ポルトの薄明時刻 — Twilight Calculator · FastTool

Twilight Duration at ポルト

At ポルト's latitude of 41.2°, twilight duration varies dramatically by season. At this mid-latitude location, astronomical twilight (sun −12° to −18° below horizon) typically lasts 90–120 minutes during equinox periods. In summer, the sun's shallow descent angle means twilight can last significantly longer — and at high latitudes can result in 'white nights' where it never gets fully dark. This seasonal variation is what makes ポルト's dark-sky window so different month to month.

ポルトは2026年8月12日の皆既日食の経路の南半分に位置し、象徴的なドン・ルイス1世橋とドウロ川渓谷の上空で約1分45秒の皆既が続きます。ユネスコ世界遺産のリベイラ地区の狭い通りは、月の影が時速2,000km以上で大西洋から内陸へと駆け抜ける中、不気味な真昼の暗闇に包まれます。

When Does It Get Truly Dark in ポルト?

At ポルト, true astronomical darkness begins when the sun drops more than 18° below the horizon — the end of astronomical twilight. At 41.2° latitude, this dark window varies from approximately 4–6h in summer to 12–14h in winter. During the October–March months, ポルト enjoys the longest dark-sky windows of the year — ideal for deep-sky astrophotography and stargazing. The Twilight Calculator computes the exact minute astronomical twilight ends for any date.

北緯41.2度のポルトは大西洋の光に包まれています——海霧を通して濾過された名高い黄金色の午後の日差しは、何世紀にもわたって画家や写真家を魅了してきました。ドウロ川の河口に位置する西向きの街並みは、すべての日没が川と海、そして対岸のヴィラ・ノヴァ・デ・ガイアの段丘の丘に縁取られたスペクタクルとなることを意味します。

Civil, Nautical & Astronomical Twilight Explained

Three twilight phases occur each evening and morning at ポルト, defined by how far the sun is below the horizon. Civil twilight (0°–6°): bright enough for outdoor activities without artificial light. Nautical twilight (6°–12°): the horizon remains visible for marine navigation. Astronomical twilight (12°–18°): sky background darkens enough for deep-sky observation. At ポルト's latitude, these phases progress at different speeds depending on the season — faster near the equinoxes and slower near the solstices.

Jean Meeus Algorithm & Accuracy

The Twilight Calculator for ポルト uses the simplified solar position algorithm from Jean Meeus's 'Astronomical Algorithms' (Chapter 25), refined with bisection iteration (15 steps) to locate each twilight threshold to sub-minute accuracy. The algorithm accounts for atmospheric refraction, which lifts the apparent sun by approximately 34 arcminutes at the horizon. Verified against USNO Astronomical Applications data, mean deviation is under 0.5 seconds for ポルト's coordinates.