They say it works on cloudy days, and when the Sun has set.

BBC: 'Viking sunstone' found in shipwreck

But for a solar eternal plane to be exactly that it needs an efficient energy harvesting, conversion and storage system that will work when the Sun has set.

BBC: NEWS | Science/Nature | 'Eternal planes' to watch over us

Unlike Antarctica, which is protected by an international agreement, numerous countries lay claim to parts of the Arctic, which makes it hard to pin down exactly how many tourists visit the region each summer, when the sun does not set below the horizon and the ice melts enough to allow expedition ships passage to the remote wilderness.

BBC: Life in the fragile, frozen Arctic

Knowing your location, the app pulls local information on when the sun will rise and set that day.

FORBES: Triggertrap Happy: A Modular, Robust App-based Intervalometer

Birky told CNN the crew was set to do a search when the sun came up, but received a call while they were en route that a good Samaritan boat found Walter Marino about 7:30 a.m.

CNN: Father, son rescued after more than 12 hours in Atlantic

When we finish tasting, the sun has begun to set.

BBC: Discovering wine in Georgia

The final set, which began shortly after 1400 BST on Wednesday and was still going seven hours later when the sun went down, is already longer than any match ever played.

BBC: Wimbledon 2010: Epic halted at 59-59 in fifth set