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The best planet duo of 2015 - Venus and Jupiter

All eyes on Venus and Jupiter - this image from Austria, June 15. H. Raab/flickr, CC BY-NC-ND

They are the two brightest planets in the night sky – the cloud-covered world of Venus and the enormous gas giant Jupiter. Put them together and it’s a double delight. We are set for a stunning sight at the end of the month that’s sure to attract attention.

Over the next fortnight, the two planets will slowly make their way towards each other in the north-western sky. They are visible each evening for a few hours after sunset.

On Saturday, June 20, the thin crescent moon will sit alongside the duo, with Venus below and Jupiter above.

After sunset on June 20, the crescent moon (enlarged in this image) alongside Venus and Jupiter. Museum Victoria/stellarium

But by July 1, the distance between the two planets will have dramatically shrunk - the pair will mimic a splendid double star.

In real terms, the two planets are over 800 million km apart but to us here on Earth they’ll appear to be almost touching.

The wanderers

In our everyday experience, there’s not much to see changing in the night sky.

The stars appear fixed within their constellations, rising and setting at the same time each year. It’s only on time scales measuring tens of thousands of years that the stars drift far enough to mess with our constellation patterns. It makes for fantastic animations as seen here, but causes no confusion for backyard stargazers.

Other changes are more of the transient kind - meteors, comets, novae and supernovae that on the right occasion can light up the sky quite spectacularly.

Such sights are impressive but there’s something to be said about the stately movements of the planets. They provide an easy focus for checking the sky each night (or month) to see which ones are about and where they sit against the backdrop of the distant stars.

Tied to the zodiac

The planets are always found within the band of constellations known as the zodiac. This marks out the plane of our solar system.

The sun follows a path in the sky that runs through the zodiac constellations.

Imagine that we could see both the sun and the stars at the same time. As the Earth moves around the sun, it causes the sun to drift against the background stars. The path the sun appears to follow is called the ecliptic and the zodiac constellations are built around it.

It then follows that since all the planets orbit the sun within roughly the same plane, they too are found closely bound to the zodiac constellations and that’s where we watch them pass by each other.

Linked to the sun

Venus, being closer to the sun than the Earth, can only be seen setting in the west after sunset or rising in the east before sunrise. It’s the same for Mercury. It’s as if the planets are tied to the sun and can never drift too far away.

The outer planets are another story. Just last February, Jupiter reached opposition, a time when the planet rises in the east as the sun sets in the west. But over the last four months, Jupiter has slowly made its way to the western sky to now meet up with Venus.

The two planets meet on July 1. Museum Victoria/stellarium

Officially the two planets reach conjunction, or appear closest together, at 2:02pm (AEST) on July 1. But they won’t be visible in the daytime sky. That evening is when we will see them at their closest and they’ll also look great a few days either side of that date.

With their dazzling brightness and their light so steady, compared to the twinkling light of the stars, it’ll be good fun to watch the planets as they approach each other over the coming nights.

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