Night and day is illustrative only; daylight hours depend on latitude and time of year.) The IDL is roughly based on the meridian of 180° longitude, roughly down the middle of the Pacific Ocean, and halfway around the world from the Greenwich meridian.
In many places, the IDL follows the 180° meridian exactly.
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In other places, however, the IDL deviates east or west away from that meridian. Aleutian Islands (Attu Island being the westernmost) and the Commander Islands, which belong to Russia. Thus, all of Russia is to the west of the IDL, and all of the United States is to the east except for the insular areas of Guam, the Northern Mariana Islands, and Wake Island.
These various deviations generally accommodate the political and/or economic affiliations of the affected areas. The IDL remains on the 180° meridian until passing the equator.
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