Every month, the traveling moon circles across the entire sky, passing through the 12 constellations of the zodiac and briefly lining up with various bright planets that currently inhabit those star groups: Jupiter in Virgo, Saturn in Ophiuchus (not an official zodiac constellation, but that’s another story) and Venus in Pisces. The alignments are called conjunctions, and tonight (June 3), the moon will line up just 1.4° north of Jupiter. That’s pretty close as conjunctions go, so I encourage you to poke your head out for a look.
During it’s 27-day orbit around the Earth, the moon swings from high to low in the sky just like the sun alternates from high in the sky in summer to low in winter and back to summer over the course of a year. These cyclic swings in altitude are a consequence of the 23.5° tilt of Earth’s axis.
If the planet rotated straight up and down with zero tilt, the sun’s path would never vary during the year. From the equator, for instance, the sun would pass directly overhead every day at noon. From mid-northern latitudes, it would always appear halfway up in the southern sky at noon and from the north and south poles, it would forever circle the horizon. We’d be stuck, astronomically speaking, on the first day of spring (or fall) 365 days of the year.
Now the curious thing about the moon is that it’s orbit is tilted 5° with respect to Earth’s orbit around the sun, so it doesn’t follow the ecliptic exactly like the sun but can appear up to 5° above or below it. Five degrees is about the width of three fingers held together at arm’s length against the sky.
A brief lesson in planet orbit tilts. Note: Saturn’s tilt is given as 3.4°, but it’s actually 2.5°. The others are correct.
The planets are also inclined to Earth’s orbit. Jupiter’s orbit is tipped 1.3°; Mars 1.85° and Saturn 2.5°. If you now put everything into motion, you can picture the planets cycling around the sun (and moon about the Earth) continually dipping above and below the ecliptic like weaving spaghetti noodles. That’s one of the reason why conjunctions between the moon and planets vary over time and are never identical.
Another is that during the 27 days the moon travels around the Earth, the Earth is moving around the sun, causing the planets and stars to drift westward. By the time the moon comes around for its next Jupiter conjunction on June 30, Jupiter has moved more than 30° to the west, so they won’t line up the same way they did the month before.
For the same reason, the conjunction might happen at 3 in the afternoon, when you can’t see the moon. By the time it’s evening, moon and planet will be further apart. Nor will the moon be in quite the same phase when it passes the planet. Since Jupiter will have moved westward during the month, the moon doesn’t have to travel as far to reach it the next time around, so its phase will be less: gibbous instead of full or thick crescent instead of half.
If this isn’t enough of a beautiful mess, the Earth and sun’s gravity torque the moon, causing its orbit to rotate clockwise around the Earth, completing one full circle every 18.6 years. Since we learned that the moon’s orbit is tilted with respect to the Earth, that means that the direction of that tilt changes over over the 18.6 years, alternately adding to or subtracting from the 23.5° tilt of Earth’s axis.
When it adds to it, the moon travels the full 5° both above and the lowest and highest points of the ecliptic, reaching its northern and southern extremes in the sky. Astronomers call this a major lunar standstill. When it subtracts from the tilt, the moon climbs to 5° below the high point and 5° above the low point, restricting its altitude range. This happens during a minor lunar standstill. Of course, the moon can be between minor and major standstills, too — that’s where we are right now.
This simplified version of the diagram above shows the range of the moon’s motion in the sky. The sun follows the ecliptic exactly, but the moon’s tilted orbit coupled with the gravitational tug of Earth and sun, cause its range to vary above and below the ecliptic with a period of 18.6 years. The orange area shows the range during a major standstill, the yellow during a minor standstill. The numbers are degrees the moon is north or south of the celestial equator (Earth’s equator extended into space) during the standstills. Credit: Sarah Fuhro/additions by the author
There are even more factors affect the appearance of conjunctions. Earth’s revolution causes the timing of the moon’s phases to lag throughout the year and the zigzags of planetary retrograde motion play cat-and-mouse with the moon, but I suspect you’ve got plenty to digest for now 😉 I wanted you to see that the variety and appearance of repeated moon-planet conjunctions is virtually infinite when you combine the moon’s tilted and torqued orbit, its changing phase, the tilts of the planets’ orbits and Earth’s revolution around the sun.
Yes, like snowflakes.