Hari Julian adalah hitungan hari yang terus menerus sejak permulaan Periode Julian dan terutama digunakan oleh para astronom.

Angka Hari Julian (JDN) adalah bilangan bulat yang ditetapkan untuk keseluruhan hari matahari dalam hari Julian dihitung mulai dari siang waktu Universal, di mana angka 0 hari Julian ditetapkan untuk hari terhitung pada siang tanggal 1 Januari 4713 SM, kalender Julian proleptik (24 November 4714 SM, dalam kalender Gregorian proleptik),^[1]^[2]^[3] suatu tanggal di mana tiga siklus multitahun mulai (yakni siklus Indiksi, Solar, dan Lunar) dan yang mendahului tanggal mana pun dalam catatan sejarah.^[4] Sebagai contoh, angka hari Julian untuk hari yang dimulai pada 12.00 UT pada 1 Januari 2000 adalah 2.451.545.^[5]

Tanggal Julian (JD) dari saat mana pun adalah angka hari Julian untuk siang sebelumnya dalam Waktu Universal ditambah fraksi hari sejak saat itu. Tanggal Julian dinyatakan sebagai angka hari Julian dengan tambahan pecahan desimal.^[6] Sebagai contoh, Tanggal Julian untuk 00.30.00.0 UT 1 Januari 2013 adalah 2.456.293,520833.^[7]

Periode Julian adalah suatu interval kronologis dari 7980 tahun; tahun 1 dari Periode Julian adalah 4713 SM.^[8] Ini telah digunakan oleh ahli sejarah sejak pengenalannya tahun 1583 untuk mengkonversi antara berbagai kalender. Tahun kalender Julian 2024 adalah tahun 6737 dari Periode Julian sekarang. Periode Julian berikutnya mulai pada tahun 3268 M.

Referensi

^ Dershowitz & Reingold 2008, 15.
^ Seidelman 2013, 15.
^ "Astronomical Almanac Online" 2016, Glossary, s.v. Julian date. Various timescales may be used with Julian date, such as Terrestrial Time (TT) or Universal Time (UT); in precise work the timescale should be specified.
^ Both of these dates are years of the Christian or Common Era (which has no year 0 between 1 BC and AD 1). Astronomical calculations generally include a year 0, so these dates should be adjusted accordingly (i.e. the year 4713 BC becomes astronomical year number −4712, etc.). In this article, dates before October 15, 1582 are in the (possibly proleptic) Julian calendar and dates on or after October 15, 1582 are in the Gregorian calendar, unless otherwise labelled.
^ McCarthy & Guinot 2013, 91–2
^ "Resolution B1" 1997.
^ US Naval Observatory 2005
^ Astronomical Almanac for the year 2017 p. B4, which states 2017 is year 6730 of the Julian Period.

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