نصف‌النهار

از ویکی‌پدیا، دانشنامهٔ آزاد
پرش به ناوبری پرش به جستجو
فارسیEnglish

نِصفُ‌النّـَهار[۱] به پارسی نیمروز خط یا نیم‌دایره متناظر فرضی است که به قطب شمال و جنوب می‌رسد و طول جغرافیایی یک نقطه را با توجه به یک پرگارهٔ نیمروزی قراردادی به عنوان مبدأ تعیین می‌کند.

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.[۲]

تاریخچه[ویرایش]

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.[۳] ایرج افشار تعیین مبدأ کنگ دژ را در سال ۱۷۶۷ پیش از میلاد، توسط زرتشت می‌داند.[۴] 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.[۵]

علامه حسن‌زاده آملی در اثر خود بنام «دروس هیئت و دیگر رشته‌های ریاضی» دربارهٔ مبدأ طول ارض آورده‌است (با اندکی تلخیص و تعدیل):

«اکثر قدماء (بخصوص یونانیان و اهالی مغرب زمین) مبدأ طول را غایت نقطه غربی یعنی از جزیره هرو یا فرو، یکی از جزائر خالدات (جزائر کاناری) گرفته‌اند. جزائر خالدات شش جزیره‌اند که در مغرب آفریقا در اقیانوس اطلس نزدیک به ساحل واقع اند. در قانون مسعودی بین آنها تا ساحل بحر ده درجه آمده‌است. بطلیموس و پیروانش مبدأ طول را از آن جزائر می‌گرفتند و بعد از آنان چون جزائر نام برده در آب فرورفته بود مبدأ طول را از ساحل بحر مغرب گرفته‌اند چنان‌که ابوریحان بیرونی در قانون مسعودی مبدأ طول بلد را ساحل گرفته‌است. جزائر خالدات (جزائر کاناری) در اطلسها و نقشه‌ها در جهت غرب آفریقا در اقیانوس اطلس شمالی قریب به عرض شمالی سی درجه مشاهده می‌شود که عرض شمالی هرو (فرو یا هیرو) ۲۷ درجه و ۴۵ دقیقه و ۸ ثانیه است؛ و نصف‌النهار مبدأ برای قدمای هند و اهل چین و ترکستان و اهالی مشرق زمین کنگ دژ بوده که در ۹۰ درجه جزایر خالدات بوده‌است.[۶]»

ابوسهل بیژن بن رستم کوهی یا ابوسهوهی ریاضیدان و ستاره‌شناس سدهٔ دهم ایرانی و اهل مازندران کتابی دربارهٔ نواقص مسائل ارشمیدس نوشت و اقدام به محاسبه نصف‌النهار نمود.[نیازمند منبع]

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.[۴]

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[ویرایش]

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.

جستارهای وابسته[ویرایش]

منابع[ویرایش]

  1. «نصف‌النهار» [نجوم رصدی و آشکارسازها] هم‌ارزِ «meridian, celestial meridian»؛ منبع: گروه واژه‌گزینی. «فارسی». در همان. دفتر هفتم. فرهنگ واژه‌های مصوب فرهنگستان. تهران: انتشارات فرهنگستان زبان و ادب فارسی. شابک ۹۷۸-۹۶۴-۷۵۳۱-۹۴-۸ (ذیل سرواژهٔ نصف‌النهار)
  2. بدیعی، ربیع (۱۳۶۲). جغرافیای مفصل ایران (جلد یک). اقبال. صص. ۱۱.
  3. گنگ دژ یا گرینویچ؟! وبگاه جام جمشید
  4. ۴٫۰ ۴٫۱ رصدخانه نیمروز: دکتر ایرج افشار (سیستانی)
  5. جهان دانش، محمد بن مسعود مسعودی، بکوشش: حسین علیزاده غریب، تهران: مؤسسه فرهنگی اهل قلم (1381)، ص 108-109.
  6. مبدأ طول بلد کجاست ؟ دروس هیئت و دیگر رشته‌های ریاضی، تألیف حسن‌زاده آملی [منبع الکترونیکی]
Meridians run between the North and South poles.

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.[1] 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 all circles of latitude. 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).

Pre-Greenwich

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.[2] 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.[3] 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.

Etymology

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.[4] 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.

Geographic

The astronomic prime meridian at Greenwich, England. The geodetic prime meridian is actually 102.478 meters east of this point since the adoption of WGS84.

