ロンドンの薄明時刻 — Twilight Calculator · FastTool

Twilight Duration at ロンドン

At ロンドン's latitude of 51.5°, twilight duration varies dramatically by season. At this high-latitude location, astronomical twilight (sun −12° to −18° below horizon) typically lasts 2–4 hours 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 ロンドン's dark-sky window so different month to month.

1675年以来グリニッジ王立天文台の本拠地であるロンドンは、本初子午線——地球全体の世界時を定義する経度0度線——の発祥の地です。地球上のすべてのタイムゾーンは、テムズ川を見下ろす丘の上のこの一点を基準に測定されており、ロンドンを文字通り世界の計時の中心地としています。

When Does It Get Truly Dark in ロンドン?

At ロンドン, true astronomical darkness begins when the sun drops more than 18° below the horizon — the end of astronomical twilight. At 51.5° latitude, this dark window varies from approximately 0–3 (white nights possible)h in summer to 14–16h in winter. During the September–April months, ロンドン 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51.5度に位置するロンドンでは、真夏と真冬の日照時間に8時間以上の極端な季節差があり、これが街のリズムを形作っています。6月の夏至には夕暮れが午後10時を過ぎても続き、12月の日中はわずか8時間の日照に縮まります——この幅は低緯度からの訪問者を常に驚かせます。

Civil, Nautical & Astronomical Twilight Explained

Three twilight phases occur each evening and morning at ロンドン, 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 ロンドン'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 ロンドン 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 ロンドン's coordinates.