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[[ImageBerkas:WorldMapLongLat-eq-circles-tropics-non.png|thumb|450px|[[Peta]] Bumi, memperlihatkan garis-garis bujur, yang nampak melengkung dan [[vertikal]] pada proyeksi ini, namun sebenarnya garis-garis bujur tersebut merupakan setengah dari sebuah [[lingkaran besar]] bumi.]]
'''Bujur''' kadangkala dinotasikan oleh [[Lambda|abjad Yunani λλ]], menggambarkan lokasi sebuah tempat di [[timur]] atau [[barat]] [[Bumi]] dari sebuah garis utara-selatan yang disebut [[Meridian Utama]]. Longitude diberikan berdasarkan [[sudut|pengukuran sudut]] yang berkisar dari 0° di Meridian Utama ke +180° arah timur dan −180−180° arah barat. Tidak seperti [[lintang]] yang memiliki [[ekuator]] sebagia posisi awal alami, tidak ada posisi awal alami untuk bujur. Oleh karena itu, sebuah dasar [[meridian]] harus dipilih. Meskipun [[kartografer]] [[Britania Raya]] telah lama menggunakan meridian [[Observatorium Greenwich]] di [[London]], referensi lainnya digunakan di tempat yang berbeda, termasuk [[Ferro]], [[Roma]], [[Kopenhagen]], [[Yerusalem]], [[Saint Petersburg]], [[Pisa]], [[Paris]], [[Philadelphia, Pennsylvania|Philadelphia]], dan [[Washington, D.C.]]. Pada [[1884]], [[Meridian Utama|Konferensi Meridian Internasional]] mengadopsi meridian Greenwich sebagai '''Meridian utama universal''' atau ''titik nol bujur''.
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Each degree of longitude is further sub-divided into 60 [[minute of arc|minutes]], each of which divided into 60 seconds. A longitude is thus specified as '''23°° 27′27′ 30"" E'''. For high accuracy, the seconds are specified with a decimal fraction. An alternative representation uses degrees and minutes, where parts of a minute are expressed as a decimal fraction, thus: '''23°° 27.500′500′ E'''. Degrees expressed as a decimal number is also used: '''23.45833°° E'''. Sometimes, the West/East suffix is replaced by a negative sign for West. Confusingly, the convention of negative for East is also sometimes seen. The preferred convention that East is positive is consistent with the right-handed ''x''-axis in the [[Cartesian coordinate system]].
A specific longitude may then be combined with a specific [[latitude]] to give a precise position on the Earth's surface.
As opposed to a degree of latitude, which always corresponds to about 111 km (69 mi), a degree of longitude corresponds to a distance from 0 to 111 km: it is 111 km times the [[cosine]] of the latitude, when the distance is laid out on a circle of constant latitude; if the shortest distance, on a [[great circle]] were used, the distance would be even a little less.
Longitude at a point may be determined by calculating the time difference between that at its location and [[Coordinated Universal Time]] (UTC). Since there are 24 hours in a day and 360 degrees in a circle, the sun moves 15 degrees per hour (360°/24 hours = 15° per hour). So if the [[time zone]] a person is in is three hours ahead of UTC then that person is near 45° longitude (3 hours ×× 15° per hour = 45°). The word ''near'' was used because the point might not be at the center of the time zone; also the time zones are defined politically, so their centers and boundaries often do not lie on [[meridian (astronomy)|meridians]] at multiples of 15°. In order to perform this calculation, however, a person needs to have a [[chronometer]] (watch) set to UTC and needs to determine local time by solar observation or astronomical observation. The details are more complex than described here: see the articles on [[Universal Time]] and on the [[Equation of time]] for more details.
A line of constant longitude is a meridian, and half of a [[great circle]].
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