From Earth, Jupiter is the superior-planet lesson pushed almost to full. The orbit ratio on NASA’s fact sheet is 5.204. The maximum phase angle on these circles is arcsin(1 / 5.204), and the minimum lit fraction is (1 + cos of that angle) / 2 — about 11.08° and 99.07%, rounded in the script from the unrounded angle. That is a gibbous sliver, not a crescent. A crescent needs a phase angle past 90°, and Earth cannot stand that far around Jupiter. A spacecraft can. Voyager 1 looked back on 24 March 1979 and recorded a crescent. Cassini’s farewell mosaic of 15 January 2001 gives the phase angle as 120°. Juno’s crescent mosaics are the same kind of viewpoint, from a craft that has left the narrow wedge Earth is stuck in. Drag the observer. The shaded wedge is every direction Earth can occupy. The opposition desk is the lineup, drawn for Mars; Jupiter reaches the same geometry once per 398.88-day synodic period on the fact sheet.
Educational schematic — circular coplanar orbits, disks exaggerated. Earth is held on the right. The Jupiter marker walks clockwise on that frozen view because Earth is faster; the orbit arrows stay counterclockwise. The ratio is 5.204. The fact sheet lists eccentricity 0.0487 and inclination 1.304°. The Galilean moons use periods from NASA’s Jovian Satellite Fact Sheet, with schematic angles, not an ephemeris of where Io is tonight. The ring of the spacecraft view is a drawing, not a trajectory. Not a magnitude. Not a date. Not for pointing a telescope. The bands are a recognition mark, not cloud data.
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
Jupiter
Earth · viewpoint held
Line of sight
0 d
Beyond Jupiter · a spacecraft can leave the wedge
Drag the observer · shaded wedge is every direction Earth can occupy
View from that observer · bright limb faces the Sun · Galilean moons share this phase angle
120°
Literacy · words for the phase
Phase angle
The angle at Jupiter between the Sun and the observer. From Earth it stays inside ±11.08° on these circles. Lit fraction = (1 + cos of the phase angle) / 2. Zero is full. Past 90° is a crescent. The spacecraft slider is that angle, signed so the two sides of the wedge are opposite crescents.
Earth’s wedge
Every direction from Jupiter that an Earth-orbit observer can occupy. It is fixed in the Sun–Jupiter frame: Earth cannot wander more than arcsin(1 / 5.204) away from the Sun. The synodic slider moves Earth inside the wedge. It cannot leave.
Opposition
Earth between the Sun and Jupiter, phase angle 0°, largest disk on these circles. NASA’s Basics of Space Flight defines opposition that way. The repeat is the fact sheet’s synodic period, 398.88 days. The opposition desk draws the lineup with Mars. The words are the same.
Quadrature
The right angle at Earth, east or west. It is the maximum phase angle, and the 99.07% minimum. Still gibbous. Eastern quadrature is the evening side of the Earth-view sketch; western is the morning side.
Spacecraft crescents
Voyager 1, 24 March 1979, a crescent look back. Cassini, 15 January 2001, a thinning crescent at a solar phase angle of 120°. Juno, 11 December 2016, a crescent with the Great Red Spot, from data taken about 285,100 miles out on the third close flyby. The 120° button uses Cassini’s published angle. It is not a claim that the other images were 120°.
75° is not yet a crescent
New Horizons observed Jupiter on 28 February 2007 at a solar phase angle of 75°. The caption says that scan was projected onto a crescent to remove distortion from Jupiter’s rotation. On this disk, 75° is still gibbous, past Earth’s 11° wedge and short of the 90° half-lit line. The 75° button is that cited angle, not a crescent preset.
Galilean moons
Io, Europa, Ganymede, and Callisto, with orbital periods 1.769138, 3.551181, 7.154553, and 16.689017 days on the Jovian Satellite Fact Sheet. On this cartoon their phase matches Jupiter’s, because they sit close to the planet compared with the observer. The little angle marks are schematic, not tonight’s positions. From Earth they stay nearly full for the same reason Jupiter does. From the dragged observer they can crescent. A Juno caption shows Io’s shadow on the cloud tops — a moon between the Sun and Jupiter — which this page does not try to animate.
Sources · public NASA pages
Cited, not invented
NASA NSSDCA — Jupiter Fact Sheet — semimajor-axis ratio 5.204; sidereal orbit period 4,332.589 days; Earth 365.256 days; synodic period 398.88 days; mean orbital velocity 13.06 km/s (Jupiter) and 29.78 km/s (Earth); eccentricity 0.0487; inclination 1.304°. The 11.08° and 99.07% figures are computed from 5.204.
NASA NSSDCA — Jovian Satellite Fact Sheet — Io, Europa, Ganymede, and Callisto orbital periods 1.769138, 3.551181, 7.154553, and 16.689017 days. Semimajor axes are on that sheet too. This drawing uses the periods only, as schematic angles.
NASA Science — Basics of Space Flight, Chapter 1 — superior planets usually appear gibbous, and full at opposition; at superior conjunction the outer planet appears near its fully illuminated phase.
NASA Science — Jupiter in Shadow (Voyager 1) — a crescent view taken 24 March 1979, assembled at JPL from three black-and-white frames. Image credit on that page: NASA/JPL-Caltech. This desk links the page and does not hotlink the file.
NASA Science — Cassini’s Farewell to Jupiter — 15 January 2001, 17 days after closest approach, Jupiter as a thinning crescent. The caption states the solar phase angle, the angle from the spacecraft to the planet to the Sun, as 120 degrees. That is the 120° preset.
NASA Science — Jupiter–Io montage (New Horizons) — infrared observation at 1:40 UT on 28 February 2007, solar phase angle 75 degrees, projected onto a crescent to remove distortion caused by Jupiter’s rotation during the scan. The 75° preset follows that caption, including the distinction between the phase angle and the projected crescent.
NASA / JPL Photojournal — PIA25013, A View of Crescent Jupiter from Juno — JunoCam mosaic from the 39th close pass, 12 January 2022. The caption says a crescent phase is impossible to see from Earth, even with a telescope, and that Jupiter always appears full from here because its orbit is outside Earth’s.
NASA — Crescent Jupiter with the Great Red Spot — citizen-scientist processing of JunoCam data from 11 December 2016, about 285,100 miles (458,800 kilometers) from the planet, during the third close flyby. Image credit on that page: NASA/JPL-Caltech/SwRI/MSSS/Roman Tkachenko.
NASA Science — Moon Shadow (Juno) — Io casts its shadow on Jupiter in an image from the 22nd close flyby, 11 September 2019. The caption compares it to a solar eclipse on Jupiter. This page cites that and does not draw the shadow.
Classroom picture of an outer orbit, the gibbous disk Earth gets, and the crescent a spacecraft gets by standing outside that wedge. Not an ephemeris. Days are counted from a schematic opposition. The 11.08° and 99.07% figures are the script’s rounding of arcsin(1 / 5.204) and (1 + cos of that angle) / 2. Spacecraft angles that are not printed on the NASA pages above are not printed here. Earthrise preview: NASA / Bill Anders (public domain).