This century’s last transit

Tomorrow you will be able to witness a celestial spectacle no one will ever see again this century: a transit of Venus. While Venus starts to slowly pass between Earth and Sun, millions of people will look in awe at the planet’s silhouette against the brilliant solar disk, beholding the actual clockwork movement of our solar system. It’s one of the most infrequent of planetary alignments, and its rarity alone should already justify your own observation of the transit. If you miss this one, you will have to wait another 105 years until the snow is falling in December 2117.

Because of its rarity, viewing the transit yourself not only connects you to the hundreds of astronomers in history who set out on perilous journeys to measure the Sun’s distance using the transit of Venus, but also to your descendants who will see it again in the next century: it will make you part of a chain of privileged people to whom Venus reveals her black profile backdropped by the solar disk.

There are four distinct stages of the transit: when Venus starts to move unto the solar disk (1) there will become visible a small black dent on the solar limb, which will grow larger when time passes by. After nearly 18 minutes, the disk of Venus is on the solar disk in its entirety (2), touching the solar limb on the inside. From then on Venus parades in about seven hours to the other side of the solar disk, where the planet will touch the solar limb first on the inside (3). Just 18 minutes later the shadow of Venus will have left the solar disk once more (4) – the transit is over.

Not all stages might be visible from where you are. Find out which part of the transit you can see using our calculator of local transit times.

During the stages at the start and end of the transit, two special phenomena may be seen. While Venus is partly off the solar disk (1 and 4), light scattered by the planet’s dense atmosphere will produce a thin, luminous ring around the silhouette. This effect, called the aureole, was already seen during the 1761 transit of Venus and has helped astronomers to investigate the composition of the atmosphere. Tomorrow astronomers will yet again turn their instruments to the aureole to analyse the structure of the Cytherean atmosphere.
When Venus touches the solar limb on the inside (2 and 3) a greyish haze between the two limbs may become apparent, depending on the quality of the applied optics. In the 18th century this optical effect caused the round shape of Venus to deform extremely, hence its name black drop effect. With larger telescopes the effect is less manifest.

Image credits: left image by Lorenzo Comolli, right image by Paul Dolk.

Whenever you have the chance, you should observe the transit yourself. But be careful: only look at the sun with proper eye protection, on pain of permanent loss of sight. Once safety precautions are met, you can enjoy the transit of Venus in several ways. Just peering through a telescope and discerning the small black dot of Venus can be a humbling experience: realising that Venus is about the same size of our own planet, our smallness in the vast Universe becomes readily visible before our own eyes. But there’s more you can do.

  • Use our free phone app to join a collective experiment to measure the Sun’s distance.
  • Picture the transit, either by sketching or photographing, and upload your creations to our gallery by sending it to
  • Enter Southern Stars’ Venus Transit Challenge, a photo contest for mobile devices.
  • Twitter about what you come across (use hashtags #tov2012 or #venustransit). The results of all these activities can be followed live using Esri’s web app, showing an interactive map with all observations recorded by users of the phone app, as well as tweets, pictures and videos appearing in social media.
  • Even if you’re clouded out, or if the transit isn’t visible from where you live, there are still many online viewing options to follow the event.
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