Moon swings by Saturn and Jupiter on late May and early June mornings
On the mornings of May 30 to June 2, 2021, watch for the waning moon to sweep by two bright planets – two outer solar system gas giants – golden Saturn and bright Jupiter. Unless you’re a night owl, you probably won’t see the moon, Saturn and Jupiter rising into your sky before your bedtime. But they’ll be glorious at dawn. Look for the threesome – the moon, Saturn and Jupiter – along the path that the sun and moon travel across your sky. They’ll be visible just before dawn, and (in the case of Jupiter and the moon), as dawn is beginning to break.
Neptune and Pluto? Though shown on our chart, they’re not visible to the eye. Yet these outer worlds are near Jupiter and Saturn, too, on our sky dome, as we stand gazing outward from Earth.
It’s pretty easy to distinguish Jupiter from Saturn, because Jupiter is much brighter. Although Saturn shines as brilliantly as a 1st-magnitude star – or one of the brightest stars in our sky – Saturn pales next to Jupiter, the largest planet in our solar system. Jupiter outshines Saturn by some 16 times. Jupiter ranks as the 4th-brightest celestial body to light up the heavens, after the sun, moon and the planet Venus, respectively. Fortunately, there’s no way to mistake Jupiter for Venus, or vice versa, because, in May 2021, Venus is up in the evening sky.
The inclination of Saturn’s rings in 2021
If you can view Saturn and Jupiter with the unaided eye, you should be able to spot Saturn’s rings and Jupiter’s four major moons with an optical aid. Saturn’s rings are inclined at about 17 degrees toward Earth in May – in other words, they’re very open now and easy to see – and they’ll be readily observable through a telescope throughout 2021. As May began, astrophotographers were beginning to glimpse Saturn’s southern hemisphere, which has been largely hidden from us due to the extreme inclination of Saturn’s rings this year (in 2021, Saturn’s rings and north pole reach their maximum tilt toward Earth of 20.9 degrees in January). It’s a peculiarity of Saturn that, when a hemisphere moves from winter to spring – as is happening now in Saturn’s southern hemisphere – that hemisphere takes on a bluish tinge. Read more: Saturn’s cold blue hemisphere
Want to know the inclination of Saturn’s rings for all of 2021? The info below is from the indispensable Observer’s Handbook (p. 225). Saturn’s ring tilt in 2021:
January 1: 20.9 degrees
February 1: 19.6 degrees
March 1: 18.5 degrees
April 1: 17.5 degrees
May 1: 16.9 degrees
June 1: 16.8 degrees
July 1: 17.3 degrees
August 1: 18.1 degrees
September 1: 19.0 degrees
October 1: 19.4 degrees
November 1: 19.3 degrees
December 1: 18.7 degrees
January 1, 2022: 17.6 degrees
Although you need a telescope to see Saturn’s rings, a favorable tilt of Saturn’s rings toward Earth in 2021 helps to make Saturn all the brighter to the eye alone. Saturn’s rotational axis is tilted at 27 degrees out of vertical, relative to the plane of Saturn’s orbit around the sun. So whenever it’s solstice time on Saturn, Saturn’s rings are inclined at a maximum of 27 degrees toward Earth. Saturn’s most recent solstice occurred in May 2017, and will next place in April 2032.
The rings circle Saturn above Saturn’s equatorial plane. So when Saturn is at an equinox, we then see the ring’s edge-on from Earth. Saturn’s next equinox will take place on May 6, 2025. At and around that time, Saturn’s rings will appear as a thin line to the eye, or possibly even disappear from view.
Don’t forget to look at Jupiter
A small backyard telescope will also show you Jupiter’s 4 major moons – sometimes called the Galilean moons – with binoculars.
Jupiter had its most recent equinox on May 2, 2021. That means we are viewing Jupiter’s equator pretty much edge-on this year. Because the orbits of the Galilean moons coincide with Jupiter’s equatorial plane, the moons of Jupiter tend to eclipse one another during a Jovian equinox year. Jupiter will swing to its next equinox on December 16, 2026.
Expect to see Jupiter’s moons lining up on or near the same plane all year long. Now and again, a moon or two could be “missing” as these moons flit in front of or behind Jupiter. This year, in 2021, it’s even possible for one moon to hide another moon.
The outermost Galilean moon – Callisto – is the only one of Jupiter’s four major moons not to swing in front of and in back of Jupiter during each orbit around the king planet. At a Jovian solstice, Jupiter’s rotational axis is most inclined toward Earth. Centered on a solstice, Callisto swings to the north and south of Jupiter, as seen from Earth, for a few years. Jupiter’s last solstice occurred on April 27, 2018, and will next happen on January 20, 2024.
Bottom line: These next several mornings – May 30 to June 2, 2021 – use the moon to guide you to the bright planets, Saturn and Jupiter.