This week, look for the ringed planet Saturn to light up the night sky from dusk till dawn. Our planet Earth is about to fly between Saturn and the sun, bringing Saturn to what astronomers call opposition on June 27, 2018. In other words, Saturn is virtually opposite the sun in Earth’s sky right now, presenting a big milestone for observing the ringed planet! We go between Saturn and the sun on June 27 at 13 UTC. For the Americas, that places Saturn’s opposition on June 27 at 10 a.m ADT, 9 a.m. EDT, 8 a.m. CDT, 7 a.m. MDT, 6 a.m. PDT, 5 a.m. Alaskan Time and 3 a.m. Hawaiian Time.
So Saturn’s special day, its yearly opposition is upon us, when Saturn lies opposite the sun in Earth’s sky. As a consequence, Saturn rises in the east around sunset, climbs highest up for the night around midnight and sets in the west around sunrise. It is visible all night, closest and brightest for this year.
Although Saturn comes closest to Earth for the year on the same date that it reaches opposition (June 27), the ringed planet comes nowhere as close to Earth as the NASA illustration at the very top of this post might lead you to believe. At present, Saturn lies some 10 times the Earth’s distance from the sun, or 9 times the Earth-sun distance from Earth. Astronomers refer to the Earth-sun distance as the astronomical unit, or AU. Saturn is now 10 AU from the sun, and 9 AU from us.
So the distance scale of the image at top is off, and so is the size scale. For a realistic depiction of Saturn’s size relative to that of Earth, see the illustration below.
Don’t assume Saturn’s opposition is a one-night-only event. The ringed planet will be in good view throughout July, August and September 2018. You can recognize Saturn because it’s in your southeast sky at dusk and nightfall. Saturn will remain a fixture of the evening sky until December 2018. All the while, golden Saturn shines in front of the constellation Sagittarius, to the north of the Teapot asterism.
Our fast movement in orbit brings Earth between Saturn and the sun every year – or, more precisely, about two weeks later every year. Two years ago, for instance, Saturn’s opposition happened on May 23, 2015. Last year, in 2016, it was June 3. This year, opposition happens on June 15, 2017. Next year, in 2018, Saturn’s opposition will come on June 27. If you recognize this golden world tonight or later this month, you’ll also enjoy it throughout the Northern Hemisphere summer, or Southern Hemisphere winter.
If you had a bird’s-eye view of the solar system today, you’d see our planet Earth passing in between the sun and Saturn. You’d see the sun, Earth, and Saturn lining up in space. But not for long. Earth moves in orbit at 18 miles (29 km) per second in contrast to about 6 miles (9 km) per second for Saturn. Soon, we’ll be pulling ahead of Saturn in the race of the planets.
The inner planets – Mercury and Venus – can never be at opposition, because they orbit the sun inside Earth’s orbit. Only the planets that orbit the sun beyond Earth’s orbit – Mars, Jupiter, Saturn, Uranus and Neptune – can ever reach opposition, that is, appear opposite the sun in Earth’s sky.
All the planets farther from the sun reach opposition every time our swifter-moving planet sweeps between the sun and these superior planets – planets that orbit the sun outside of Earth’s orbit. Mars returns to opposition every other year. Jupiter’s opposition happens about one month later each year, whereas Saturn’s opposition occurs about two weeks later yearly. The farther that a planet resides from the sun, the shorter the period of time between successive oppositions.
Saturn, the 6th planet outward from the sun, is the most distant world that’s easily visible to the unaided eye. Telescopes revealed its rings in the 17th century. Spacecraft in the 20th century revealed that what we thought of as three rings around Saturn to be thousands of thin, finely detailed rings – made of tiny chunks of ice. Saturn also has 62 moons with confirmed orbits. Only 53 of Saturn’s moons have names, and only 13 have diameters larger than 50 kilometers (about 30 miles).
Saturn is truly a wondrous world of rings and moons. It’s everyone’s favorite thing to celestial object to gaze at through a small telescope, so if there’s a public astronomy night near you this month – go!
Bottom line: Look for Saturn at and around opposition – June 27, 2018. It will be shining in the east this evening, fairly close to the moon. Can’t see Saturn tonight? No problem. It’ll be in an excellent place to observe throughout June, July and August 2018.