The U.S. Naval Observatory in Augusta, Maine indicates that the sun set between 4:30 p.m. and 5:20 p.m. Eastern Standard Time during October 2015. For any given month, the sun sets when it becomes no longer visible above the western horizon due to the rotation of the earth.
The time at which the sun sets depends on the several factors such as the geographic location of the observer in terms of latitude and longitude and the observer's altitude.
Factors that contribute to varying times of sunset at the same location include the 23.4-degree axial tilt of the earth relative to its poles, the elliptic, rather than circular, shape of the earth’s orbit and the 7-degree angle of the earth’s orbital plane in relation to the sun’s equator. These factors cause the length of daylight to become progressively shorter and sunset to occur at earlier times each day during October in the Northern Hemisphere and for the Southern Hemisphere to experience increasingly longer days during the month.