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Twilight occurs according to the solar elevation angle θ s, which is the position of the geometric center of the Sun relative to the horizon. There are three established and widely accepted subcategories of twilight: civil twilight (nearest the horizon), nautical twilight, and astronomical twilight (farthest from the horizon).
After the Sun has also set for these altitudes at the end of nautical twilight, the intensity of light emanating from earlier mentioned lines decreases, until the oxygen-green remains as the dominant source. When astronomical darkness has set in, the green 557.7 nm oxygen line is dominant, and atmospheric scattering of starlight occurs.
Blue hour at the Old Cathedral of the Holy Name of Jesus, Bragança in Portugal. The blue hour (from French l'heure bleue; [1] [a] pronounced [lœʁ blø]) is the period of twilight (in the morning or evening, around the nautical stage) when the Sun is at a significant depth below the horizon.
For example, a typical day during civil polar twilight in Vadsø, Norway will begin with night, astronomical twilight, nautical twilight, and civil twilight in that order (with each successive phase including more light than the last). Following civil twilight, the day will progress through the other phases in the opposite order (nautical ...
Graphical and textual descriptions of navigational light sequences and colours are displayed on nautical charts and in Light Lists with the chart symbol for a lighthouse, lightvessel, buoy or sea mark with a light on it. Different lights use different colours, frequencies and light patterns, so mariners can identify which light they are seeing. [1]
The front cover of a List of Lights volume. A list of lights is a publication describing lighthouses and other aids to maritime navigation. Most such lists are published by national hydrographic offices. Some nations, including the United Kingdom and the United States, publish lists that cover the whole world in many volumes. Other nations ...
Leading lights, also known as range lights in the United States, are a pair of light beacons used in navigation [2] to indicate a safe passage for vessels entering a shallow or dangerous channel; they may also be used for position fixing. At night, the lights are a form of leading line that can be used for safe navigation. The beacons consist ...
A large PEL sector light installation at the Port of Durban Original Porirua Harbour PEL The PEL sector light is a projector style marine beacon which is used to guide maritime vessels. It does this by projecting a colored sector out to sea with very sharp color changes as the mariner transitions from one sector to the next.
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