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Our solar months, January, February, etc, are based on the movement of the sun, and the lunar months on the movement of the moon. The lunar months are based on the synodic lunar month, the period between two dark moons; in other words, how we see the moon from Earth. The same is true of the months for Mercury, and the other four bright planets, and they help us follow their movement around our sky.
The planet months feature in the 2018 Moonwise Diaries for the first time, and the month names also feature in abbreviated form around the sky map on the calendar.
Mercury’s synodic year is the period from one inferior conjunction to the next. Inferior conjunction is when Mercury passes this side of the sun, and the year is quite variable, due to the planet’s eccentric orbit. It can be anything from 105 to 129 days long. This is split into four months. More specifically, it is split into eight fortnights, with four of the fortnights starting at the conjunctions and greatest elongations. The months start at the midpoints between these four, so that Mercury in the Morning has the greatest elongation west somewhere in its middle. Mercury years start with Mercury in the Evening, and 22 of them make a cycle of seven Gregorian years minus about seven days. Similar viewing possibilities happen at the same point in every cycle.
Venus’ synodic year is about 584 days, and is split into twenty months, in a similar way, but this time with months of equal length. The month is reckoned to start on the day (starting 05:00 UTC) when the exact 20th of the year begins. Five Venus years form a cycle of almost exactly eight Gregorian years.
Mars’ synodic year — from one opposition to the next, in this case — is about 780 days, and is split into 26 equal length months. The year starts with Mars in the Morning L, and finishes with Mars Far, when Mars is back behind the sun. The midpoint of the year is the glory of Mars opposition, when Mars can outshine Jupiter. The cycle is 7 or 8 years, with Year 1 containing the glorious opposition nearest Mars perihelion.
Jupiter’s synodic year of about 399 days is divided into 13 equal months, and its opposition is in the middle of Jupiter Night. During that month, the planet rises around sunset and sets around sunrise, and shines at its brightest. In the Far months, Jupiter is around the other side of the sun. The cycle is of 11 years, sometimes ten, with Year 1 being the first opposition with the sun in Aries.
Saturn’s synodic year is about 378 days, not much longer than the Gregorian year, as Saturn moves so slowly through the stars. It is split into thirteen slightly shorter months, and has a cycle of 28 or 29 years, corresponding to 29 or 30 Gregorian ones, again starting with the first opposition when the sun is in Aries. Years 1 and 15 also correspond roughly with the time Saturn’s rings are edge on, and hard to see.