The December 18, 2017 moon is new, or most nearly between the Earth and sun for this month. Astronomers mark the beginning of each lunar month, a period of approximately 29.5 days, at new moon. Lunar months are slightly different lengths, however, and this particular new moon ushers in the longest lunar month of the 21st century (2001 to 2100).
A lunar month (also called a lunation or synodic month) is defined as the period of time between successive new moons. Although the mean length of the lunar month lasts 29.53059 days (29d 12h 44m 03s), this upcoming lunar month will be more than 7 hours longer than the mean, having a duration of 29 days 19 hours 47 minutes.
By the way, it’s no coincidence that the new moon on December 18, 2017, and the following new moon on January 17, 2018, coincide quite closely with lunar apogee – the moon’s most distant point from Earth in its monthly orbit.
December 2017 new moon: Dec 18 at 6:30 UTC
December 2017 lunar apogee: Dec 19 at 1:27 UTC
January 2018 new moon: Jan 17 at 2:17 UTC
January 2018 lunar apogee: Jan 15 at 2:10 UTC
Also, the December 2017 apogee (406,603 km) is the farthest of 2017’s 13 apogees; and the January 2018 apogee (406,464 km) gives us the most distant of 2018’s 13 apogees. What’s more, the Earth will reach perihelion – its closest point to the sun in its yearly orbit – on January 3, 2018.
All these events – successive new moons near apogee and Earth at perihelion – add up to give us the longest lunar month of the 21st century. Click here for an explanation.
The century’s shortest lunar month will take place in between the new moons of June and July 2053, a period of 29 days 6 hours and 35 minutes. That’s 6 hours and 9 minutes shorter than the mean, or 13 hours and 12 minutes shorter than the century’s longest lunar month of 29 days 19 hours 47 minutes.
Again, it’s no coincidence that these new moons in June and July 2053 coincide quite closely with lunar perigee – the moon’s closest point to Earth in its monthly orbit. And, again, it’s no coincidence Earth swings out to aphelion – its most distant point from the sun – on July 3, 2053.
Bottom line: The longest lunar month of the 21st century (2001 to 2100) happens between the new moons of December 18, 2017, and January 17, 2018.
Botton line: Lunar months are roughly 29.5 days. The one between the December 18, 2017 and January 17, 2018 new moons will be 7 hours longer than the mean. It’ll be this century’s longest lunar month.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.