Saturn at opposition with Venus and Jupiter

Saturn is the most distant planet that can be seen with the naked eye and this weekend brings it closest to Earth for 2015. Seen as a small star, with a steady light and a slightly yellow-tinge, the planet is currently rising in the east as the sun sets in the west.

In other words, Saturn is directly opposite the sun in the sky – what is known as opposition.

The planet officially reaches opposition at 11:22am on Saturday morning, May 23 (AEST). Being opposite the sun, means that Saturn is visible all night long.

The planet is going to be a lovely sight in the sky over the coming winter months.

Closer but not especially brighter

Opposition also brings Saturn and Earth closer together, as the two planets find themselves on the same side of the sun. But for the next few years, oppositions of Saturn are not as favourable as they can be.

This year’s opposition sees Saturn 10 million km further away than last year. And for the next few years, each opposition will continue to be slightly more distant.

This is because Saturn is approaching aphelion in 2018, that’s the point in its orbit when Saturn is furthest from the sun. Saturn takes 29.5 years to orbit the sun, so it’ll be about a decade before there’s an opposition where Saturn has moved close enough to Earth for it to shine more brightly.

Outshone by the rings

Of course, what makes Saturn stand-out against all the other planets is its lovely ring system that can be seen using binoculars or a small telescope.

Saturn’s rings are currently looking fantastic and are set to get even better still.

From Earth, we can watch the orientation of Saturn’s rings change from being wide-open, when the full extent of the rings can be seen, to being edge-on, when the rings seem to disappear. It’s captured in a simple but fascinating animation seen here.

Simulated views showing Saturn at opposition and the changing tilt of the rings.Tom Ruen

At the present time, the rings are tipped so they appear 24 degrees wide, which is pretty close to the maximum tilt of 27 degrees that will occur in 2017.

Saturn’s changing seasons

This changing view of the rings is linked to Saturn’s seasons. Like Earth, Saturn experiences seasons because its axis is tilted relative to its orbit around the sun.

During its 29.5 year orbit, each hemisphere spends time tilted towards the sun – currently it’s the northern hemisphere’s turn – and the rings appear wide-open from Earth.

And, just like Earth, twice per orbit Saturn reaches equinox when neither hemisphere is tilted towards the sun. Instead the sun shines directly over the equator and the rings appear edge-on.

Night falls on Saturn’s rings at the equinox, August 2009.NASA/JPL/Space Science Institute

In 2009, NASA’s Cassini spacecraft in orbit around Saturn, observed a Saturn equinox for the first time. The rings plunged into a continuous night lasting around four Earth days as the sun climbed from the southern to northern hemisphere.

Trio of planets

Saturn is not the only planet that can be seen in the evening sky. If you turn around and face west, it’s impossible to miss two very brilliant and eye-catching planets.

Venus is the brighter of the two, found closer towards the north-west horizon. The other is Jupiter, which sits higher in the sky and further around to the north. The crescent moon will pair up with Venus for the next two nights and then moves to sit above Jupiter on Sunday, May 24.

The north-west is a very rich part of the sky, that includes the bright star Sirius, its companion Procyon, and the prominent constellation of Orion.

Follow the moon in the north-west sky at sunset.Museum Victoria/Stellarium

In fact, its the last chance to catch Orion in the sky before it sets for the winter. Turning back to the east, you’ll see Saturn rising at the head of Scorpius, as if adding an extra ‘claw’ to the scorpion.

Here playing out before us in the sky is the ancient Greek story of the scorpion forever chasing down the mighty hunter for his conceited ways.