Phrases like "once in a lifetime" have a tendency to get bandied about with more than a touch of recklessness. Is that hamburger really so delicious that eating it is an experience the likes of which you'll never have again?

What's happening up in the sky on Tuesday, on the other hand, may be definition of a once in a lifetime event.

Beginning at 3:06pm and ending at 9:47pm, Venus will pass directly in front of the Sun in a manner visible to observers on earth. Its called the "Transit of Venus" and is a celestial occurrence that won't happen again until 2117.

(SCROLL DOWN FOR PHOTOS)

The phenomenon was first discovered by English astronomer Jeremiah Horrocks in 1639. After predicting its existence, Horrocks used a helioscope to project the image onto a piece of paper and watch the small shadow cast by Venus move slowly across the sun.

The event happens in pairs every 100 to 120 years. The last Transit of Venus occurred in 2004, but was only visible on the other side of the planet. (So technically its a "twice in a lifetime" event if you recently moved here from Australia.)

"You might think it might happen more often," Wayne State Univerity's Jeff Conn explained to the Huffington Post. "The reason it doesn't is...Venus is not quite in the same plane [as the earth] -- and every time Venus is lined up, it is usually a little above or a little below the sun."

The Transit of Venus isn't just an interesting thing to observe. It held a special scientific purpose for generations of astronomers who used it to refine measurements of the relative sizes and distances between things in the solar system.

Horrocks used it to determine Venus's size and distance from the Earth. Later astronomers enlisted data gathered from the transit to do things like more accurately calculate the distance between the Earth and the Sun and fine tune maps. Info from the 1769 transit caused map makers to shift the location of California about 350 miles to the east.

Even today, when more sophisticated instruments like radio telescopes provide generate most astronomical data, scientists are still using the Transit of Venus to discover new things.

"Astronomers in the 18th and 19th centuries observed transits of Mercury and Venus to help measure the distance from Earth to sun," Frank Hill, director of the National Solar Observatory’s Integrated Synoptic Program, told HuffPost. "We have that number nailed down now, but transits are still useful. This one will help us calibrate in several different instruments, and hunt for extrasolar planets with atmospheres."

Watching planets briefly dim the light from the stars they're orbiting is how astronomers typically discover planets outside of our solar system. At the same time, the planet's atmosphere also filters light from the star as it passes in transit. Scientists watching the Transit of Venus can combine what they know about our planetary neighbor's atmosphere with observations of how its atmosphere filters light during said transit to gain knowledge that could be applied to determining the atmospheric composition of other, far away planets.

The Bay Area has a long history of being packed with great viewing sites for the transit. The San Francisco Chronicle notes:

The transit of 1882 also enthralled residents of the Bay Area and the West Coast. "Residents of San Francisco were noticed yesterday with a piece of smoked glass to their eye, looking curiously at the sun," The Chronicle reported at the time. It was, the paper reported, "a grand and beautiful spectacle."

Here are few local places to both view and learn more about the Transit of Venus. Just remember to not look directly at the sun without protective eyewear:

College of San Mateo, San Mateo: 3-7pm. Professor Darryl Stanford will be leading viewing sessions and answering questions.

Exploratorium, San Francisco: 3pm. Showing live webcast of the transit broadcast from Mauna Loa, Hawaii.

Mt. Diablo Summit, Walnut Creek: 3pm. The Mount Diablo Astronomical Society will have telescopes set up for safe viewing.

Chabot Space and Science Center, Oakland: 3-10pm. Telescopes will be set up along with a NASA broadcast of the transit.

Lawrence Hall of Science, Berkeley: 2:30-8:30pm. Solar telescopes set up in the main lobby and a planetarium will be showing a program about the transit.

Check out this slideshow about the history of the Transit of Venus:

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  • Venus Transit Across The Sun

    IN SPACE - JUNE 5: In this handout image provided by NASA, the SDO satellite captures an ultra-high definition image of the Transit of Venus across the face of the sun on June 5, 2012 from space. The last transit was in 2004 and the next pair of events will not happen again until the year 2117 and 2125. (Photo by SDO/NASA via Getty Images)

  • Clouds partially obscure the sun during the transit of Venus June 5, 2012 as seen from Riverside Park on the west side of Manhattan in New York. Astronomers around the world are training their telescopes on the skies to watch Venus pass in front of the Sun, a once-in-a-lifetime event that will not be seen for another 105 years. (STAN HONDA/AFP/GettyImages)

