A solar-powered plane on a round-the-world journey has passed a point about a third of the way to California after taking off from Hawaii.

Solar Impulse 2 pilot Bertrand Piccard called out the sunrise after cruising over the north Pacific on battery power during darkness, according to a live stream from a website documenting the journey.

"Absolutely fantastic moment. That's a sunrise I will remember all my life," he said.

The plane was cruising over the cold northern Pacific on Friday - Earth Day - an occasion the team planned to mark with a live call between Mr Piccard and United Nations secretary-general Ban Ki-moon.

The trans-Pacific leg is the riskiest part of the plane's global travels due to the lack of emergency landing sites.

After some uncertainty about winds, the plane took off from Hawaii on Thursday morning and was on course to land in Mountain View, California, over the weekend. The crew that helped it take off is clearing out of its Hawaiian hangar and heading for the mainland for the weekend arrival.

At one point passengers on a Hawaiian Air jet caught a glimpse of the Solar Impulse 2 before the airliner sped past the slow-moving aircraft.

The Solar Impulse 2 landed in Hawaii in July and was forced to stay in the islands after the plane's battery system sustained heat damage on its trip from Japan.

The aircraft started its round-the-world journey in March last year from Abu Dhabi, the capital of the United Arab Emirates, and made stops in Oman, Burma, China and Japan. It is on the ninth leg of its circumnavigation.

Mr Piccard said the destination in the heart of Silicon Valley is fitting, as the plane will land "in the middle of the pioneering spirit". Co-pilot Andre Borschberg flew the leg from Japan to Hawaii.

The team was delayed in Asia as well. When first attempting to fly from Nanjing, China, to Hawaii, the crew had to divert to Japan because of unfavourable weather and a damaged wing.

A month later, when weather conditions were right, the plane departed from Nagoya in central Japan for Hawaii.

The plane's ideal flight speed is about 28mph, though that can double during the day when the sun's rays are strongest. The carbon-fibre aircraft weighs more than 5,000lb, or about as much as a mid-size truck.

The wings of Solar Impulse 2, which stretch wider than those of a Boeing 747, are equipped with 17,000 solar cells that power propellers and charge batteries. The plane runs on stored energy at night.