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.[5]

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.[6][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).[7] 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.[8] 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[9]

Magnetic Meridian

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.[10] 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.[11] The angle between the magnetic and the true meridian is the magnetic declination, which is relevant for navigating with a compass.[12] Navigators were able to use the azimuth (the horizontal angle or direction of a compass bearing)[13] of the rising and setting Sun to measure the magnetic variation (difference between magnetic and true north).[14]

True Meridian

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.[15][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.[16]

Meridian Passage

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.[17]

Latitude = (90o – noon altitude + declination)

The declination of major stars are their angles north and south from the celestial equator.[18] 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.[19]

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.[20]

Tools of Measurement

  • Visual and Photographic Zenith Tubes
  • Circumzenithals
  • Danjon Astrolabes
  • Transit Telescopes - Airy Transit Circle

See also

References

  1. ^ Withers, Charles W. J. (2017), WITHERS, CHARLES W. J. (ed.), ""Absurd Vanity": The World's Prime Meridians before c. 1790", Zero Degrees, Geographies of the Prime Meridian, Harvard University Press: 25–72, ISBN 9780674088818, JSTOR j.ctt1n2ttsj.6 |access-date= requires |url= (help)
  2. ^ Withers, Charles W. J. (2017), WITHERS, CHARLES W. J. (ed.), ""Absurd Vanity": The World's Prime Meridians before c. 1790", Zero Degrees, Geographies of the Prime Meridian, Harvard University Press: 25–72, ISBN 9780674088818, JSTOR j.ctt1n2ttsj.6 |access-date= requires |url= (help)
  3. ^ Withers, Charles W. J. (2017), WITHERS, CHARLES W. J. (ed.), "PROLOGUE", Zero Degrees, Geographies of the Prime Meridian, Harvard University Press: 1–4, ISBN 9780674088818, JSTOR j.ctt1n2ttsj.4 |access-date= requires |url= (help)
  4. ^ First Teachings about the Earth; its lands and waters; its countries and States, etc. 1870.
  5. ^ Withers, Charles W. J. (2017), WITHERS, CHARLES W. J. (ed.), "Ruling Space, Fixing Time", Zero Degrees, Geographies of the Prime Meridian, Harvard University Press: 263–274, ISBN 9780674088818, JSTOR j.ctt1n2ttsj.12 |access-date= requires |url= (help)
  6. ^ "What is the Prime Meridian? - Definition, Facts & Location - Video & Lesson Transcript | Study.com". study.com. Retrieved 2018-07-25.
  7. ^ "A&G Volume 56 Issue 5, Full Issue". Astronomy & Geophysics. 56 (5): ASTROG. 2015-09-22. doi:10.1093/astrogeo/atv173. ISSN 1366-8781.
  8. ^ Smith, Humphry M. (1976-01-01). "Greenwich time and the prime meridian". Vistas in Astronomy. 20: 219–229. doi:10.1016/0083-6656(76)90039-8. ISSN 0083-6656.
  9. ^ Smith, Humphry M. (1976-01-01). "Greenwich time and the prime meridian". Vistas in Astronomy. 20: 219–229. doi:10.1016/0083-6656(76)90039-8. ISSN 0083-6656.
  10. ^ "Induction effects of geomagnetic disturbances in the geo-electric field var...: EBSCOhost". web.b.ebscohost.com. Retrieved 2018-07-25.
  11. ^ Haughton, Graves C. (1843). "On the Relative Dynamic Value of the Degrees of the Compass; and on the Cause of the Needle Resting in the Magnetic Meridian. [Abstract]". Abstracts of the Papers Communicated to the Royal Society of London. 5: 626. JSTOR 110936.
  12. ^ "RESEARCH ON MAGNETIC DECLINATION IN LITHUANIAN TERRITORY.: EBSCOhost". web.a.ebscohost.com. Retrieved 2018-07-26.
  13. ^ "Definition of AZIMUTH". www.merriam-webster.com. Retrieved 2018-07-28.
  14. ^ Huth, John Edward (2013-01-15). The Lost Art of Finding Our Way. Cambridge, MA and London, England: Harvard University Press. doi:10.4159/harvard.9780674074811. ISBN 9780674074811.
  15. ^ "Difference Between True Meridian and Magnetic Meridian - DifferenceBetween.com". DifferenceBetween.com. 2011-08-14. Retrieved 2018-07-28.
  16. ^ McLean, Albert F. (1968). "Thoreau's True Meridian: Natural Fact and Metaphor". American Quarterly. 20 (3): 567–579. doi:10.2307/2711017. JSTOR 2711017.
  17. ^ Huth, John (2013). "Lost Art of Finding Our Way". Cambridge: Harvard University Press: 99–200 – via ProQuest Ebook Central.
  18. ^ Huth, John (2013). "Lost Art of Finding Our Way". Cambridge; Harvard University Press – via ProQuest Ebook Central.
  19. ^ Jassal, Reeve. "What is the difference of noon position and meridian passage? - MySeaTime". www.myseatime.com. Retrieved 2018-07-28.
  20. ^ Malys, Stephen; Seago, John H.; Pavlis, Nikolaos K.; Seidelmann, P. Kenneth; Kaplan, George H. (2015-08-01). "Why the Greenwich meridian moved". Journal of Geodesy. 89 (12): 1263–1272. doi:10.1007/s00190-015-0844-y. ISSN 0949-7714.

External links