I think you mean the western and eastern boundaries. And that would be the Cascades Mountain range.
Labeling boundaries in liquid water is difficult, so these boundaries are usually marked by latitude. Sixty degrees S marks the northern-most edge of the Southern Ocean and the southern-most edges of the South Atlantic, the South Pacific and the South Indian oceans.
The Huang He River marks the eastern border of China's Shensi province. The Danube River helps form the border between Romania and Bulgaria.
The Continental Divide marks the eastern border of the Ute reservation.
The December solstice, which is the winter solstice in the northern hemisphere marks the beginning of northern winter.
The Ural Mountains serve as a distinct geographic boundary between Europe and Asia, marking the eastern border of Europe and the western border of Asia.
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The prime meridian at longitude 0° marks the eastern boundary of the Western Hemisphere The corresponding anti-meridian at Longitude 180° marks the western boundary of the Western Hemisphere. (In practical terms you could also consider the international date line to be the boundary - just not the official boundary.)
Yes, the Arctic Circle is in both the eastern and western hemispheres, and it's also completely in the northern one.
The likely word is surveyor (one who measures and marks lands and boundaries).
It's called "the Terminator".
The winter solstice in Eastern Standard Time in 2008 occurred on December 21st. This is the shortest day and longest night of the year in the Northern Hemisphere. It marks the official start of winter.