What it isThe angle between two planets in Earth's sky (the delta angle Δλ), and how fast each planet is moving along the zodiac, drawn every day for 330 years. Conjunction = the two share a longitude (Δλ→0); opposition = they sit opposite (Δλ→180).
RetrogradePlanets never really reverse; from a moving Earth the outer planets APPEAR to drift backwards for weeks each year. That is retrograde. Here it is exact: the daily longitude rate dλ/dt going negative. The middle pane's line below zero, and the ribbon's red, both mark it; the depth is how fast backwards.
StationsA zero-crossing of the rate line is a station — the moment a planet pauses and turns (direct→retrograde or back). The event markers also flag which planets were retrograde at each conjunction/opposition.
Delta-angle shapeEach pair traces a triangle wave: down to 0° at every conjunction, up to 180° at every opposition. Mars–Jupiter repeats every ~2.24 years; Jupiter–Saturn every ~19.86 years (the "great conjunction" cycle).
The 2020 great conjunctionZoom to 2020 and the Jupiter–Saturn delta line dips to ≈0° on 2020-12-21 — the closest the two had come since 1623 (0.10° apart). Its conjunction marker sits exactly on that dip.
Triple conjunctionsWhen the faster planet is retrograde near alignment, the pair can meet three times in a few months (the delta line kisses 0° three times). Mars–Jupiter: 1789-90, 1836-37, 1979-80. Jupiter–Saturn: 1940-41, 1980-81.
Ribbon = second viewThe line pane gives the exact speed; the ribbon shows the same retrograde as coloured bands so you can see the rhythm across the whole 330 years without zooming. Each row is scaled to its own planet's normal speed.
Reading real numbersThe left axis of the top pane is degrees (0–180) with fixed ticks at 30°, 45°, 60°, 90°, 120°, 135°, 180° (classic aspect levels); the middle pane is deg/day with a labelled zero line. Tap any line for the exact value on the date; shift-tap (or long-press) drops a date line reading every series at once, on all three panes.
Isosceles triangle (with Earth)Gold diamond markers flag days when Earth + one charted pair form an isosceles triangle in space: Earth–Mars–Jupiter or Earth–Jupiter–Saturn. Sides are geocentric distances (AU) plus the planet–planet chord from the true angular separation. Two sides equal within 0.02 AU at a local residual minimum. Apex = the body where the equal sides meet. Mars–Saturn is not used. Each pair has its own Isos M–J / Isos J–S toggle; markers also hide when that pair's delta-angle series is toggled off (and reappear when the delta is turned back on, if the isos toggle is still on).
Measure toolThe msr chip on either line pane: tap two points to read the change (in degrees, or deg/day) and the real time between them — invaluable for a retrograde's depth and duration.
Source & frameNASA JPL Horizons, apparent geocentric ecliptic-of-date longitude of the Mars / Jupiter / Saturn barycenters, corrected for light-time and aberration. Daily grid, 120,531 days, 1766-01-01 → 2096-01-01, no gaps. Great-circle separation uses latitude too, so a conjunction's closeness is real.
ColoursMars red, Jupiter blue, Saturn green; the Mars–Jupiter delta is purple and Jupiter–Saturn orange — all maximally distinct. Toggle any series with its pill; Select all / Deselect all clear or restore them in one tap.
ControlsEvery pane has its own height grip, its own vertical stretch, full screen, Save-PNG and reset in the top-right corner, plus the shared window buttons. Nothing blanks: the data is continuous, so any window and any zoom always draws.