نیمروزها همگی برابر میباشند؛ و چون هیچ برتری نسبت به یکدیگر ندارند برای استفاده از آنها باید یکی را به عنوان نیمروز مبدأ برگزید و بقیه را بر مبنای آن نامگذاری کرد. نصفالنهار گرینویچ اکنون نصفالنهار مبدأ میباشد.
نصف النهارها دوایری هستند فرضی و از هر نقطه روی زمین یک نصف النهار میگذرد.
ایرانیان باستان کنگدژ، مابین سیستان و خراسان قدیم در عرض سیوسهونیم درجه شمالی از استوا را نصفالنهار مبدأ (نود درجه) قرار دادند. این ناحیه را نیمروز (به معنی نصفالنهار) گفته شدهاست و این ناحیه دقیقاً وسط چین و اروپا و همه سرزمینهای مسکون شناخته شده آنروز است و از جانب شمال-جنوب نیز از این حیث در میانه میافتد. ایرج افشار تعیین مبدأ کنگ دژ را در سال ۱۷۶۷ پیش از میلاد، توسط زرتشت میداند. آغاز آبادانی (ربع مسکون) جزایر خالدات (جزایر قناری در غرب آفریقا) و مرز شرقی آن ماچین (ژاپن) و میانه آن نیمروز (کنگ دژ) دانسته شدهاست. نصفالنهار مبدأ را جزایر خالدات (یا ساحل غربی آفریقا) یا کنگ دژ بودهاست. بعضی نیز بقعه مکه را برای مبدأ نصفالنهار در نظر گرفتهاند.
علامه حسنزاده آملی در اثر خود بنام «دروس هیئت و دیگر رشتههای ریاضی» دربارهٔ مبدأ طول ارض آوردهاست (با اندکی تلخیص و تعدیل):
در دورهای از شکوفایی علمی اسلام در راستای مطالعات نجوم اسلامی، نصفالنهار مکه (کعبه) نیز به عنوان نصفالنهار مبدأ قلمداد میشدهاست.[نیازمند منبع]
اروپاییان تا آخر سده هفدهم میلادی، جزایر خالدات و سپس پاریس را مبدأ نصف النهارها قرار دادند.
در سال ۱۸۸۴، در یک توافق بینالمللی تصمیم گرفته شد که نصفالنهاری که از گرینویچ میگذرد، نصفالنهار مبدأ (نصفالنهار صفر) بهشمار آید؛ و با انتخاب نصفالنهار مبدأ بقیهٔ نصفالنهارها بر اساس محلشان نسبت به نصفالنهار مبدأ به نصفالنهار شرقی و غربی تقسیم شدند.
نصفالنهار مغناطیسی ، خطوطی فرضی اند که قطب شمال و جنوب را به هم متصل میکنند؛ که میتوان آنها را به عنوان مؤلفههای خطوط نیروی مغناطیسی در روی سطح زمین در نظر گرفت؛ و همواره سوزن یک قطبنما موازی با نصفالنهار مغناطیسی میایستد. زاویهٔ بین نصفالنهار مغناطیسی و نصفالنهار واقعی، میل مغناطیسی نامیده میشود که در مسائل مربوط به ناوبری بسیار با اهمیت است.
A (geographic) meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian. The position of a point along the meridian is given by that longitude and its latitude, measured in angular degrees north or south of the Equator. Each meridian is perpendicular to the Equator. Each is also the same length, being half of a great circle on the Earth's surface and therefore measuring 20,003.93 km (12,429.9 miles).
The first prime meridian was set by Eratosthenes in 200 BCE. This prime meridian was used to provide measurement of the earth, but had many problems because of the lack of latitude measurement. Many years later around the 19th century there was still concerns of the prime meridian. The idea of having one prime meridian came from William Parker Snow, because he realized the confusion of having multiple prime meridian locations. Many of theses geographical locations were traced back to the ancient Greeks, and others were created by several nations. Multiple locations for the geographical meridian meant that there was inconsistency, because each country had their own guidelines for where the prime meridian was located.
The term meridian comes from the spanish meridies, meaning "midday"; the subsolar point passes through a given meridian at solar noon, midway between the times of sunrise and sunset on that meridian. Likewise, the Sun crosses the celestial meridian at the same time. The same Latin stem gives rise to the terms a.m. (ante meridiem) and p.m. (post meridiem) used to disambiguate hours of the day when utilizing the 12-hour clock.
International Meridian Conference
Because of a growing international economy, there was a demand for a set international prime meridian to make make it easier for worldwide traveling which will, in turn, enhance international trading across countries. As a result, a Conference was held in 1884, at Washington D.C. twenty-six countries were present at the International Meridian Conference to vote on an international prime meridian. Ultimately the outcome was as follow: there will only be a single meridian, the meridian was to cross and pass at Greenwich (which was the 0°), there will be two longitude direction up to 180° (east being plus and west being minus), there will be a universal day, and the day begins at the mean midnight of the initial meridian. 
