|[Note Guidelines] Photographer's Note|
|Today another astronomy lesson and new word to learn - occultation.|
Year and a half ago, on May 2007, at late evening Saturn was occulated by The Moon. I was ready on my balcony, weather was good, and I took few quite good photos of this phenomenon. On two top pictures you can see moment before occultation. Saturn is small white dot at left side of The Moon. Bottom pictures show moment few minutes after occultation, after Saturn's passing behind The Moon. Saturn is - surprise! - dot from right side this time. Pictures are less or more overexposured - it was necessary to show Saturn, on pictures with well exposured Moon, Saturn is too dark and disapears. On top right picture you can see ashen glow also.
Occultations by the Moon:
The term occultation is most frequently used to describe those relatively frequent occasions when the Moon passes in front of a star during the course of its orbital motion around the Earth. Since the Moon has no atmosphere and stars have no appreciable angular size, a star that is occulted by the moon will disappear or reappear very nearly instantaneously on the moon's edge, or limb. Events that take place on the Moon's dark limb are of particular interest to observers, because the lack of glare allows these occultations to more easily be observed and timed.
The Moon's orbit is inclined to the ecliptic, and any stars with an ecliptic latitude of less than about 6.5 degrees may be occulted by it. There are three first magnitude stars that are sufficiently close to the ecliptic that they may be occulted by the Moon and by planets -- Regulus, Spica and Antares. Occultations of Aldebaran are presently only possible by the Moon, because the planets pass Aldebaran to the north. Neither planetary nor lunar occultations of Pollux are currently possible. However, in the far future, occultations of Aldebaran and Pollux will be possible, as they were in the far past. Some deep-sky objects, such as the Pleiades, can also be occulted by the moon.
Within a few kilometres of the edge of an occultation's predicted path, referred to as its northern or southern limit, an observer may see the star intermittently disappearing and reappearing as the irregular limb of the Moon moves past the star, creating what is known as a Grazing lunar occultation. From an observational and scientific standpoint, these "grazes" are the most dynamic and interesting of lunar occultations.
The accurate timing of lunar occultations is performed regularly by (primarily amateur) astronomers. Lunar occultations timed to an accuracy of a few tenths of a second have various scientific uses, particularly in refining our knowledge of lunar topography. Photoelectric analysis of lunar occultations have also discovered some stars to be very close visual or spectroscopic binaries. Early radio astronomers found occultations of radio sources by the Moon valuable for determining their exact positions, because the long wavelength of radio waves limited the resolution available through direct observation.
Several times during the year, someone on Earth can usually observe the Moon occulting a planet. Since planets, unlike stars, have significant angular sizes, lunar occultations of planets will create a narrow zone on earth from which a partial occultation of the planet will occur. An observer located within that narrow zone could observe the planet's disk partly blocked by the slowly moving moon.
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