Mars orbits outside Earth, so it is a superior planet. From here the disk never becomes a crescent. The phase angle — the angle at Mars between the Sun and Earth — tops out where the line of sight makes a right angle at Earth. On these circles that maximum is arcsin(1 / 1.524), and the lit fraction there is (1 + cos of that angle) / 2. The drawing shows a gibbous disk, short of full by enough to see. Opposition is the full disk and the close one, Earth in the middle. Conjunction is full again, small, and lost near the Sun. NASA’s Basics of Space Flight says superior planets usually appear gibbous, and full only at opposition. The lineup words live on the opposition desk. The backward loop is retrograde. This page is the phase. Jupiter’s version of the same limit, much closer to full, is the Jupiter desk, including the crescent only a spacecraft sees.
Educational schematic — circular orbits, one plane, disks exaggerated. Earth is held on the right so the angle at Earth is easy to read. The Mars marker walks clockwise on that frozen view because Earth is faster; the orbit arrows stay counterclockwise. Circle radii use the fact-sheet ratio 1.524. That sheet lists eccentricity 0.0935 and inclination 1.848°. Not an ephemeris. Not a date of the next Mars opposition. Not a magnitude. Not for pointing a telescope.
Interactive · From Earth
North up · Earth held on the right · circles at the fact-sheet ratio
As seen from Earth · east to the right · ghost ring is the largest size on these circles
Sun
Mars
Earth · viewpoint held
Line of sight
0 d
Literacy · words for the phase
Superior planet
NASA’s Basics of Space Flight: Mars, Jupiter, Saturn, Uranus, and Neptune are superior planets because their orbits are farther from the Sun than Earth’s. They usually appear gibbous, and full at opposition. At superior conjunction the chapter says the outer planet appears near its fully illuminated phase. Inferior planets — Mercury and Venus — can show crescents. Mars cannot, from Earth.
Phase angle
The angle at Mars between the Sun and Earth. Zero is full. The maximum on these circles is arcsin(1 / 1.524). Illuminated fraction = (1 + cos of the phase angle) / 2. Both numbers in the readout are that arithmetic, rounded to two decimals.
Opposition
Earth between the Sun and Mars. Basics of Space Flight: the planet and Earth on the same side of the Sun, all three in a line. The phase angle here is 0°. The disk is as large as these circles get. The geometry, without this phase shading, is the opposition desk.
Quadrature
A right angle at Earth, east or west of the Sun. On these circles that is also the maximum phase angle. Eastern quadrature is the evening side of this drawing; western quadrature is the morning side. The presets sit on that right angle.
Conjunction
For a superior planet, only a superior conjunction. The Sun is in the middle. The phase returns toward full, the disk is as small as these circles allow, and Mars is in the glare. That is not Venus’s inferior conjunction.
Why no crescent
A crescent is a phase angle past 90°. From Mars, Earth never strays more than arcsin(1 / 1.524) from the direction of the Sun, because Earth is on the inner orbit. The same limit, tighter, is why Jupiter looks full. A spacecraft can leave the wedge. Earth cannot. That demonstration is on the Jupiter desk.
Sources · public NASA pages
Cited, not invented
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°. The maximum phase angle and the minimum lit fraction are computed from 1.524. They are not a separate NASA table.
NASA Science — Basics of Space Flight, Chapter 1 — superior planets usually appear gibbous, and appear full only when at opposition; at superior conjunction the outer planet appears near its completely illuminated full phase; opposition is the planet and Earth on the same side of the Sun, all three in a line.
NASA Science — Mars oppositions, 1995–2007 — opposition occurs when the Sun and Mars are on opposite sides of Earth, resulting in a full phase for Mars, similar to a full moon. The years, the distances, and the “every 26 months” on that page are those oppositions, not this slider.
Classroom picture of an outer orbit and the gibbous-to-full phase it shows from Earth. Not an ephemeris and not a pointing aid. Days are counted from a schematic opposition. The 41.01° and 87.73% figures are the script’s rounding of arcsin(1 / 1.524) and (1 + cos of that angle) / 2. Earthrise preview: NASA / Bill Anders (public domain).