Thursday, November 26, 2009

Pleiades occultation by Moon oN Dec 1st, 2009

Eclipse of stars in night

Imagine moon covering the sun to bring darkness in broad daylight, what would that event be? Eclipse, you would say but is it really eclipse? Well give it a thought. We know eclipse as “An astronomical object coming under shadow of another astronomical body and seen from other astronomical body” but the solar eclipse is not coming under this category because we are on the object where shadow is falling. So then what is this event? In astronomy we call it an “occultation” which means that a celestial body is directly coming in front and blocking the light from another body. Solar eclipse is a perfect example of occultation.
Now the greatest eclipse of the century is history and why we are discussing this again? Moon, our nearest neighbor, keeps on coming in front of so many stars in night that each night one can see numerous occultations. So one does not have to wait for a solar eclipse to happen to see an occultation. Now the whole talk about occultation goes to an interesting juncture where moon comes in front of a group of stars and in span of few hours you see 10-15 stars being occulted.  As the occultation can and only happen with stars and planets and lying on the path of ecliptic, some interesting events occur. Pleiades star cluster happens to be in Taurus and it provides us humans a rare treat in visual astronomy when moon comes in front of it.
December 1st 2009 night is one of those nights when moon is coming in front of the Pleiades as seen from earth and we get a chance to see this event though the moon is almost full and most of the Pleiades stars get washed out in the brilliance of it.

Occultation of Pleiades photographed by SPACE team on March 30, 2009

Occultations have been used to know more about the celestial mechanics as well as these have helped us finding new things in the skies. The discovery of the Uranian Rings was made by astronomers James L. Elliot, Edward W. Dunham, and Douglas J. Mink on March 10, 1977, using the Kuiper Airborne Observatory, and it was an unexpected event. They planned to use the occultation of the star SAO 158687 by Uranus to study the planet's atmosphere. However, when their observations were analyzed, they found that the star disappeared briefly from view five times both before and after it was eclipsed by the planet. They deduced that a system of narrow rings was present. Observations of occultation by Neptune were observed by UP state Observtaory, nainitaal and they confirmed presence of rings around Neptune. Stellar occultations by Moon gives us more and more accurate data about the Lunar orbit.
On December 1st 2009, around 7:00 pm IST get hold of any optical astronomical instrument you have. Binocular, spotting scope, small refracting scope or a telescope. These all will show you the moon sitting pretty in front of Pleiades.

Moon with Pleiades at 7 pm from Delhi

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