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Transit of Venus today

June 5, 2012

Begins: 5:03:55 pm in Delaware. The transit lasts nearly seven hours, but the sun sets before it ends.
Webcast: Best bet 5:45 pm, NASA webcast from Mauna Kea, Hawaii. They will see the whole transit from beginning to end.

Today, most of the world will have the opportunity to witness a rare and scientifically important astronomical event: the transit of Venus. This transit occurs when Venus comes between us and the Sun and appears as a small black disk traveling (transiting) across the face of the Sun.

The full transit will take nearly 7 hours. In northern Delaware, the transit begins at 5:03:55 pm, so the sun will set before the transit is complete. The full transit can be seen on webcasts around the world, which start at different times depending on the observing point. The primary NASA webcast is from Mauna Kea, Hawaii.

Whatever you do, make sure you follow the safety rules for observing the Sun:

  • Don’t look directly at the Sun.
  • Don’t look at the Sun through any old sunglasses or colored glass you have lying around. It might make it more comfortable to look at the Sun but it won’t stop the invisible radiation from damaging your eyes, probably permanently.
  • Especially don’t look directly at the Sun through a telescope or binoculars, unless it is operated by an expert with the appropriate equipment.

There are a number of ways to look at the Sun safely. I’m not going to describe them here, but you can find them on the web. Here’s a link to some safe viewing advice from NASA.

Perhaps the best way to watch the transit is on a webcast, especially if it is cloudy where you are. I’m planning to watch it on the live NASA webcast from the summit of Mauna Kea, Hawaii. If I have a line of sight I may go out and experiment with an ad hoc pinhole projector.

A transit of Venus is a rare event. According to Space.com:

The reason for the wide interest in the Venus transit is clear: This is the last time anyone alive today will have a chance to see Venus cross the face of the sun. The next transit of Venus won’t occur until the year 2117. Venus transits occur in pairs eight years apart. The last one occurred in 2004, making the 2012 transit the last in the current series.

And from wikipedia:

Transits of Venus are among the rarest of predictable astronomical phenomena. They occur in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.5 years and 105.5 years.

Click here for a great informative video (30 minutes).

Venus of course is the second rock from the Sun, and so has the opportunity to get between us and the Sun. When the Moon does this, we call it a solar eclipse, because the Moon blocks the entire Sun. But Venus, although larger than the Moon, is also much further away, so we see it as a dot that doesn’t perceptibly block any sunlight at all. The only other planet that can do this is Mercury, the first rock from the Sun. Transits of Mercury happen much more frequently.

Transits of Venus are of great scientific interest. The first reasonably accurate attempts to measure the distance from the Earth to the Sun were calculated by observing the transit of Venus. In 1619, Kepler published his tables of planetary orbits, based on his observation of elliptical orbits rather than the circular orbits which had been assumed. With this more accurate model, Jeremiah Horrocks refined Kepler’s calculations and successfully predicted the 1639 transit, allowing Horrocks to observe the transit and estimate the distance to the Sun as well as the size of Venus.

In the transit of 1861, Mikhail Lomonosov observed refraction of light around Venus, and inferred the existence of an atmosphere.

In today’s transit, the Hubble Space Telescope will also be looking at the atmosphere of Venus, but not directly. The Hubble is not equipped for solar viewing and cannot be pointed anywhere near the Sun, or it would be destroyed. Instead, the HST will be pointed at the Moon, and will observe the spectrum of the Sun’s light reflected from the moon. During the transit, any change in the spectrum of light reflected from the moon must be caused by the atmosphere of Venus, so that change in spectrum can be analyzed to determine the composition of the atmosphere.

We already know a great deal about the atmosphere of Venus, but we are practicing this technique because it can also be used to analyze the atmosphere of exoplanets when they transit their stars, and potentially detect atmospheres that might support life. Yes, the instruments are that good.

The Hubble will have the opportunity to calibrate this technique again in 2014, when the Earth transits the sun from the viewpoint of Jupiter. The HST will observe the light reflected from Jupiter while Earth is between the Sun and Jupiter, and will be able to refine the spectrum-analyzing technique further.

It’s not widely known in the US that a series of Soviet probes (the Venera spacecraft) were launched to Venus from 1961 through 1984 and transmitted data. Some of the probes simply crash-landed or parachuted through the atmosphere collecting data, but some of the later probes successfuly landed intact. Venera 13 transmitted these remarkable images of the planet’s surface:

The success of these probes was even more remarkable, considering that Venus is one of the most extreme environments in the solar system. The surface temperature of Venus is nearly 900° F, at a pressure of 90 Earth atmospheres, with thick clouds of sulfur dioxide. In fact it is often said that the surface of Venus resembles traditional descriptions of Hell.

Play us out, boys!

  1. John Young
    June 5, 2012 at 7:30 am

    I predict a million people alive today will be alive in 2117.

  2. June 5, 2012 at 7:52 am

    It would suck to be in the last generation to die.

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