Today's historic Venus transit is a marathon event lasting nearly seven hours, but skywatchers who don't have that kind of time can break it down into a handful of key milestones.

Venus treks across the sun's face from Earth's perspective today (June 5; June 6 in much of the Eastern Hemisphere), marking the last such Venus transit until 2117. Few people alive today will be around to see the next transit, which makes the rare celestial sight a premier event in the astronomical and skywatching communities.

The Venus-sun show will begin around 6 p.m. EDT (2200 GMT) and end at roughly 12:50 a.m. EDT (0450 GMT) Wednesday, with the exact timing varying by a few minutes from point to point around the globe.

First contact

Before you even attempt to observe the transit of Venus, a warning: NEVER stare at the sun through binoculars or small telescopes or with the unaided eye without the proper safety equipment. Doing so can result in serious and permanent eye damage, including blindness.

Astronomers use special solar filters on telescopes to view the sun safely, while No. 14 welder's glass and eclipse glasses can be used to observe the sun directly. [How to Safely Photograph the Venus Transit]

With that warning stated, here's a look at the first major stage of the transit of Venus.

The transit officially commences when the leading edge of Venus first touches the solar disk, an event astronomers call "Contact I" or "ingress exterior." This milestone occurs at 6:03 p.m. EDT (2203 GMT) for observers in eastern North America, while skywatchers on the other side of the continent will see it a few minutes later, at 3:06 p.m. PDT.

Second contact and beyond

Next up is "Contact II," or "ingress interior" the moment when Venus moves fully onto the sun's face. This will happen 18 minutes after Contact I. [Venus Transit of 2004: 51 Amazing Photos]

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Transit of Venus Explained: The Stages of Today's Rare Sun Crossing