  • 8-year-old Alex Olling smiles as he uses makeshift sunglasses to watch Venus crossing the sun's face on June 5, 2012 as seen from College Park, Maryland. Astronomers around the world trained their telescopes on the skies Tuesday to watch Venus pass in front of the Sun, a once-in-a-lifetime event that will not be seen for another 105 years. (JEWEL SAMAD/AFP/GettyImages)

  • People wait to view the transit of Venus across the face of the sun on June 5, 2012, in Medellin, Antioquia department, Colombia. Astronomers around the world are pointing their telescopes on the skies to watch Venus pass in front of the Sun, a once-in-a-lifetime event that will not be seen for another 105 years. (RAUL ARBOLEDA/AFP/GettyImages)

  • A passenger plane passes Venus (black spot on the right) in the planet's transit across the face of the sun seen from Los Angeles, California on June 05, 2012. Astronomers around the world are training their telescopes on the skies to watch Venus pass in front of the Sun, a once-in-a-lifetime event that will not be seen for another 105 years.AFP PHOTO /JOE KLAMAR (Photo credit should read JOE KLAMAR/AFP/GettyImages)

  • A woman and her child take a peek at a telescope during the transit of Venus across the Sun as at the Universum Museum in the National University (UNAM) in Mexico City, Mexico, Tuesday, June 5, 2012. Despite a very cloudy day, thousands of people gathered at the museum to get a glimpse of the event that will only be seen again in 2117. (AP Photo/Dario Lopez-Mills)

  • San Luis Obispo, Calif.

    Dr. Joe Schwartz observes the transit of Venus through a telescope from his home in San Luis Obispo, Calif., on June 5, 2012.

  • Venus Transit 2012

    <a href="http://www.huffingtonpost.com/social/Eric_Ebling"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/794169724/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Eric_Ebling">Eric Ebling</a>:<br />took this series over a few hours on my rooftop. Santa Monica California

  • Getting ready in Traverse City, Michigan.

  • 'Setting up shop in Texas'

    Celestron CPC 100 with MallinCam video system and white filter for sun viewing. (Submitted by FiberInspector.)

  • My Teenage Daughter Eclipsing the Eclipse

    <a href="http://www.huffingtonpost.com/social/sexypathdoc"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/sexypathdoc">sexypathdoc</a>:<br />

  • San Luis Obispo, Calif.

    Dr. Joe Schwartz, left, and Barbara Boom view the transit of Venus in San Luis Obispo, Calif.

  • Venus begins to pass in front of the sun, as visible from New York, Tuesday, June 5, 2012. From the U.S. to South Korea, people around the world turned their attention to the daytime sky on Tuesday and early Wednesday in Asia to make sure they caught the once-in-a-lifetime sight of the transit of Venus, which won't be seen for another 150 years. (AP Photo/Seth Wenig)

  • San Luis Obispo, Calif.

    Brian P. Lawler writes: "About 100 people stood in line Tuesday afternoon to see Venus' transit of the Sun at California Polytechnic State University in San Luis Obispo, Calif. Manning the telescope on the left is Physics and Astronomy Professor David Mitchell."

  • Indian Pramod Kumar Pandey, Director of Jawahar Planetarium checks a telescope as he makes preparations for people to watch the transit of Venus in Allahabad, India, Tuesday, June 5, 2012. Stargazers around the world are setting up special telescopes and passing out cardboard eclipse glasses to view the once-in-a-lifetime celestial cameo of Venus passing in front of the sun. Venus is Earth's second-closest neighboring planet. (AP Photo/Rajesh Kumar Singh)

  • Brian P. Lawler writes: Carina Hessmer, a graduating senior at California Polytechnic State University, San Luis Obispo, Calif., looks through an 80mm telescope at Venus' transit of the Sun. Behind the telescope is Cal Poly Physics and Astronomy Professor David Mitchell. Over 100 people stood in line this afternoon to see the astronomical event that won't repeat for over 100 years.

  • Venus transit over Dallas

    <a href="http://www.huffingtonpost.com/social/davidworthington"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/davidworthington">davidworthington</a>:<br />Venus in transit shot from Dallas Texas at appox 6 p.m.

