When to see the planets

Morning star and evening star

A planet east of the Sun sets after it and is seen in the west after sunset; a planet west of the Sun rises before it and is seen in the east before sunrise. Mercury and Venus, inside the Earth’s orbit, swing from one side of the Sun to the other and never get far from it: Mercury at most about twenty-eight degrees, Venus about forty-seven. Mars, Jupiter and Saturn go all the way round. After they pass behind the Sun they come back in the morning sky, rise a little earlier every night until they stand opposite the Sun and are up all night, and then linger in the evening sky, sinking into the dusk until they meet the Sun again.

Why the lanes look so different

Each planet repeats its pattern after one lap against the Sun as we see it, and the laps differ a great deal. Mercury takes about 116 days, so its lane is cut into short bands: three evening showings and three morning ones in most years, each lasting a few weeks. Venus takes about 584 days, more than a year and a half. It spends roughly eight or nine months on one side of the Sun, disappears for a week or two when it passes between us and the Sun, and for about two months when it goes round the far side, because there it moves across the sky almost as fast as the Sun does and takes a long time to pull clear. On this chart, which counts Venus only when it is 10° or more from the Sun, the far-side gap shows as nearer three months.

Mars is the slow one. A lap takes about 780 days, a little over two years, and the Sun overtakes it so gradually that Mars can be lost in the glare for three months or more, then spends the better part of a year climbing out of the morning sky. Some calendar years have no Mars opposition at all, and the lane shows no diamond. Jupiter, at about 399 days, and Saturn, at about 378, are the regular ones: roughly a month hidden, half a year in the morning sky, half a year in the evening, with each opposition falling a few weeks later than the one before. The long view of two of these laps is on the Venus cycle and Mercury cycle pages.

Telling a planet from a star

Once the chart says a planet is up, three things help to pick it out. It lies along the zodiac, the same strip of sky the Sun and Moon follow, and never far to the north or south of it. It shines with a steadier light than a star, which flickers more, most of all near the horizon. And it is usually bright: Venus outshines everything in the night sky except the Moon, Jupiter comes next, and Mars near opposition turns a plain orange that no bright star quite matches. Saturn is the quiet one, a steady yellowish point about as bright as the brighter stars. When two of them draw close together, the dates are on the Conjunctions page.

How it is worked out

For every day of the year, at noon universal time, the page takes the angle between each planet and the Sun as seen from the centre of the Earth, and which side of the Sun the planet is on, from the same astronomy as the rest of the site. A day counts as visible when the angle passes the rule of thumb for that planet. The meetings with the Sun and the oppositions are found to the minute. The page does not know your horizon or your weather; far from the equator a summer sky that never gets dark can hide even a bright planet, and how steeply the zodiac meets the horizon changes how high a planet stands at the same distance from the Sun.

What it claims

The chart says where to look and when, and nothing more. The tradition has long given meaning to a planet rising before the Sun or setting after it; that is described on the pages for each planet, and this page predicts nothing.