Twilight. Partial illumination by sunlight during interval between Sun's upper limb being on horizon and his centre being 18° below it. Caused by the scattering, refraction, and reflection of light by Earth's atmosphere. See 'Civil', 'Nautical', and 'Astronomical' twilights.
Twilight Arch. Upper boundary of a slightly illuminated tract of sky when Sun is below visible horizon.
Twilight Stars. Stars suitably placed for observation during morning or evening twilight.
Twine. Two or more threads twisted together to make a small line. 2. The linen thread with which canvas is sewn.