New Zealanders will be in for an astronomical treat on Thursday when the largest planet in our solar system, Jupiter, disappears behind the dark edge of the Moon at the convenient time of 6.15pm in Auckland and slightly later elsewhere.
The spectacular naked eye event will be easily visible throughout most of the country and children especially will enjoy seeing the collision of the two brightest objects in the night sky.
Their keen eyes will readily detect the slow dimming of the giant planet and will beat adults at spotting its reappearance - a much harder task - an hour or so later on the bright side of the Moon. No special equipment is needed, but good binoculars or a telescope will enhance the experience.
They may also enable watchers to see some of the reddish bands across the silvery disc of Jupiter and will certainly show the main four of its many moons: Io, Europa, Ganymede and Callisto.
These Galilean satellites, as they are called, were discovered in 1609 by Galileo, who at first thought they were nearby stars.
When observation showed that he was seeing moons orbiting Jupiter, he understood that the accepted view of the universe - that all celestial objects revolved around the Earth - was wrong.
Such proof helped to sweep away the old ideas of classical cosmology and lay the rational foundations for modern astronomy.
Thursday's event is known as a Jovian occultation - Jovian from Jupiter, and occultation meaning hidden. An occultation happens when a moon or star or asteroid passes in front of another celestial object (eclipses are when one object is obscured by the shadow of another).
By timing and observing occultations and eclipses, astronomers can derive an enormous amount of information, from the shape of an asteroid to the temperature and structure of a planet's atmosphere.
Before accurate maritime clocks were invented, timing eclipses of Jupiter's moons was one of the few ways to measure longitude at sea.
Captain Cook's outstanding navigational skills, for example, are directly attributable to his skill with astronomical instruments and tables.
Graham Blow, director of the Royal Astronomical Society of New Zealand's occultation section, says timing the exact instant an occultation occurs is one of the most important ways in which amateurs can contribute to science and he is keen to encourage people to give it a go. No special equipment is necessary, although digital technology including webcams, videos and GPS devices are proving useful.
And if gazing heavenwards turns your thoughts to the spiritual, don't be surprised.
Historian and astronomer Dr Michael Molnar suggests that it was a significant event like this, a Jovian occultation, which is the true explanation of the Christmas Star.
BY JUPITER
* Jupiter is the solar system's largest planet; it is about 140,000km across - 11 times Earth's diameter.
* It is made up mostly of water, hydrogen, helium, nitrogen and other gases.
* Its surface appears striped because of winds in its upper atmosphere.
* 63 moons orbit Jupiter, though new ones are discovered occasionally.
Astronomical treat right on teatime
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