The 51st parallel north is a circle of latitude that is 51 degrees north of the Earth's equatorial plane. It crosses Europe, Asia, the Pacific Ocean, North America, and the Atlantic Ocean.
At this latitude the sun is visible for 16 hours, 33 minutes during the summer solstice and 7 hours, 55 minutes during the winter solstice.
The catchment area of London, the capital city of England and the United Kingdom, can be broadly defined by the 51st and 52nd parallels.
51st parallel north Wikipedia
Starting at the Prime Meridian (just north of the Sheffield Park Garden in East Sussex, England) and heading eastwards, the parallel 51° north passes through:
In 1799 Paul I, Tsar of the Russian Empire, issued a ukase creating the Russian-American Company (RAC). It was granted monopolistic control north of the 55th parallel north, which had been the Russian claim since 1790, as well as the right to operate and occupy territory to the south as long as the lands had not been previously occupied, or dependent on any other nation. In 1821 the RAC's charter was renewed and at the same time an ukase proclaimed that Russian sovereignty extended south to the 51st parallel, and that waters north of that line were closed to foreign shipping. The ukase was met with strong objections by the United States and Great Britain. Subsequent negotiations resulted in a clear and permanent boundary for Russian America, the southward terminus of which was established at 54°40′ north.