Triple conjunction facts for kids

A triple conjunction is a cool event in space! It happens when two planets, or a planet and a star, seem to meet each other three times in a short period. This happens because of how they move in their orbits around the Sun.

Imagine watching a planet in the night sky. Usually, it moves forward (this is called prograde motion). But sometimes, it looks like it slows down, moves backward for a bit (this is called retrograde motion), and then moves forward again. A triple conjunction happens when this "backward and forward" dance makes the planet line up with another object three times.

Sometimes, three planets can line up in a special way. This is a rare type of triple conjunction.

Triple Conjunctions: Mercury and Venus

Almost every time Venus passes behind the Sun (called a superior conjunction), it has a triple conjunction with Mercury. But often, you can't see the middle part of this event. This is because both planets are too close to the Sun in the sky. When they are too close, the Sun's bright light makes them hard to spot.

Triple conjunctions between Mercury and Venus can also happen when both planets are passing between Earth and the Sun at the same time. This is much rarer. Again, the middle part of this event is usually hidden by the Sun's glare.

Triple Conjunctions: Inner and Outer Planets or Stars

When Mars is on the opposite side of the Sun from Earth, it often has a triple conjunction with Mercury or Venus. If Mercury is involved, the middle part of the conjunction is usually invisible. This is because Mercury is too close to the Sun. The other parts are also hard to see because Mars is far away from Earth and looks very dim.

For a Mars–Venus triple conjunction, you can almost always see all three parts. However, Mars will still look faint because it's so far away from the Earth.

Triple conjunctions can also happen between the inner planets (Mercury and Venus) and the outer planets like Jupiter, Saturn, Uranus, Neptune, or even the dwarf planet Pluto. They can also happen with bright stars. These events occur when Mercury or Venus are passing between Earth and the Sun, and the other planet or star is also lining up with the Sun.

Often, the second part of these conjunctions is too close to the Sun to be seen. But the other parts are usually easy to spot, especially if Jupiter or Saturn are involved, or if it's a bright star. It's harder to see these events with dim planets like Uranus, Neptune, or Pluto because they are already faint and also close to the Sun.

Triple conjunctions involving Mercury or Venus with the outer planets happen fairly often, about once every 10 years.

Triple Conjunctions: Two Outer Planets

These are some of the most exciting triple conjunctions to watch! You can usually see all three parts of the event very clearly. This is because the planets involved are far from the Sun in the sky.

Triple conjunctions between bright outer planets are very rare. For example, Mars and Jupiter had triple conjunctions in 1789–1790, 1836–1837, and 1979–1980. The next ones won't happen until 2123 and 2169–2170.

Triple conjunctions between Mars and Saturn happened in 1779, 1877, and 1945–1946. The next ones will be in 2148–2149, 2185, and 2187. Sometimes, two triple conjunctions between Mars and Jupiter or Mars and Saturn can happen just two years apart.

Conjunctions between Jupiter and Saturn are called great conjunctions. Sometimes, these are triple conjunctions. This has happened seven times between the years 1200 and 2400. The three parts of these conjunctions happen several months apart.

One very famous triple conjunction was between Jupiter and Saturn in 7 BCE-5 BCE. Some people think this event might explain the star of Bethlehem mentioned in history. The last triple conjunctions between Jupiter and Saturn were in 1682–1683, 1821, 1940–1941, and 1981. The next one won't be until 2238–2239.

Triple conjunctions of Jupiter with Uranus or Neptune happen more often. They might not be as spectacular, but they are a great chance for amateur astronomers to find these dim planets. The last triple conjunction between Jupiter and Uranus was in 2010–2011, and the next will be in 2037–2038. The last between Jupiter and Neptune was in 2009, and the next will be in 2047–2048.

Because of how planets appear to loop in the sky, triple conjunctions also happen between a planet and some stars when the planet is in opposition (opposite the Sun from Earth). Triple conjunctions between planets and bright stars near the zodiac (the path the Sun, Moon, and planets appear to follow) are not very frequent, happening about twice every 10 years.

Astronomers measure positions in the sky using two main ways: right ascension (like longitude on Earth) and ecliptic longitude (position along the Sun's apparent path). A triple conjunction might happen in one way of measuring but not the other.

Some Triple Conjunctions Between 2100 and 3000

See also

• Celestial mechanics
• Great conjunction
• Positional astronomy