  • Clouds partially obscure the sun during the transit of Venus June 5, 2012 as seen from Riverside Park on the west side of Manhattan in New York. Astronomers around the world are training their telescopes on the skies to watch Venus pass in front of the Sun, a once-in-a-lifetime event that will not be seen for another 105 years. (STAN HONDA/AFP/GettyImages)

  • Venus (top) begins to cross the sun's face during the transit of Venus June 5, 2012 as seen from the west side of Manhattan in New York. Astronomers around the world are training their telescopes on the skies to watch Venus pass in front of the Sun, a once-in-a-lifetime event that will not be seen for another 105 years. (STAN HONDA/AFP/GettyImages)

  • Venus VS Sun

    MY favorite Fuji with 30X zoom

  • Venus sailing across the sun

    <a href="http://www.huffingtonpost.com/social/AdamJohn89"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1410774186/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/AdamJohn89">AdamJohn89</a>:<br />Venus sailing across the sun. Picture taken at 7PM from Havre de Grace Maryland.

  • Projection of Venus using binoculars.

    <a href="http://www.huffingtonpost.com/social/Maharlika77"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/Maharlika77">Maharlika77</a>:<br />View of Venus from Columbia, MO.

  • Venus Transit in Milton Ontario using binoculars.

    <a href="http://www.huffingtonpost.com/social/Mike_Druiven"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1510503789/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Mike_Druiven">Mike Druiven</a>:<br />Photo by Sandra Voisin. Steady hand by Mike Druiven

  • In this photo provided by Ole Miss Communications, astronomy enthusiasts view the transit of Venus in front of the sun at the Kennon Observatory at The University of Mississippi in Oxford, Miss., Tuesday, June 5, 2012. From the U.S. to South Korea, people around the world turned their attention to the daytime sky on Tuesday and early Wednesday in Asia to make sure they caught the once-in-a-lifetime sight of the transit of Venus, which won't be seen for another 150 years. (AP Photo/Ole Miss Communications, Nathan Latil)

  • Hong Kong stargazers use telescopes to observe the transit of Venus along the Victoria Habour in Hong Kong Wednesday, June 6, 2012. Stargazers around the world are setting up special telescopes and passing out cardboard eclipse glasses to view the once-in-a-lifetime celestial cameo of Venus passing in front of the sun. (AP Photo/Vincent Yu)

  • The Starlight Astronomy Club in Altoona, Pa., gears up for the Venus transit with telescopes modified for solar viewing.

  • Tom Kasner of the Starlight Astronomy Club in Altoona, Pa. getting ready for the Venus transit.

  • Venus, upper right, transits the sun as seen through a dark glass from Quito, Ecuador, Tuesday, June 5, 2012. From the U.S. to South Korea, people around the world turned their attention to the daytime sky on Tuesday and early Wednesday in Asia to make sure they caught the once-in-a-lifetime sight of the transit of Venus, which won't be seen for another 150 years. (AP Photo/Dolores Ochoa)

  • A cameraman records the Transit of Venus seen on a screen at the Chapultepec Castle in Mexico City on June 5, 2012. (ALFREDO ESTRELLA/AFP/GettyImages)

  • Venus In Albuquerque, NM

    <a href="http://www.huffingtonpost.com/social/Azulla"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1106103997/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Azulla">Azulla</a>:<br />

  • High Ridge, Mo.

    Taken with a Cannon PowerShot SX230 HS starting at 5:10 p.m. Central time.

  • Kyle, Tex.

    <a href="http://www.huffingtonpost.com/social/Tasheena_Ashton"></a><a href="http://www.huffingtonpost.com/social/Tasheena_Ashton">Tasheena Ashton</a>:<br />

  • In this photo made using a red filter, Venus begins to pass in front of the sun, as visible from from Overland Park, Kan.on Tuesday, June 5, 2012. From the U.S. to South Korea, people around the world turned their attention to the daytime sky on Tuesday and early Wednesday in Asia to make sure they caught the once-in-a-lifetime sight of the transit of Venus, which won't be seen for another 150 years. (AP Photo/Charlie Riedel)

  • Venus in Transit

    <a href="http://www.huffingtonpost.com/social/casaroonc"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/casaroonc">casaroonc</a>:<br />I took this Photo with my Canon Power Shot that was looking through a pair of Binoculars that were looking through my welding Helmet. Venus is the green dot at the 1 O'Clock mark. Eric DeYoung Wilmington, NC

  • Meggan Wood's Homemade Solar Filter

    Meggan Wood writes: "NexStar 5SE telescope with a homemade solar filter and my Canon Rebel Xsi attached with a basic T-ring and adapter."

  • 4:59 p.m.

