(6 a.m. - 5:55 p.m. ET) Tomorrow, Discovery Channel presents a nature series marathon of its own, televising the entire Frozen Planet, a sequel to this series – which, like Frozen, was a co-production by the BBC and Discovery Channel. The two series have something else in common: while narrated by Sir David Attenborough in the U.K., both were televised in the U.S. by Discovery with substitute narrators: Sigourney Weaver for Planet Earth, and Alec Baldwin for the Frozen Planet, which just ended its run on Discovery. I’ve complained about this needless substitution both times, and relief is at hand. Today’s Planet Earth marathon on BBC America, is shown three times (April 21, 2012 from 6am-5:55pm ET, then repeating 5:55pm-9:05am and 9:05am-9pm ET Sunday), and is followed at 9 p.m. ET Sunday by The Making of Planet Earth. And this time, all of it is televised with the original Attenborough narration – as is tomorrow’s Frozen Planet marathon on Discovery. A victory for two Davids.