ASTRONOMICAL UNIT
An astronomical unit, or AU, is equal to the mean (average) distance from the Earth to the Sun , about 93 million miles (150 million km). It takes a beam of light about 8.3 minutes to travel 1 AU. |
 BLACK DROP EFFECT
The black drop effect is an optical effect that happens early in a transit of Venus , when Venus just travels in front of the solar disk. The light from the Sun is bent around Venus (it is refracted by Venus' dense atmosphere) making Venus look stretched-out.
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CONJUNCTION
Conjunction is the apparent close approach of a planet to the Sun (or another planet), from the point of view of an observer on the Earth. A planet is in conjunction when the Sun is exactly between that planet and the Earth or, for Mercury and Venus (the two inferior planets), when that planet, the Sun , and the Earth are lined up. Mercury and Venus have two positions of conjunction: when either planet is directly between the Earth and the Sun, it is in inferior conjunction; when either planet is on the far side of the Sun from the earth, it is in superior conjunction. During conjunction, a planet cannot be seen from Earth (unless it is in transit ); it is either behind the Sun or is lost in the glare of the Sun. |
ECLIPTIC
The ecliptic is the plane defined by the Earth's orbit around the Sun; conversely, in the course of a year, the sun traces a path in the sky along the ecliptic. Most of the planets in our solar system appear close to the ecliptic plane from Earth. The Earth's axis is tilted at a 23.5° from the ecliptic (which causes the seasons).
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