    Photo by Meggan Wood

  • A crescent Venus shines in an ultraviolet snapshot taken by the NASA-ESA Hubble Space Telescope in 1995. As Venus circles the sun, it appears to go through phases that mimic those of our moon when seen through a telescope. Visit National Geographic's gallery of Venus transit photos <a href="http://news.nationalgeographic.com/news/2012/06/pictures/120604-transit-of-venus-2012-pictures-sun-earth-planets-space/" target="_hplink">here</a>, and learn more <a href="http://news.nationalgeographic.com/news/2012/06/120604-transit-of-venus-2012-sun-planet-hubble-space-science-how-when/" target="_hplink">here</a>.

  • Photo by Marcus Holmes.

  • Marcus Holmes' Rig

  • NASA's SDO Satellite Captures First Image of the Venus Transit

    On June 5-6 2012, SDO is collecting images of one of the rarest predictable solar events: the transit of Venus across the face of the sun. This event happens in pairs eight years apart that are separated from each other by 105 or 121 years. The last transit was in 2004 and the next will not happen until 2117. (NASA/SDO, HMI)

  • Transit of Venus

    <a href="http://www.huffingtonpost.com/social/Tammye_McDuff_Dunn"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1642065951/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Tammye_McDuff_Dunn">Tammye McDuff Dunn</a>:<br />Using the technology of a Discovery 8" Dobsonian and a solar filter and my Sony digital camera, we caught amazing photos!

  • Venus Sailing across the Sun

    <a href="http://www.huffingtonpost.com/social/hagjk_09"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/524358076/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/hagjk_09">hagjk 09</a>:<br />JHK Photography New Oxford Pa 17350

  • Venus hangs low in the evening twilight near a razor-thin crescent moon in an undated picture taken from Troms County in northern Norway. Visit National Geographic's gallery of Venus transit photos <a href="http://news.nationalgeographic.com/news/2012/06/pictures/120604-transit-of-venus-2012-pictures-sun-earth-planets-space/" target="_hplink">here</a>, and learn more <a href="http://news.nationalgeographic.com/news/2012/06/120604-transit-of-venus-2012-sun-planet-hubble-space-science-how-when/" target="_hplink">here</a>.

  • Transit of Venus

    <a href="http://www.huffingtonpost.com/social/Kelby_Roberson"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/1817589103/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Kelby_Roberson">Kelby Roberson</a>:<br />This image was taken on June 5, 2012 in the United States.

  • Venus and an airplane transit the sun

    <a href="http://www.huffingtonpost.com/social/grumpykel"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/grumpykel">grumpykel</a>:<br />Venus and an airplane cross infront of the sun.

  • Venus Transit 2012

    <a href="http://www.huffingtonpost.com/social/Richard_MysticalSun"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/507162091/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Richard_MysticalSun">Richard MysticalSun</a>:<br />I took these two photos in the SF Bay Area using a custom made kit.

  • Venus transit over Carbondale IL

    <a href="http://www.huffingtonpost.com/social/debbiethechicken"><img style="float:left;padding-right:6px !important;" src="http://s.huffpost.com/images/profile/user_placeholder.gif" /></a><a href="http://www.huffingtonpost.com/social/debbiethechicken">debbiethechicken</a>:<br />Photo by Jason Thomas. Shot with a Canon S2IS (old) Inside a projection box. Images captured on Mac with a USB over Ethernet connection and Canon Image Capture.

  • veiw of venus

    <a href="http://www.huffingtonpost.com/social/RobHTexera"><img style="float:left;padding-right:6px !important;" src="http://images.huffingtonpost.com/twitter_profile_img/1876464.png" /></a><a href="http://www.huffingtonpost.com/social/RobHTexera">RobHTexera</a>:<br />venus passes by the sun 6/5/2012

  • Venus Transit from Vashon

    <a href="http://www.huffingtonpost.com/social/Mark_Cavener"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/764384573/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Mark_Cavener">Mark Cavener</a>:<br />Taken @ 3:39PM PDT from Manzanita Beach - Vashon Island, WA

  • Venus Transit 2012

    <a href="http://www.huffingtonpost.com/social/Richard_MysticalSun"><img style="float:left;padding-right:6px !important;" src="http://graph.facebook.com/507162091/picture?type=square" /></a><a href="http://www.huffingtonpost.com/social/Richard_MysticalSun">Richard MysticalSun</a>:<br />Taken from the SF Bay Area using my custom solar astrophotography kit.