因為恆星的赤經和赤緯會因歲差的原因而改變，天文學家經常指定某一特定的紀元作為參考點。B1950.0標準已經被繼任標準J2000.0紀元所取代，J2000.0是現在使用的紀元標準。
前綴"B"代表這是一個貝塞耳紀元，而非儒略紀元。
參考
- Aoki, S.; Soma, M.; Kinoshita, H.; Inoue, K. Conversion matrix of epoch B 1950.0 FK4-based positions of stars to epoch J 2000.0 positions in accordance with the new IAU resolutions. Astronomy and Astrophysics. 1983-12-01, 128 [2023-01-18]. Bibcode:1983A&A...128..263A. ISSN 0004-6361. （原始內容存檔於2022-10-03）.
- Standish, E. M., Jr. Conversion of positions and proper motions from B1950.0 to the IAU system at J2000.0. Astronomy and Astrophysics. 1982-11-01, 115 [2023-01-18]. Bibcode:1982A&A...115...20S. ISSN 0004-6361. （原始內容存檔於2022-11-13）.
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