The Northern Lights glow green in a spectacular light show above a field of trees frozen solid with snow. (Photo by Jaak Sarv/Solent News & Photo Agency)

A paper mache pig meant to mock Donald Trump sits on display at a protest against the Republican presidential candidate before the start of the Republican National Convention in Cleveland, Ohio, USA, 17 July 2016. Numerous protest groups are expected in Cleveland throughout the four-day-long convention. (Photo by Jim Lo Scalzo/EPA)

A US Army M1A2 SEP tank participates in the Strong Europe Tank Challenge 2017 at the military training area in Grafenwoehr, Germany, 12 May 2017. Six North Atlantic Treaty Organization (NATO) nations and partner nations, Austria, France, Germany, Poland, Ukraine and the USA are taking part in this competing exercise. (Photo by Christian Bruna/EPA)

Couples take part in a competition during a mass wedding of 64 doctoral student couples at Harbin Institute of Technology, a university in Harbin, Heilongjiang province, China, June 4, 2017. (Photo by Reuters/Stringer)

Nicki Minaj performs on stage during the 2017 NBA Awards Live On TNT on June 26, 2017 in New York City, USA. (Photo by Michael Loccisano/Getty Images for TNT)

Fish are thrown onto a truck during a winter fishing festival held on the frozen Wulungu Lake in Fuhai county, Xinjiang Uighur Autonomous Region, January 18, 2015. (Photo by Reuters/China Daily)

An employee of the National Park Service takes a selfie with President Barack Obama, left, in the background meeting with the crowd after a tour of Everglades National Park on Earth Day, Wednesday, April 22, 2015, in Florida. Obama used the visit to warn of the damage that climate change is already inflicting on the nation's environmental treasures. (Photo by Lynne Sladky/AP Photo)

Princess Diana relaxes on the sand during a visit to the beach on the Caribbean Island of Nevis January 4, 1993. (Photo by Mark Cardwell/Reuters)