How to watch The Great Conjunction in Whanganui

The Ward Observatory will be open on Monday. Photo / Bevan Conley

Whanganui's Ward Observatory will be opening on Monday as the solar system's largest planets will look as though they are going to collide with each other in what's called The Great Conjunction.

The phenomenon occurs between December19 and 23. Saturn and Jupiter are closest together on December 21.

The Ward Observatory will be open from 9pm, if the weather is clear, so people can see them. A visit costs an adult $2, and children are free.

"It's a beautiful thing to watch, particularly since the moons of both planets will be intermingling," Whanganui Astronomical Society president Ross Skilton said.

On that night Jupiter and Saturn will look closer together than they have for 400 years. This has prompted Northern Hemisphere people to call the resulting point of light the Christmas Star or Star of Bethlehem - because it appears so close to Christmas.

Jupiter and Saturn will be visible on the western horizon about half an hour after sunset, to the right of the setting sun. They will move east and disappear from view by about 10.30pm.

For several days they will be unusually close together and visible to the naked eye. They will also be worth looking at through binoculars, for those who cannot get to the observatory.

Keen sky watchers will want to make the visit, Skilton said. For the rest of the summer there will not be many bright planets to look at in our night sky.

On December 21 there will also be a waxing crescent moon.

The moon and planets will be viewable through the observatory's refurbished Cooke Telescope, the largest unmodified refractor telescope still in use in New Zealand. The society will also have smaller portable telescopes working.

The last time Saturn and Jupiter were visible close together from Earth was in 1623. The won't be another conjunction until 2080. So for most people seeing this one will be a once-in-a-lifetime chance.

Such is the excitement about the event that Whanganui Astronomical Society secretary Mark Lee has even been asked whether there is any danger of the two planets colliding. There isn't - they will be millions of kilometres apart even when they look close together to us.

On December 21, Jupiter will be 763 million km from Earth and Saturn will be another 856 million km beyond.

The conjunction happens at the Northern Hemisphere winter solstice and for us it's the summer solstice - the longest day of daylight in our year, with the sun highest in the sky. The solstice and conjunction coming together are a complete coincidence.

Saturn and Jupiter are the biggest planets in our solar system. They are gas giants, and each of them has moons that are not usually easy to see. Jupiter is the closer and brighter of the two.