00 Phobos Time. You check the Orbit Counter: Orbit 2,847 of your mission. That's... 38 Mars days. You've been here 5 weeks.
The Morning Race
You stumble to the observation window, and there it is: Mars. ENORMOUS. UNMOVING. Taking up nearly half the sky, its red-orange surface covered in craters, canyons, and polar ice caps. You can see individual volcanoes. You can see Valles Marineris like a scar across the face of the planet.
Mars doesn't rise or set. It just... sits there. Rotating. You watch carefully and can see the surface moving—the Tharsis Bulge is rotating into view. In a few hours, Olympus Mons will be visible.
But forget Mars for a moment. You need to catch the sunrise.
You check your watch: 07:15. The sun will rise in the WEST in about 10 minutes. You suit up quickly and head to the western viewing platform.
Sunrise #1: Western Sunrise
07:23 - The sun breaks the western horizon.
It's surreal. Every instinct screams that sunrises happen in the east. But on Phobos, the sun rises in the west because Phobos orbits Mars faster than Mars rotates. From Phobos's perspective, Mars appears to rotate backwards, and the sun gets "left behind" and rises from the "wrong" direction.
You watch the sun climb for 15 minutes, then head inside. You have work to do.
4 Hours of Daylight
Your shift begins. You work for 4 hours on experiments, maintenance, and sample collection. Through the window, the sun arcs across the black sky. It moves fast—you can see it moving if you watch for a few minutes.
Mars, meanwhile, barely rotates. The Tharsis region has moved slightly, but Mars's rotation is SLOW compared to Phobos's frantic orbit.
11:45 - Sunset #1. The sun touches the eastern horizon and disappears. You've had 4 hours and 22 minutes of sunlight.
First Sunset, Second Sunrise
You eat a quick lunch. Outside, it's dark. The stars are brilliant. Mars is in "Gibbous" phase—not quite full—casting a dim red light across Phobos's surface.
You work through the darkness for 2 hours. Then—
14:02 - Sunrise #2. The sun rises in the west again.
You've seen two sunrises before most people on Mars have finished their morning coffee.
Watching Mars Turn
Your colleague, Dr. Reeves, is obsessed with watching Mars rotate. She's set up a camera to track surface features.
"Look," she says, pointing at the monitor. "Olympus Mons was on the eastern edge this morning. Now it's nearly centered. By tonight, it'll be on the western edge."
One full Mars rotation = 24.6 hours. One Phobos orbit = 7.66 hours. So Mars rotates about 120° during each of your orbits. You complete 3.2 orbits while Mars completes one rotation.
It's dizzying to think about.
Sunset #2, Sunrise #3
18:26 - Sunset #2. The sun sets in the east again. You've now seen two complete sunrises and two complete sunsets, and it's not even dinnertime.
You take a break, grab dinner, review data. Outside, darkness again. Mars continues its slow rotation.
20:45 - Sunrise #3 of the day.
"Third sunrise!" announces the base AI cheerfully. "You are experiencing your third solar dawn of this Martian sol."
Dr. Reeves laughs. "Nobody on Mars has had three sunrises today except us."
Time Becomes Meaningless
Later, you check the calendar:
- Earth time: You've been here 38 days
- Mars time: You've been here 37 sols (Mars days)
- Phobos orbits: You've experienced 2,847 orbits
- Phobos sunrises: You've seen ~5,400 sunrises
How do you count your life here? In sunrises? In orbits? In Mars days?
The base keeps Earth time for sleep schedules, but it's absurd. You go to sleep during "daylight" and wake up in "darkness" because Phobos's day-night cycle doesn't care about your sleep schedule.
The View That Never Changes, And Always Changes
You return to the window before bed. Mars is there, massive, unchanging in position. But the surface has rotated—Olympus Mons is now on the western limb, about to rotate out of view.
The sun has set for the third time today. It will rise in the west again in a few hours, while you sleep.
Tomorrow, you'll see three more sunrises. Three more sunsets. Mars will complete another rotation. Phobos will complete another 3.2 orbits.
Time on Phobos doesn't flow—it races. And standing on this doomed moon, watching the sun rise in the west for the third time today, you realize: time is whatever the universe wants it to be.
Thought Experiments
If you tried to use Phobos for a "24-hour day" schedule...
It would be impossible to sync with natural light! A 24-hour day on Phobos would span more than THREE complete Phobos orbits. You'd see the sun rise and set THREE TIMES during your "day." You'd have lunch during "night" (eclipse), dinner during "morning," and wake up during "afternoon." The only solution: ignore the sun entirely and use artificial lighting.
What if you traveled from Phobos to Deimos (Mars's outer moon)?
You'd experience time whiplash! Phobos orbits in 7.66 hours and rises in the west. Deimos orbits in 30.3 hours and rises in the east (slower than Mars's rotation). On Deimos, you'd see one rise and set per 2.7 MARS DAYS. Coming from three sunrises per day to one sunrise per three days would be psychologically shocking!
How would you celebrate New Year on Phobos?
Which New Year? Earth years (365 days)? Mars years (687 days)? Phobos orbits (every 7.66 hours)? If you celebrated every Phobos orbit, you'd have 1,140 "New Years" per Earth year—three per day! More likely, you'd celebrate Mars New Year (687 days) while living through over 2,160 Phobos orbits between celebrations.
The Science of Time on Phobos
The Science of Phobos's Frantic Time
Why Phobos Rises in the West
Phobos is the only large moon in the solar system that rises in the west. This happens because:
1. Phobos orbits FASTER than Mars rotates:
- Phobos orbital period: 7 hours 39 minutes
- Mars rotation period: 24 hours 37 minutes
2. From Mars's surface, Phobos appears to move "backwards" relative to the stars
3. The sun gets "left behind", causing Phobos to lap it
4. Result: From Phobos's perspective, the sun appears to rise in the west
The Doomed Moon
Phobos is spiraling inward due to tidal forces:
- Tidal deceleration slows Phobos's orbit by ~1.8 cm/year
- In 50 million years, Phobos will either crash into Mars or break apart
- Mars may get rings from Phobos's debris!
Tidal Locking Despite Fast Orbit
Despite its rapid orbit, Phobos is tidally locked. This happened quickly because:
- Close proximity to Mars = strong tidal forces
- Irregular shape = asymmetric mass distribution
- Result: Same face always points to Mars
The View from Mars
From Mars's surface, Phobos would be an incredible sight:
- Rises in the west every 11 hours
- Moves visibly across the sky - you can see it moving
- Goes through phases like our Moon
- Causes brief eclipses but Phobos is too small to fully block the sun
- Not visible from polar regions - Phobos's orbit is equatorial
The Fastest Natural Satellite
Phobos holds records:
- Shortest orbital period of any moon in the solar system (relative to parent planet's surface)
- Only large moon that rises in the west
- Only moon experiencing tidal acceleration (all others are slowly moving away from their planets)
Time Dilation... Sort Of
Phobos orbits at 2.138 km/s. This creates minimal relativistic time dilation (time moves ~0.000000003% slower on Phobos due to its speed), but the psychological time dilation from experiencing three sunrises per day is overwhelming!