We go between Saturn and the sun

Illustration above via NASA; not to scale.

Our planet Earth flies this week between Saturn and the sun, bringing Saturn to what astronomers call opposition. The exact date and time of opposition are June 27, 2018, at 13:00 UTC. That is June 27 at 10:00 a.m ADT, 9:00 a.m. EDT, 8:00 a.m. CDT, 7:00 a.m. MDT, 6:00 a.m. PDT, 5:00 a.m. Alaskan Time and 3:00 a.m. Hawaiian Time; click here to translate UTC to your time. And don’t worry about exact times too much. Just know that – around now – Saturn is more or less opposite the sun in Earth’s sky, rising in the east around sunset, climbing highest up for the night around midnight and setting in the west around sunrise. When opposite the sun, Saturn is visible all night.

Opposition is a big milestone each year for observing the ringed planet Saturn, or any superior planet (planet orbiting the sun outside Earth’s orbit). When we fly between a superior planet and the sun, the planet is generally closest and brightest for that year.

Viewing Saturn’s rings soon? Read me 1st

You need a telescope to see Saturn’s rings. But you can easily spot the planet with the eye alone with this week. How? Let the moon guide you. The moon is full on the night of June 27-28, and what is a full moon if not a moon opposite the sun? Like a planet at opposition – opposite the sun – a full moon always rises in the east at sunset, climbs highest around midnight and sets in the west around sunrise. For that reason, around the time of its opposition, a superior planet is always near the moon when it turns full.

This particular full moon will be closest to Saturn on the exact date of opposition as shown on the chart below:

Watch for the bright moon to swing near Saturn and then Mars from June 27-30, 2018. On the night of full moon – June 27-28, 2018 – the moon will be near Saturn. June 27 is also the date of Saturn’s yearly opposition.

To understand why Saturn at opposition is near the full moon, compare this image to the one at the very top of this post, showing Saturn at opposition. At every full moon, the sun, Earth and moon line up, with the moon opposite the sun. We on Earth (between the sun and the moon) see the sun setting in the west and the moon rising in the east at roughly the same time. Likewise, we on Earth will see Saturn at opposition rising at sunset. Image via RASC Calgary Centre.

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, and nine 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. Heavens Above gives information about the present distances of the planets from the sun and Earth.

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.

Contrasting the size of Saturn and its rings with our planet Earth via Hubble Heritage Team.

Contrasting the size of Saturn and its rings with our planet Earth via Hubble Heritage Team.

Also, 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.

More tips on seeing Saturn throughout 2018

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The brightness of Saturn at opposition is partly determined by the orientation of its rings with respect to Earth. In 2016, the rings are wide open, tilted by 26-26.8 degrees, showing their northern face to Earth. Image via Hubble Heritage.

The brightness of Saturn at opposition is partly determined by the orientation of its rings with respect to Earth. In 2018, the rings are wide open, showing their northern face to Earth. Image via Hubble Heritage.

Our fast movement in orbit brings Earth between Saturn and the sun every year – or, more precisely, about two weeks later every year. Three years ago, for instance, Saturn’s opposition happened on May 23, 2015. In 2016, it was June 3. In 2017, it was June 15. This year, 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 planets that orbit the sun inside of Earth’s orbit – Mercury and Venus – can never be at opposition. 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.

Full moon passing Saturn on June 9, 2017. A full moon is opposite the sun, too, so it makes sense it would pass a planet near that planet’s opposition. Photo by Sue Christopher in San Dimas, California.

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 celestial object to gaze at through a small telescope, so if there’s a public astronomy night near you this month – go!

Crescent moon Rhea occults - or passes in front of - a crescent Saturn, as seen by the orbiting Cassini spacecraft.  Image via Cassini Imaging Team, SSI, JPL, ESA, NASA.

The Cassini spacecraft, which has been orbiting Saturn since 2004, has obtained almost unbelievably stunning images of the planet. Here, a crescent moon, Rhea, occults – or passes in front of – a crescent Saturn. Image via Cassini Imaging Team, SSI, JPL, ESA, NASA. See more images of Saturn from Cassini.

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.

Stunning images of Saturn’s moons Dione and Enceladus

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Bruce McClure