How can you tell a planet from a star just by looking?

There are two easy clues: planets wander and stars twinkle. The ancient Greeks noticed the first. Most lights in the night sky keep the same pattern from night to night, the fixed stars. But a few lights move across that backdrop, and the Greeks called them wanderers, planētēs, which is where our word planet comes from. In antiquity that meant the Sun, the Moon and five points of light: Mercury, Venus, Mars, Jupiter and Saturn.

The second clue works in a single glance. Stars twinkle because they are so far away that each appears as a point of light, and Earth's turbulent atmosphere easily disturbs it, bending the light's path like a lens. A planet is much closer. It shows a tiny disc, so the different bits of its light average out, and twinkling usually does not make a planet flicker appreciably. A steady, unblinking light is probably a planet.

Twinkling gets stronger low in the sky, because light near the horizon has a longer path through the atmosphere, so be careful with anything just above the trees. The brightest planet is the easiest. Venus can shine at magnitude minus 4.2, brighter than Sirius, the brightest star, at minus 1.46. And Venus has a human story: the Greeks first thought its morning and evening appearances were two separate stars, Phosphorus and Hesperus.

The test is simple: a bright point of light that is steady, and that has moved against the stars after a few weeks, is a planet.