Moon-Planet Conjunctions — The Possibilities Are Infinite!

The waxing gibbous moon will shine just 1.4° above the brightest evening planet, Jupiter, this evening (June 3). The ecliptic is the invisible celestial highway the moon, the planets and the sun follow around the sky, each at their own pace. The moon completes the circle in under a month, the sun in a year and Jupiter in 12 years. Created with Stellarium
By happy chance, there will be a double transit of Jupiter’s moons Io and Ganymede across the planet tonight. You can see their shadows as black dots on the clouds below. Io’s shadow enters the planet starting at 9:12 CDT and leaves the disk at 11:23 p.m. Ganymede’s shadow — noticeably larger than Io’s — enters the disk at 9:23 p.m. and departs at 11:39 p.m. Credit: Meridian software

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.

Earth’s axis maintains a 23.5° tilt as it orbits the sun, but its changing position in orbit causes the axis to point toward, away and sideways to the sun during the year. Credit: Sonoma University with additions by the author

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.

The path the sun takes in the sky is caused by the tilt of Earth’s axis. In summer, the sun rides high and days are long; in winter its path is much closer to the horizon and days are short. Credit: Wiki

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.

The moon’s orbit is tilted about 5° with respect to the ecliptic plane, defined as the Earth’s orbit around the sun. Not only does it affect how one conjunction varies from the next, but it’s also the reason we only have couple lunar and solar eclipses a year: most the time the moon is out of line with the Earth and sun. Credit: Bob King

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.

The moon changing position and phase when it conjuncts with Jupiter in the next three months as seen from U.S. Midwest. Because the moon never stops moving (to the east or left in these pictures), it can appear a little closer or further from the planet depending upon your location. For instance, on June 30, skywatchers on the West Coast will see the moon a full degree closer to Jupiter when it gets dark there because of the two-hour time difference. Created with Stellarium

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.

The clockwise turning of the moon’s orbit around the Earth combined with the tilt of that orbit cause the moon’s range in altitude in the sky to vary with a period of 18.6 years. Credit: Matt/Wikipedia/CC0

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.