東京の薄明時刻 — Twilight Calculator · FastTool

Twilight Duration at 東京

At 東京's latitude of 35.7°, 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 東京's dark-sky window so different month to month.

東京のスカイラインからわずか60km西、奥多摩地域はボートル3〜4の空を提供し、晴れた夜には天の川が肉眼で見えます。世界最大級の都市圏で本物の暗い空にこれほど近いことは極めて稀であり、国立天文台三鷹キャンパスが公開観望会を通じて積極的に推進しています。

When Does It Get Truly Dark in 東京?

At 東京, true astronomical darkness begins when the sun drops more than 18° below the horizon — the end of astronomical twilight. At 35.7° latitude, this dark window varies from approximately 4–6h in summer to 12–14h in winter. During the October–March months, 東京 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.

東京・三鷹にある国立天文台は1878年以来天体を追跡し続けており、アジアで最も歴史のある継続的な天文学機関の一つです。その20cm屈折望遠鏡は1918年に小惑星「東京」を発見し、太陽塔望遠鏡は現在も大都市の中心から日々黒点活動を監視しています。

Civil, Nautical & Astronomical Twilight Explained

Three twilight phases occur each evening and morning at 東京, 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 東京'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 東京 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 東京's coordinates.