When planets meet

What a conjunction is

Two planets are in conjunction when they stand at the same longitude on the zodiac as seen from the Earth. They are not near each other in space: at a great conjunction Jupiter and Saturn are still hundreds of millions of kilometres apart, one behind the other. Because the planets’ orbits are tilted a little against each other, most meetings pass a degree or more apart, one planet north of the other, and only a few in a decade come closer than the width of the full Moon.

How often each pair meets

The slower the two planets, the rarer the meeting. Jupiter and Saturn come together about every twenty years. Mars catches Saturn about every two years and Jupiter about every twenty-seven months. Mercury and Venus are a different case: they never go far from the Sun, so over time they keep the Sun’s pace round the zodiac, and each of them passes Mars, Jupiter and Saturn roughly whenever the Sun does. That is why so many of their meetings with the outer planets happen in the glare, near the outer planet’s own conjunction with the Sun, and why the good ones, well out in a dark sky, are worth marking. When one of a pair turns retrograde near the meeting point, the two can pass each other three times in a few months instead of once.

Judging the gap, and why dates differ

The full Moon is about half a degree wide. A little finger held at arm’s length covers about one degree, and a closed fist about ten, so a separation of two degrees on the list is a couple of finger-widths in the sky. Seen from the southern hemisphere the pair is the other way up, with the planet listed as north standing below.

Other calendars sometimes give a slightly different day for the same meeting. Many astronomical almanacs time a conjunction by right ascension, measured along the celestial equator, while this page, like astrology, uses longitude along the zodiac. Others list the moment the two are closest on the sky, called an appulse. All three describe the same pass, and they can fall hours or a day or two apart. How a conjunction between planets in a birth chart is read is on the Conjunction page.

How it is worked out

For each of the ten pairs the page compares the two longitudes day by day and halves down to the minute they are equal, from the same astronomy as the rest of the site. That minute is the conjunction, and its date is the one listed. At that minute the page measures the true angle between the two planets on the sky as seen from the centre of the Earth, which at equal longitude is simply how far one stands north of the other, and how far each stands from the Sun. The two are not always closest then: when one of them is slow or about to turn, the gap can go on closing for hours or a few days, so where they come at least three minutes of arc nearer than at the meeting, the list gives that distance and its date as well. A pass in which one planet turns back just short of the other is not a conjunction by longitude, and it is not listed. Closer than about twelve degrees a planet is usually lost in the Sun’s glare. That is a single rough rule for the whole list, where the visibility page uses one for each planet, and the horizon, the weather and your latitude all matter. Dates and times are by this device’s clock.

What astrology makes of it

A conjunction is read in astrology as the two planets’ themes blending, and the meetings of Jupiter and Saturn were long used to mark eras: for about two centuries they fall in signs of one element, and the meeting of 2020 was the first of a run in air signs. That is the tradition, described here. The page gives the dates and distances and predicts nothing.