雅思阅读文章3

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　　TRANSIT OF VENUS

　　A transit of Venus is the passage of the planet Venus across the visible disc of the Sun, as observed from the Earth. Such events are rare, with no more than two occurring every century. The most recent transit of Venus occurred on 8 June 2004, 122 years after the previous transit was observed. The reason for the rarity of this event is the inclination of the orbit of Venus with respect to that of the Earth.

　　If the Venusian orbit were not tilted we would expect to see a transit every 584 days. However, due to the relative tilt of the orbits of Venus and Earth, Venus is almost always either above or below the ecliptic when Venus and the Earth are at the same positions in their orbits. Only when Venus and the Earth lie close to the line of nodes indicated in the diagram above, will we observe a transit.

　　Transits of Venus commonly occur in pairs spaced about 8 years apart which correspond to one of the nodes. These are separated by the next pair on the opposite node after an interval of either 105.5 or 121.5 years.

　　The following table gives the dates for past and future transits of Venus within a few hundred years of our present date. Dates and times are all referenced to Universal Time. Eastern Australian Standard Time is UT plus 10 hours. Western Australian Standard Time is UT plus 8 hours. Note that some transits go over the UT day boundary. In this case, the start time refers to the first date, and the end time refers to the second date.

　　At the present time, the significance of a transit of Venus lies primarily in the rarity of the event. No-one saw such an event in the 20th century.