ˌsemi-diˈurnal, a.
[semi- 6 a.]
1. Astron. Pertaining to, consisting of, or performed in, half the time between the rising and setting of a celestial body. Chiefly in semi-diurnal arc.
| 1594 Blundevil Exerc. iii. xviii. (1597) 154 It deuideth the artificial day and artificiall night each of them into two parts, that is to say, into two semi-diurnall and into two seminocturnall parts. Ibid. l. 176 b, There will remaine 9. houres 4{p}8. which is the length of the artificiall day, when the Sunne is in the first degree of Scorpio, the one halfe wherof is called the semi-diurnall Arke of that artificial day. 1664 Wakely Mariners-Compass rectified (1694) 93 Astronomical Tables of Semidiurnal and Seminocturnal Arches. 1725 Watts Geog. & Astron. xix, The difference between the sun or star's semidiurnal arc and a quadrant or ninety degrees. 1852 Hind Astron. Vocab. 52 Semi-diurnal Arc, is half the arc described by a heavenly body between its rising and setting. 1867 G. F. Chambers Astron. Vocab. 801. |
2. Occurring every twelve hours. Chiefly of the tides.
| 1794 R. J. Sulivan View Nat. I. 414 The tides are semi-diurnal polar effusions, as the general currents of the ocean are semi-annual. 1813 J. Forbes Oriental Mem. I. 309 The sole cause then of these semidiurnal breezes, being the capacity which the earth has for acquiring a higher temperature than that of the sea. 1866 Lockyer Guillemin's Heavens 59 The semi-diurnal oscillatory movement of the waters of the ocean,—the tides. |
3. Ent. Partly diurnal, flying at twilight.
| In recent Dicts. |