Mars does not stop and turn around. Earth does the passing. We ride a smaller orbit, and we move faster, so every so often we catch the outer planet and look past it. For a stretch the line of sight swings backward against the stars. That apparent reversal is retrograde. The orbit arrows on this cartoon never flip. Pair it with the solar map (where the planets actually sit), with seasons (Earth’s axis, not this passing), and with gravity, the pull that keeps both worlds on those tracks.
Educational schematic — circular orbits, flattened onto one plane, disks exaggerated so a phone can see them. Circle radii use the semimajor-axis ratio on NASA’s Mars Fact Sheet, 1.524, as if both paths were circles. They are not. That sheet lists Mars’s eccentricity as 0.0935 and its orbit inclination as 1.848°. Not an ephemeris. Not a date. Not for pointing a telescope from Campbell or anywhere else.
Interactive · Top-down
North up · both orbits counterclockwise · dashed ray is the line of sight
Apparent ecliptic longitude · the backward stretch is the dip
Sun
Earth · inner, faster
Mars · outer planet
Westward stretch · schematic
0 d
Literacy · words for the passing
Prograde
The usual apparent drift: west to east through the stars, night to night. StarChild uses that name for the ordinary direction. On this cartoon it is the rising parts of the longitude strip. Both planets are also moving prograde around the Sun the whole time — counterclockwise here — including during the backward stretch.
Retrograde
An apparent east-to-west drift against the stars. It is not a reversal of the orbit. StarChild: it happens roughly when a faster moving planet catches up to and passes a slower one. For Mars, that faster planet is Earth. APOD says the same loop shows up each time Earth laps a planet orbiting farther from the Sun. Jupiter and Saturn do this too. This drawing uses Mars as the outer planet.
Stationary
The pause between those directions, when the line of sight almost stops changing ecliptic longitude. There is one as Earth closes in, and one as Earth pulls away. The orbit arrow does not stop. “Stationary” is about the sky, not about a planet hanging in space.
Opposition
NASA’s Basics of Space Flight: Earth and the outer planet on the same side of the Sun, all three in a line. Night Sky Notes says the same thing from the ground — the planet is directly opposite the Sun as seen from Earth. On this slider that lineup is the middle of the westward stretch. The space-flight chapter calls opposition a good time to observe, and says the planet is then nearest. Mars’s eccentricity on the fact sheet is 0.0935, so a real closest approach can miss that instant. These circles do not time it.
Superior conjunction
The far-side lineup. Basics of Space Flight: Mars is a superior planet because its orbit is farther from the Sun than Earth’s, and superior planets have only superior conjunctions with the Sun — Earth and Mars on opposite sides, nearly in a straight line, Sun between. That is not the backward loop. The Conjunction preset jumps there. The strip stays on the passing, so the playhead leaves the zigzag.
Synodic period
The repeat of the same Sun–Earth–Mars alignment. NASA’s Mars Fact Sheet lists the synodic period as 779.94 days. Sidereal orbit periods on that sheet are 365.256 days for Earth and 686.980 days for Mars. This slider can walk one schematic lap using those mean periods on circles. It is not a calendar of the next opposition.
Apparent motion
What the line of sight does against the stars, not what the orbit does. StarChild’s classroom picture: stand beside a friend, walk faster, and as you pass they appear to drift backward even though they never turned around. The longitude strip is that illusion, unwrapped. A real retrograde also wanders a little in latitude and draws a loop on the sky — APOD’s photograph is that loop. This page keeps the orbits coplanar, so you get the east–west zigzag and not a star chart.
Sources · public NASA education
Cited, not invented
NASA GSFC StarChild — What does it mean for Mercury to be in retrograde? — retrograde is an apparent change, not a planet physically moving backwards in its orbit; prograde is west-to-east through the stars; retrograde is east-to-west; it occurs roughly when a faster planet catches a slower one; the Mars figure is Earth moving faster in its orbit than Mars.
Astronomy Picture of the Day — Mars in the Loop (2025 May 30) — Mars did not reverse the direction of its orbit; the apparent backward motion against the stars reflects Earth’s own orbit; retrograde shows each time Earth overtakes and laps a planet farther from the Sun, Earth moving more rapidly on its closer-in orbit. The dates and the 96 million kilometers on that page are that one apparition, not this slider.
NASA Science — January’s Night Sky Notes: The Red Planet — Mars appears to change direction because Earth is passing the slower-moving Mars; opposition is when a planet is directly opposite the Sun as seen from Earth. The December 2024–February 2025 dates on that page belong to that season.
NASA Science — Basics of Space Flight, Chapter 1 — outer planets are superior planets because their orbits are farther from the Sun than Earth’s; superior conjunction puts Earth and the planet on opposite sides of the Sun; opposition puts them on the same side, in a line. That chapter also calls opposition a nearest-point observing window. This cartoon does not use that as a distance forecast.
NASA NSSDCA — Mars Fact Sheet — semimajor-axis ratio 1.524; sidereal orbit periods 686.980 days (Mars) and 365.256 days (Earth); synodic period 779.94 days; mean orbital velocity 24.08 km/s (Mars) and 29.78 km/s (Earth); eccentricity 0.0935; inclination 1.848°. Those means set the circles. They are not a nightly position.
Classroom picture of a faster inner orbit overtaking a slower outer one. Not an ephemeris, not a pointing aid, and not a Campbell observing plan. Days on the slider are a coordinate of these circles, counted from a schematic opposition. Speeds, periods, and the 779.94-day synodic figure are the ones NASA publishes on the pages above. Earthrise preview: NASA / Bill Anders (public domain).