The comet that has stunned star gazers into silence blazing across the skies is now promising an even more spectacular sight.
This week, Comet McNaught will provide a dramatic glimpse of its tail, which is about 30 million kilometres long, about one-fifth of the distance between Earth and the sun.
Dr Grant Christie, an astronomer at the Stardome Observatory at One Tree Hill, said the tail was caused by a combination of pressure from sunlight and the comet's motion.
He said the only reason the tail could be seen was because it was being illuminated by the sun.
"If you were in the middle of the tail, it would look pretty much like a vacuum," he said.
"It's actually material stretched over a long distance and is visible because space is so empty.
"It's very tenuous - almost like cigarette smoke - as opposed to thick, ashy cloud."
Dr Christie said the frozen nucleus of the comet - "mainly water ice with a bit of rock and dust all mixed in" - was about the size of Auckland.
The comet's head could be many thousands of kilometres across.
"The head of the comet will be rotating and, as it does, it's usually squirting out debris through fissures in the surface, creating these little jets," he said.
"And they get wrapped around in arcs of their own."
Throngs of comet-watchers, armed with binoculars, cameras and telescopes, have become a common sight around Auckland's higher vantage points in the past week.
Trafeena Chang, who works at the Orbit restaurant in the Auckland casino's Sky Tower, said it was hard to tell whether people had packed into the eatery for the cuisine or for a ringside view of the comet.
"Maybe the people here are looking at it - you probably couldn't get a better view anywhere else," she said.
"We have heard a few people talking about it but I haven't seen it myself."
Dr Christie said people did not need high-powered telescopes to view the comet.
"A pair of binoculars would do just fine," he said. "A telescope isn't necessarily going to show you any more."
Police and the Fire Service said they had not received any comet-related distress calls since Thursday night, when many people phoned to report strange objects in the sky.
The comet was discovered by Australian astronomer Rob McNaught at Siding Springs Observatory in New South Wales on August 7 last year.
It is expected to be visible to the naked eye until the end of this month.
* This week, the best viewing times are between 9pm and 9.30pm, looking southwest.
* For the best vantage point head to a beach like Piha or Karekare where there is an unobstructed view of the horizon. Alternatively head up high - somewhere like Mt Eden.
* Cloud cover will make it difficult to spot the comet. As well, the brighter the moon gets, the less visible it will become.
* The weather will be patchy during the week, with cloud predicted today from Northland to Manawatu, and rain and drizzle from tomorrow.
* Next week, there will be a new moon in the western sky that will make viewing difficult.