When a planet is closest to the sun–in terms of where we see it in the sky, not where it is in orbit–it is in solar conjunction. Sometimes, astronomers shorten the description to simply say it is in conjunction. Most of the planets are on the far side of the sun when they are in solar conjunction.
However, Mercury and Venus orbit closer to the sun than Earth. They can pass between the earth and the sun. Such a conjunction is called inferior conjunction. When Mercury or Venus is on the other side of the sun, it’s said to be in superior conjunction. In these cases, the uses of inferior and superior to describe the planets’ conjunctions comes from meanings of the words that have to do with being lower than or higher than other things. To say that one conjunction is better or worse than another doesn’t make sense and isn’t what’s meant by the words.
Usually, Mercury and Venus don’t pass directly between the earth and sun during inferior conjunction. When they do, they create the appearance of a black dot crossing the sun’s disk. The event is called a transit. The last transit of Mercury was in 2016. The next one will be in 2019. The last transit of Venus was in 2012. The next one will be in 2117.
Here are solar conjunction times of the planets during 2017. All times are in Universal Time.
March 7, 2017 – 04:39 Mercury 1.7° from Sun. Superior conjunction.
April 20, 2017 – 09:15 Mercury 1.6° from Sun. Inferior conjunction.
June 21, 2017 – 11:59 Mercury 1.1° from Sun. Superior conjunction.
August 27, 2017 – 02:30 Mercury 4.2° from Sun. Inferior conjunction.
October 9, 2017 – 01:32 Mercury 1.1° from Sun. Superior conjunction.
December 12, 2017 – 23:35 Mercury 1.7° from Sun. Inferior conjunction.
March 25, 2017 – 17:00 Venus 8.3° from Sun. Inferior conjunction.
July 27, 2017 – 00:34 Mars 1.1° from Sun.
October 26, 2017 – 18:04 Jupiter 1.0° from Sun.
December 21, 2017 – 21:10 Saturn 0.9° from Sun.
April 14, 2017 – 05:29 Uranus 0.6° from Sun.
March 2, 2017 – 02:46 Neptune 0.8° from Sun.