Lunar day

Lunar day

In space exploration, a lunar day is the period of time it takes for the Moon to complete one full rotation on its axis "with respect to the Sun". Equivalently, it is the time it takes the Moon to make one complete orbit around the Earth and come back to the same phase. It is marked from a New Moon to the next New Moon.

With respect to the stars, the Moon takes 27 days 7 hours 43.2 minutes to go around in its orbit; but since the Earth-Moon system has advanced around the Sun in the meantime, the Moon must keep going a while longer to get back to the same phase. On average, this synodic period is 29 days, 12 hours, 44 minutes and 3 seconds long. This is an average figure, since the speed of the Earth-Moon system around the Sun varies slightly over a year, due to the eccentricity of the orbit; the Moon's own orbit also undergoes a number of periodic variations about its mean value because of the gravitational perturbations of the Sun.

In oceanography, a lunar day is the period of time it takes for the Earth to complete one full rotation on its axis "with respect to the moon". This is the time from one lunar zenith to the next, about 24 hours 50 minutes. This period is also called the tidal day.

Lunar calendars

In some lunar calendars such as the Hindu calendar, a lunar day or tithi is defined as 1/30 of a lunar month, or the time it takes for the longitudinal angle between the moon and the sun to increase by 12°. By this definition, lunar days vary in duration (see tithi).

External links

* [http://lunarium.co.uk Lunar Days and other lunar data for many different cities]


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  • lunar day — Day Day (d[=a]), n. [OE. day, dai, dei, AS. d[ae]g; akin to OS., D., Dan., & Sw. dag, G. tag, Icel. dagr, Goth. dags; cf. Skr. dah (for dhagh ?) to burn. [root]69. Cf. {Dawn}.] 1. The time of light, or interval between one night and the next; the …   The Collaborative International Dictionary of English

  • lunar day — noun the period of time taken for the moon to make one full rotation on its axis (about 27.3 sidereal days) • Hypernyms: ↑day * * * ˌlunar ˈday 7 [lunar day] noun (astronomy) the ti …   Useful english dictionary

  • lunar day — in describing tides, the time of rotation of the Earth with respect to the moon, or the interval between two successive upper transits of the moon over the meridian of a place. The mean lunar day is approximately 24.84 solar hours in length. Also …   Dictionary of ichthyology

  • lunar day —    another name for the tidal day, a unit of time equal to 24 hours 50 minutes used in tidal predictions …   Dictionary of units of measurement

  • lunar day — a division of time that is equal to the elapsed time between two consecutive returns of the same terrestrial meridian to the moon. [1680 90] * * * …   Universalium

  • lunar day — noun the interval between two successive crossings of the meridian by the moon (roughly 24 hours and 50 minutes) …   English new terms dictionary

  • Day — (d[=a]), n. [OE. day, dai, dei, AS. d[ae]g; akin to OS., D., Dan., & Sw. dag, G. tag, Icel. dagr, Goth. dags; cf. Skr. dah (for dhagh ?) to burn. [root]69. Cf. {Dawn}.] 1. The time of light, or interval between one night and the next; the time… …   The Collaborative International Dictionary of English

  • Day after day — Day Day (d[=a]), n. [OE. day, dai, dei, AS. d[ae]g; akin to OS., D., Dan., & Sw. dag, G. tag, Icel. dagr, Goth. dags; cf. Skr. dah (for dhagh ?) to burn. [root]69. Cf. {Dawn}.] 1. The time of light, or interval between one night and the next; the …   The Collaborative International Dictionary of English

  • Day blindness — Day Day (d[=a]), n. [OE. day, dai, dei, AS. d[ae]g; akin to OS., D., Dan., & Sw. dag, G. tag, Icel. dagr, Goth. dags; cf. Skr. dah (for dhagh ?) to burn. [root]69. Cf. {Dawn}.] 1. The time of light, or interval between one night and the next; the …   The Collaborative International Dictionary of English

  • Day by day — Day Day (d[=a]), n. [OE. day, dai, dei, AS. d[ae]g; akin to OS., D., Dan., & Sw. dag, G. tag, Icel. dagr, Goth. dags; cf. Skr. dah (for dhagh ?) to burn. [root]69. Cf. {Dawn}.] 1. The time of light, or interval between one night and the next; the …   The Collaborative International Dictionary of English

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