Five Lights That Wander

A planet makes no light of its own. It shines because sunlight bounces off it, the same way the Moon shines. Venus looks brilliant because it is near us and wrapped in bright clouds — not because it burns like a star.

Two of the five, Mercury and Venus, circle the Sun on paths smaller than Earth's. From here they never appear far from the Sun: look for them low in the west just after sunset, or low in the east before dawn. Mars, Jupiter, and Saturn travel paths larger than ours, so they can appear anywhere along the Sun's track — even high in the sky at midnight.

The five naked-eye planets orbit the Sun in nearly the same flat plane Earth does, so they always appear near the ecliptic, the Sun's yearly track that folio 4 mapped. That one fact shrinks your search from the whole sky to a single narrow band.

The band holds two families. Mercury and Venus are inferior planets — their orbits sit inside Earth's — so their angle from the Sun has a strict maximum, the greatest elongation: about 28 degrees for Mercury, about 47 for Venus. You meet them only in twilight, morning or evening. Mars, Jupiter, and Saturn are superior planets, orbiting outside ours, and nothing prevents them from standing directly opposite the Sun — at opposition — rising at sunset and staying up all night.

Opposition is also the moment of closest approach: Earth has caught up and sits directly between the Sun and the planet, so an opposition planet is near its biggest and brightest. Mars gains the most — its distance from us swings so widely that it brightens dozens of times over as opposition nears. Oppositions repeat on each planet's synodic period: about 26 months for Mars, about 13 for Jupiter.

The inferior planets run a different calendar. Venus takes about 19 months per full cycle: months climbing out of the evening twilight, a few swift weeks plunging back, a pass between Earth and Sun, then a reappearance before dawn. The Greeks counted it as two objects — Hesperus the evening star, Phosphorus the morning star — before recognizing both as one wanderer.

Most lights in the night sky are stars. Five are not. They are planets — other worlds that go around the Sun, the way Earth does. They look like bright stars, but they do not stay put.

Watch one for many nights. The star shapes around it never change. The planet slowly slides past them. That is how you catch one: stars hold still, planets wander.

The elongation limit is one line of trigonometry. From Earth at 1 au, a planet orbiting at radius a < 1 au reaches greatest elongation where the sightline runs tangent to its orbit, so sin ε = a. Venus, at 0.72 au, gives 46 degrees; Mercury's eccentric orbit spans 0.31 to 0.47 au, giving anywhere from 18 to 28. At the tangent point the planet shows exactly half phase.

Galileo's telescope showed Venus running the full set of phases, crescent through full — impossible if Venus circled Earth, inevitable if it circled the Sun. Superior planets, seen always from inside their orbits, never show crescents: Mars at most reaches gibbous, and its brightness swings come from distance, roughly 0.5 au at opposition against 2.5 au near conjunction.

Apparitions are clocked by the synodic period, 1/S = |1/P − 1/P_E| — the relation that set the Moon's 29.5-day phase cycle in folio 5. Venus returns S ≈ 584 days, and five synodic cycles miss eight Earth years by only about two days, a near 8:13 commensurability of orbital periods that repeats the pattern of Venus apparitions against the calendar for generations.

Scintillation, too, is quantitative. Turbulent cells some centimeters across impose random phase tilts; a star, angularly minute, arrives through effectively one cell at a time and flickers. A planetary disk several arcseconds wide spans many independent turbulence paths, and their fluctuations average toward zero — the observer's steady-light rule is aperture averaging, performed by the source instead of the telescope.