astronomical

astronomical

[‚as·trə′näm·ə·kəl]

(astronomy)

Of or pertaining to astronomy or to observations of the celestial bodies. Also known as astronomic.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

tropical year

The period between two successive vernal equinoxes, the time the earth takes to complete one orbit around the sun relative to the first point of Aries. Its length was 365 days 5 h 48 min and 45.19 s in 2000, and it is changing by 0.00000006162 x y days (y in Julian years from 2000), or about 5 milliseconds/year. The tropical year differs from the solar year by 1 part in 26,000, since this is the period of the earth's precession about its rotational axis combined with the precession of the perihelion of the earth's orbit. Also called an astronomical, equinoctial, natural, or solar year. It contains one complete cycle of seasons.

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twilight

The period of incomplete darkness following sunset and preceding sunrise. Twilight is designated as civil, nautical, or astronomical, as the darker limit occurs when the center of the sun is 6°, 12°, and 18°, respectively, below the celestial horizon. The period of civil twilight before sunrise is known as morning civil twilight and after the sunset as evening civil twilight. The air almanac lists the local mean times of the beginning of morning civil twilight and the end of evening civil twilight in exactly the same format as sunrise and sunset tables. The twilight tables are printed on the right-hand pages facing the sunrise and sunset tables, with each pair covering the same block of calendar dates.

An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved