According to Space.com
, a conjunction occurs when two or more astronomical objects come close together in the sky even though in reality they are very far apart in space. During a conjunction two or more celestial bodies are seen from observer's position near one another in the sky.
Astronomers sometimes use the term "appulse" to describe the event. The impression of closeness in space of the celestial bodies is an effect from the viewer's perspective. Astrologers have for centuries looked upon conjunctions as portents, but astronomers insist there are no scientific grounds for the belief and that a conjunction is nothing more than a rare and beautiful sight to behold, and perhaps an opportunity for photography.
According to EarthSky.org
, the Moon’s appearance of nearness to Spica and Saturn is a line-of-sight illusion
. The Moon in reality is very far from Spica. It is only about one light-second from Earth, while Spica is 260 light-years away, EarthSky.org
reports. Saturn, though closer to Spica, is still far from the star, at a distance of 73 light-minutes from Earth.
In the triple conjunction expected on Thursday night, the movement of Saturn as seen from the Earth is relatively slow. According to Space.com
, Saturn has been in the constellation Virgo
, close to the bright star Spica
, for a number of years. The Moon is much closer to the Earth and appears to move much faster across the sky.
During Thursday's conjunction, sky watchers will be able to observe the Moon as it moves from one night to the next. On Thursday night, the Moon will be seen to the right of Saturn and Spica but by Friday night it will be displaced to the left of Saturn and Spica.
There have been other remarkable conjunctions in recent years. In late April 2002, a rare conjunction
of Saturn, Jupiter, Mars, Venus and Mercury occurred in the west-northwest sky shortly after sundown. This is expected to happen again in early July 2060, but it will occur in the east-northeast shortly before dawn.
A conjunction occurred on December 1, 2008, when Venus, Jupiter and the crescent moon converged to form a tight triangle in the evening after sunset. The conjunction was visible worldwide. Viewers described the conjunction as looking like a happy "V" face.