If you visit the Public Observatory during its daytime hours in May (1–3pm on Wednesday through Saturday, weather permitting), you can use the 16” telescope to observe an object which looks a lot like the Moon.  Hanging in a blue sky, it shines with yellowish reflected sunlight.  We can currently only see part of its illuminated side, giving it a crescent shape.  You won’t spot any craters, though, and it looks a little fuzzy.  It’s not the Moon, but the Earth’s twin, Venus — the planet which is most similar in orbit and size to the Earth.  All eyes are on Venus now as it prepares for the show of the century: a transit across the face of the Sun.

Venus as seen through telescopes at the Public Observatory on April 25, 2012.

Venus and Mercury,  the only two planets which orbit closer to the Sun than the Earth, can be seen in crescent phase when they start to pass between the Sun and the Earth.  The only source of illumination is the Sun, so when Venus is between the Sun and the Earth, we see mostly the dark night side and only a sliver of the daylit side.

Phases of Venus. Image Credit: NASA

When Venus passes between the Earth and the Sun (its new phase), it is invisible because we are looking at its nighttime side.  Venus is no more than a few degrees away from the Sun at this point, so anyone attempting to observe Venus when it is new or a slim crescent should be careful to not point their telescope at the Sun, even for an instant.  Permanent eye damage could result from such an accident.

Does Venus go directly between the Earth and the Sun and cast its shadow on the Earth?  Usually not.  The orbit of Venus is tipped 3.3° with respect to Earth’s, so when Venus passes through its new phase, it usually goes above or below the Sun.  Sometimes, however, the orbits line up.  When Venus crosses directly between the Sun and the Earth, it blocks only 0.1% of the Sun’s light. The drop in overall sunlight is not noticeable, but when viewed through a telescope (safely!), the silhouette of Venus appears as a dark dot in front of the bright Sun.  This event is called a transit.

The 2004 transit of Venus, observed by NASA's TRACE satellite. The faint halo on the lower left edge of Venus is sunlight shining through its atmosphere.

Transits of Venus occur at regular intervals, but they are rare.  They come in pairs eight years apart, and more than a century passes between pairs of transits.  There was a transit in 2004, and the second one in that pair occurs on June 5-6, 2012.  The next transit will not happen until 2117.  For nearly all of us, this is our last chance to see this event.

The transit can only be viewed by safely observing the Sun.  During the 6-hour transit, Venus will be silhouetted against the disk of the Sun.  Looking at the Sun with the naked eye can hurt the eyes, and pointing a telescope without a safe solar filter at the Sun will cause immediate telescope damage or permanent eye damage.

Here at the Public Observatory in Washington, DC, the transit will start shortly after 6 pm on June 5, 2012. The dome of the Observatory will already be in the shadow of the National Air and Space Museum building, so if the weather is clear, we will set up safe solar telescopes just outside the Museum’s entrance facing the National Mall.  We will follow the transit until the Sun gets too low in the sky to observe.  Sunset is at 8:31 PM that day, and we will see less than half of the transit from Washington, DC.  We will also stream a live image to the Web, and tweet updates at @SIObservatory. The event is paired with a free presentation by Museum staff about the history and science of transits of Venus, a free lecture inside the Museum on detecting the transits of planets in front of other stars and, later, nighttime observing in and around the Observatory.

Visitors using a safe solar telescope outside the Museum in Washington, DC.

Check with your local observatory or astronomy club for a public transit viewing event near you (whether it is visible depends on your location in the world), or check the Solar Dynamics Observatory’s map.  There are ways to observe the Sun safely at home if using proper equipment.  You can use eclipse glasses to safely observe the Sun, though the dot of Venus against the unmagnified Sun is at the limit of the eye’s resolution.  Finally, NASA is planning to stream the event live from the summit of Mauna Kea in Hawai’i, which is very likely to have good weather.  The entire transit will be visible from that location.

On June 5, 2012, solar telescopes around the world and in space will point to the Sun, marking another beat in the centuries-long dance of the planets.  Wherever you are in the world, whether your skies are clear or cloudy, it’s not an event to miss!

Geneviève de Messieres is an astronomy educator at the National Air and Space Museum.