Toward the ending of the 12th century there were two main locations that were acknowledged as the geographic location of the meridian, France and Britain. These two locations often conflicted and a settlement was reached only after there was an International Meridian Conference held, in which Greenwich was recognized as the 0° location.
The meridian through Greenwich (inside Greenwich Park), England, called the Prime Meridian, was set at zero degrees of longitude, while other meridians were defined by the angle at the center of the earth between where it and the prime meridian cross the equator. As there are 360 degrees in a circle, the meridian on the opposite side of the earth from Greenwich, the antimeridian, forms the other half of a circle with the one through Greenwich, and is at 180° longitude near the International Date Line (with land mass and island deviations for boundary reasons). The meridians from West of Greenwich (0°) to the antimeridian (180°) define the Western Hemisphere and the meridians from East of Greenwich (0°) to the antimeridian (180°) define the Eastern Hemisphere.[unreliable source?] Most maps show the lines of longitude.
The position of the prime meridian has changed a few times throughout history, mainly due to the transit observatory being built next door to the previous one (to maintain the service to shipping). Such changes had no significant practical effect. Historically, the average error in the determination of longitude was much larger than the change in position. The adoption of WGS84 ("World Geodetic System 84") as the positioning system has moved the geodetic prime meridian 102.478 metres east of its last astronomic position (measured at Greenwich). The position of the current geodetic prime meridian is not identified at all by any kind of sign or marking (as the older astronomic position was) in Greenwich, but can be located using a GPS receiver.
Effect of Prime Meridian (Greenwich Time)
It was in the best interests of the nations to agree to one standard meridian to benefit their fast growing economy and production. The disorganized system they had before was not sufficient for their increasing mobility. The coach services in England had erratic timing before the GWT. U.S. and Canada were also improving their railroad system and needed a standard time as well. With a standard meridian, stage coach and trains were able to be more efficient. The argument of which meridian is more scientific was set aside in order to find the most convenient for practical reasons. They were also able to agree that the universal day was going to be the mean solar day. They agreed that the days would begin at midnight and the universal day would not impact the use of local time. In the "Transactions of the Royal Society of Canada a report was submitted, dated 10 May 1894; on the Unification of the Astronomical, Civil and Nautical Days" it states that:
civil day- begins at midnight and ends at midnight following
astronomical day- begins at noon of civil day and continue until following noon
nautical day- concludes at noon of civil day, starting at preceding noon
The magnetic meridian is an equivalent imaginary line connecting the magnetic south and north poles and can be taken as the horizontal component of magnetic force lines along the surface of the earth. Therefore, a compass needle will be parallel to the magnetic meridian. However, a compass needle will not be steady in the magnetic meridian, because of the longitude from east to west being complete geodesic. The angle between the magnetic and the true meridian is the magnetic declination, which is relevant for navigating with a compass. Navigators were able to use the azimuth (the horizontal angle or direction of a compass bearing) of the rising and setting Sun to measure the magnetic variation (difference between magnetic and true north).
The true meridian is the plane that passes through true north poles and true south poles at the spot of the observer. The difference between true meridian and magnetic meridian is that the true meridian is fixed while the magnetic meridian is formed through the movement of the needle.[unreliable source] True bearing is the horizontal angle between true meridian and a line.
Henry D. Thoreau classified this true meridian versus the magnetic meridian in order to have a more qualitative, intuitive, and abstract function. He used the true meridian since his compass varied by a few degrees. There were some variations. When he noted the sight line for the True Meridian from his family's house to the depot, he could check the declination of his compass before and after surveying throughout the day. He noted this variation down.
The meridian passage is the moment when a celestial object passes the meridian of longitude of the observer. At this point, the celestial object is at its highest point. When the sun passes two times an altitude while rising and setting can be averaged to give the time of meridian passage. Navigators utilized the sun’s declination and the sun’s altitude at local meridian passage, in order to calculate their latitude with the formula.
Latitude = (90o – noon altitude + declination)
The declination of major stars are their angles north and south from the celestial equator. It is important to note that the Meridian passage will not occur exactly at 12 hours because of the inclination of the earth. The meridian passage can occur within a few minutes of variation.
Measurement of Earth Rotation
Many of these instruments rely on the ability to measure the longitude and latitude of the earth. These instruments also were typically effected by local gravity, which paired well with existing technologies such as the magnetic meridian. To measure latitute you would need to divide the earth from the equator to the north and south pole to 90°. So the other degree of latitude would be between 0 to 90 degree based on the arc of the earth from the equator.. To measure the longitude we would take the same approach and instead north and south it would west and east. However before the International Meridian Conference there was no set location to measure longitude so the conference decided on the place which was Greenwich.
Tools of Measurement