Planets in the night sky

On most nights, weather permitting, at least one bright planet is in the sky, and two or three are commonly visible in the hours around sunset.^[1] Five planets are easy to see without a telescope: Mercury, Venus, Mars, Jupiter and Saturn. Uranus is barely visible to people with excellent eyesight under dark skies, if they know where to look.^[1]

Telling a planet from a star

Planets are as bright as or brighter than most stars, and they tend to shine with a steady light, where stars often twinkle.^[1] Over days and weeks they also move against the background stars, which are much farther away.^[1]

Brightness is measured in magnitude, where a lower number means a brighter object; a star of magnitude 1 looks 100 times brighter than one of magnitude 6.^[2] The star Sirius shines at magnitude -1.46, and Venus can shine brighter than magnitude -4, bright enough to see from the middle of a city.^[2]

Mercury and Venus: morning and evening

Mercury and Venus orbit closer to the Sun than Earth does, so they always appear near the Sun, in the morning or evening sky.^[3] Their angle from the Sun as seen from Earth is called elongation.^[3] Because they can pass between Earth and the Sun, they go through phases like the Moon, from a thin crescent to nearly full.^[3]

Mars, Jupiter and Saturn: opposition

The outer planets are best seen around opposition, when Earth passes between the Sun and the planet and all three line up. The planet is then opposite the Sun in the sky, so it rises around sunset and is visible for much of the night.^[4] Around opposition a planet is also at its closest to Earth for the year, so it looks brightest.^[5]

Each planet reaches opposition on its own schedule. Mars comes to opposition about every two years, when Earth's orbit catches up with it.^[5] Jupiter's oppositions come every 13 months.^[6] Because orbits are not perfect circles, the closest approach does not always fall on the date of opposition, and some oppositions are much closer than others.^[6]

Conjunctions

A conjunction is when two objects appear to pass closest together in the sky.^[3] They are not close in space: two planets in conjunction only appear close together and remain very far apart.^[7] A planet is in solar conjunction when it passes closest to the Sun as seen from Earth, with all three nearly in a straight line.^[3] Occasionally four or five bright planets can be seen at once with the naked eye, sometimes called a planet parade; this is not exceedingly rare but does not happen every year.^[1]

How to look

No equipment is needed for the bright planets. Binoculars are an excellent first instrument, easy to use and more versatile than most telescopes.^[1] With good binoculars on a steady mount, Jupiter's central band and three or four of its largest moons can be seen.^[6] Where the Moon fits into the evening sky is covered in The Moon.