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Venus transit app lets users track the planet's rare voyage

It won't happen again for 105 years, so pay attention: on Tuesday, you can see Venus traverse the sun. And a new app called VenusTransit can help you record this "last such opportunity in our lifetimes."

Screenshot from NASA Science's video showing what the transit of Venus looks like. NASA Science
Screenshot of the VenusTransit mobile app. VenusTransit

One of the rarest celestial events viewable from Earth will occur Tuesday -- the planet Venus will make a trek across the face of the sun. Such an event has only been observed six times in recorded history, and the next occurrence won't come until 2117, so space buffs are gearing up for the big show.

One way to stay tuned -- and also help scientists record Venus' voyage -- is by using an app called VenusTransit. With this app, which is available on Android and iOS, amateur astronomers can join the ranks of historic explorers like Capt. James Cook along with current NASA scientists.

"In centuries past, explorers traveled around the globe to time the transit of Venus to determine the size of the solar system," Steven van Roode, who helped conceive of the app, wrote in the description of VenusTransit. "The phone app will allow citizens around the world to witness this rare phenomenon and to contribute their observation to a collective experiment to measure the sun's distance. This will literally be the last such opportunity in your lifetime."

The app comes with a built-in timer to calculate how long Venus takes to cross the Sun, beginning with "ingress" -- just as the little black dot starts its trek -- and ending with "egress" at the tail end of the crossing. It also has simulation and visibility sections to tell users when the transit will begin based on their GPS coordinates.

Venus' ramble between the sun and the Earth will be viewable starting at sunset on the East Coast of North America and earlier for other parts of the U.S. According to NASA Science News, observers on all seven continents will be in a position to see it.

"The timing favors observers in the mid-Pacific where the sun is high overhead during the crossing," NASA Science's Tony Phillips wrote in a blog post. "In the USA, the transit will at its best around sunset. That's good, too. Creative photographers will have a field day imaging the swollen red sun 'punctured' by the circular disk of Venus."

The importance of Venus' transit has been a topic of discussion for astronomers for centuries, because it has helped them estimate the distance between the Earth and the Sun, while also allowing them to figure out the relative size of the solar system.

Once Venus has made its trek Tuesday, all of the data from users tracking it with the VenusTransit app will be sent to central servers to be compiled. An interactive map with this information will be posted the Astronomers Without Borders Web site.

Here is a video from NASA Science about Venus' extraordinary journey: