Reference
Mercury through Saturn, without a telescope. What each one looks like, how bright it gets, and how often it is actually worth looking at.
Brightness is given at its best. The synodic period is the gap between one favourable showing and the next — the number that decides whether a given year is worth planning around.
| Planet | Best magnitude | Synodic period | Apparent size | What you get |
|---|---|---|---|---|
| Mercury | −2.5 | 116 d | 5–13″ | A hard target, never more than 28° from the Sun |
| Venus | −4.9 | 584 d | 10–63″ | Bright enough to cast a shadow; phases show in binoculars |
| Mars | −2.9 | 780 d | 4–25″ | A disc only near opposition, an orange dot the rest of the time |
| Jupiter | −2.9 | 399 d | 30–50″ | Four moons visible in steady binoculars, changing nightly |
| Saturn | −0.5 | 378 d | 15–20″ | Rings need about 30× before they separate from the disc |
Planets hold a steady light; stars twinkle. The reason is angular size — a star is a point source, so a single parcel of turbulent air deflects all of its light at once, while a planet is a small disc whose many points average out. Close to the horizon even Jupiter will flicker, so judge it high in the sky.
The second test is position. Planets stay within a narrow band, the ecliptic, the same line the Sun and Moon follow. Anything bright well away from that band is a star.
Visible for a couple of weeks at a time, low in twilight, a handful of times a year. Find a horizon that is genuinely clear to the west or east, and start looking about 40 minutes after sunset or before sunrise.
The brightest thing in the sky after the Sun and Moon, and the usual explanation behind reported sightings of something else. It shows a crescent when nearest to us, which is also when it is largest and brightest.
Oppositions run in a 26-month rhythm and vary a great deal, because the orbit is noticeably elliptical. A close opposition shows a disc with surface markings; a distant one disappoints at any magnification.
The best value in the sky for small optics. Brace binoculars against a fence and the Galilean moons line up as four faint points. Sketch them across a week and you have repeated the 1610 observation that started all of this.
Dimmer than people expect — comparable to a bright star. The rings tilt over a 29-year cycle, and when they turn edge-on they vanish for months even in a large telescope.
Uranus sits right at the naked-eye limit under a genuinely dark sky and is easy in binoculars once you know the field. Neptune needs a telescope and a chart, and looks exactly like a star when you find it.